Content
Data
license:
Version: 2.4.7 (in Debian 10)
Developer / owner: James Yonan
Short description:
Manual page and help for the openvpn Linux command. OpenVPN is an open source VPN daemon from James Yonan. Because OpenVPN is a universal VPN device that offers great flexibility, there are many options on this manual page. If you are not already familiar with OpenVPN, you may want to skip ahead to the examples section, where you will see how to create simple VPNs from the command line without the need for a configuration file.
Man page output
man openvpnopenvpn(8) System Manager's Manual openvpn(8)
NAME
openvpn - secure IP tunnel daemon.
SYNOPSIS
openvpn [ options ... ]
INTRODUCTION
OpenVPN is an open source VPN daemon by James Yonan. Because OpenVPN tries to be a universal VPN tool offering a great deal
of flexibility, there are a lot of options on this manual page. If you're new to OpenVPN, you might want to skip ahead to
the examples section where you will see how to construct simple VPNs on the command line without even needing a configura‐
tion file.
Also note that there's more documentation and examples on the OpenVPN web site: http://openvpn.net/
And if you would like to see a shorter version of this manual, see the openvpn usage message which can be obtained by run‐
ning openvpn without any parameters.
DESCRIPTION
OpenVPN is a robust and highly flexible VPN daemon. OpenVPN supports SSL/TLS security, ethernet bridging, TCP or UDP tunnel
transport through proxies or NAT, support for dynamic IP addresses and DHCP, scalability to hundreds or thousands of users,
and portability to most major OS platforms.
OpenVPN is tightly bound to the OpenSSL library, and derives much of its crypto capabilities from it.
OpenVPN supports conventional encryption using a pre-shared secret key (Static Key mode) or public key security (SSL/TLS
mode) using client & server certificates. OpenVPN also supports non-encrypted TCP/UDP tunnels.
OpenVPN is designed to work with the TUN/TAP virtual networking interface that exists on most platforms.
Overall, OpenVPN aims to offer many of the key features of IPSec but with a relatively lightweight footprint.
OPTIONS
OpenVPN allows any option to be placed either on the command line or in a configuration file. Though all command line op‐
tions are preceded by a double-leading-dash ("--"), this prefix can be removed when an option is placed in a configuration
file.
--help Show options.
--config file
Load additional config options from file where each line corresponds to one command line option, but with the leading
'--' removed.
If --config file is the only option to the openvpn command, the --config can be removed, and the command can be given
as openvpn file
Note that configuration files can be nested to a reasonable depth.
Double quotation or single quotation characters ("", '') can be used to enclose single parameters containing white‐
space, and "#" or ";" characters in the first column can be used to denote comments.
Note that OpenVPN 2.0 and higher performs backslash-based shell escaping for characters not in single quotations, so
the following mappings should be observed:
\\ Maps to a single backslash character (\).
\" Pass a literal doublequote character ("), don't
interpret it as enclosing a parameter.
\[SPACE] Pass a literal space or tab character, don't
interpret it as a parameter delimiter.
For example on Windows, use double backslashes to represent pathnames:
secret "c:\\OpenVPN\\secret.key"
For examples of configuration files, see http://openvpn.net/examples.html
Here is an example configuration file:
#
# Sample OpenVPN configuration file for
# using a pre-shared static key.
#
# '#' or ';' may be used to delimit comments.
# Use a dynamic tun device.
dev tun
# Our remote peer
remote mypeer.mydomain
# 10.1.0.1 is our local VPN endpoint
# 10.1.0.2 is our remote VPN endpoint
ifconfig 10.1.0.1 10.1.0.2
# Our pre-shared static key
secret static.key
Tunnel Options:
--mode m
Set OpenVPN major mode. By default, OpenVPN runs in point-to-point mode ("p2p"). OpenVPN 2.0 introduces a new mode
("server") which implements a multi-client server capability.
--local host
Local host name or IP address for bind. If specified, OpenVPN will bind to this address only. If unspecified, Open‐
VPN will bind to all interfaces.
--remote host [port] [proto]
Remote host name or IP address. On the client, multiple --remote options may be specified for redundancy, each re‐
ferring to a different OpenVPN server. Specifying multiple --remote options for this purpose is a special case of
the more general connection-profile feature. See the <connection> documentation below.
The OpenVPN client will try to connect to a server at host:port in the order specified by the list of --remote op‐
tions.
proto indicates the protocol to use when connecting with the remote, and may be "tcp" or "udp".
For forcing IPv4 or IPv6 connection suffix tcp or udp with 4/6 like udp4/udp6/tcp4/tcp6.
The client will move on to the next host in the list, in the event of connection failure. Note that at any given
time, the OpenVPN client will at most be connected to one server.
Note that since UDP is connectionless, connection failure is defined by the --ping and --ping-restart options.
Note the following corner case: If you use multiple --remote options, AND you are dropping root privileges on the
client with --user and/or --group, AND the client is running a non-Windows OS, if the client needs to switch to a
different server, and that server pushes back different TUN/TAP or route settings, the client may lack the necessary
privileges to close and reopen the TUN/TAP interface. This could cause the client to exit with a fatal error.
If --remote is unspecified, OpenVPN will listen for packets from any IP address, but will not act on those packets
unless they pass all authentication tests. This requirement for authentication is binding on all potential peers,
even those from known and supposedly trusted IP addresses (it is very easy to forge a source IP address on a UDP
packet).
When used in TCP mode, --remote will act as a filter, rejecting connections from any host which does not match host.
If host is a DNS name which resolves to multiple IP addresses, OpenVPN will try them in the order that the system
getaddrinfo() presents them, so priorization and DNS randomization is done by the system library. Unless an IP ver‐
sion is forced by the protocol specification (4/6 suffix), OpenVPN will try both IPv4 and IPv6 addresses, in the or‐
der getaddrinfo() returns them.
--remote-random-hostname
Prepend a random string (6 bytes, 12 hex characters) to hostname to prevent DNS caching. For example, "foo.bar.gov"
would be modified to "<random-chars>.foo.bar.gov".
<connection>
Define a client connection profile. Client connection profiles are groups of OpenVPN options that describe how to
connect to a given OpenVPN server. Client connection profiles are specified within an OpenVPN configuration file,
and each profile is bracketed by <connection> and </connection>.
An OpenVPN client will try each connection profile sequentially until it achieves a successful connection.
--remote-random can be used to initially "scramble" the connection list.
Here is an example of connection profile usage:
client
dev tun
<connection>
remote 198.19.34.56 1194 udp
</connection>
<connection>
remote 198.19.34.56 443 tcp
</connection>
<connection>
remote 198.19.34.56 443 tcp
http-proxy 192.168.0.8 8080
</connection>
<connection>
remote 198.19.36.99 443 tcp
http-proxy 192.168.0.8 8080
</connection>
persist-key
persist-tun
pkcs12 client.p12
remote-cert-tls server
verb 3
First we try to connect to a server at 198.19.34.56:1194 using UDP. If that fails, we then try to connect to
198.19.34.56:443 using TCP. If that also fails, then try connecting through an HTTP proxy at 192.168.0.8:8080 to
198.19.34.56:443 using TCP. Finally, try to connect through the same proxy to a server at 198.19.36.99:443 using
TCP.
The following OpenVPN options may be used inside of a <connection> block:
bind, connect-retry, connect-retry-max, connect-timeout, explicit-exit-notify, float, fragment, http-proxy,
http-proxy-option, link-mtu, local, lport, mssfix, mtu-disc, nobind, port, proto, remote, rport, socks-proxy, tun-mtu
and tun-mtu-extra.
A defaulting mechanism exists for specifying options to apply to all <connection> profiles. If any of the above op‐
tions (with the exception of remote ) appear outside of a <connection> block, but in a configuration file which has
one or more <connection> blocks, the option setting will be used as a default for <connection> blocks which follow it
in the configuration file.
For example, suppose the nobind option were placed in the sample configuration file above, near the top of the file,
before the first <connection> block. The effect would be as if nobind were declared in all <connection> blocks below
it.
--proto-force p
When iterating through connection profiles, only consider profiles using protocol p ('tcp'|'udp').
--remote-random
When multiple --remote address/ports are specified, or if connection profiles are being used, initially randomize the
order of the list as a kind of basic load-balancing measure.
--proto p
Use protocol p for communicating with remote host. p can be udp, tcp-client, or tcp-server.
The default protocol is udp when --proto is not specified.
For UDP operation, --proto udp should be specified on both peers.
For TCP operation, one peer must use --proto tcp-server and the other must use --proto tcp-client. A peer started
with tcp-server will wait indefinitely for an incoming connection. A peer started with tcp-client will attempt to
connect, and if that fails, will sleep for 5 seconds (adjustable via the --connect-retry option) and try again infi‐
nite or up to N retries (adjustable via the --connect-retry-max option). Both TCP client and server will simulate a
SIGUSR1 restart signal if either side resets the connection.
OpenVPN is designed to operate optimally over UDP, but TCP capability is provided for situations where UDP cannot be
used. In comparison with UDP, TCP will usually be somewhat less efficient and less robust when used over unreliable
or congested networks.
This article outlines some of problems with tunneling IP over TCP:
http://sites.inka.de/sites/bigred/devel/tcp-tcp.html
There are certain cases, however, where using TCP may be advantageous from a security and robustness perspective,
such as tunneling non-IP or application-level UDP protocols, or tunneling protocols which don't possess a built-in
reliability layer.
--connect-retry n [max]
Wait n seconds between connection attempts (default=5). Repeated reconnection attempts are slowed down after 5 re‐
tries per remote by doubling the wait time after each unsuccessful attempt. The optional argument max specifies the
maximum value of wait time in seconds at which it gets capped (default=300).
--connect-retry-max n
n specifies the number of times each --remote or <connection> entry is tried. Specifying n as one would try each en‐
try exactly once. A successful connection resets the counter. (default=unlimited).
--show-proxy-settings
Show sensed HTTP or SOCKS proxy settings. Currently, only Windows clients support this option.
--http-proxy server port [authfile|'auto'|'auto-nct'] [auth-method]
Connect to remote host through an HTTP proxy at address server and port port. If HTTP Proxy-Authenticate is re‐
quired, authfile is a file containing a username and password on 2 lines, or "stdin" to prompt from console. Its con‐
tent can also be specified in the config file with the --http-proxy-user-pass option. (See section on inline files)
auth-method should be one of "none", "basic", or "ntlm".
HTTP Digest authentication is supported as well, but only via the auto or auto-nct flags (below).
The auto flag causes OpenVPN to automatically determine the auth-method and query stdin or the management interface
for username/password credentials, if required. This flag exists on OpenVPN 2.1 or higher.
The auto-nct flag (no clear-text auth) instructs OpenVPN to automatically determine the authentication method, but to
reject weak authentication protocols such as HTTP Basic Authentication.
--http-proxy-option type [parm]
Set extended HTTP proxy options. Repeat to set multiple options.
VERSION version -- Set HTTP version number to version (default=1.0).
AGENT user-agent -- Set HTTP "User-Agent" string to user-agent.
CUSTOM-HEADER name content -- Adds the custom Header with name as name and content as the content of the custom HTTP
header.
--socks-proxy server [port] [authfile]
Connect to remote host through a Socks5 proxy at address server and port port (default=1080). authfile (optional) is
a file containing a username and password on 2 lines, or "stdin" to prompt from console.
--resolv-retry n
If hostname resolve fails for --remote, retry resolve for n seconds before failing.
Set n to "infinite" to retry indefinitely.
By default, --resolv-retry infinite is enabled. You can disable by setting n=0.
--float
Allow remote peer to change its IP address and/or port number, such as due to DHCP (this is the default if --remote
is not used). --float when specified with --remote allows an OpenVPN session to initially connect to a peer at a
known address, however if packets arrive from a new address and pass all authentication tests, the new address will
take control of the session. This is useful when you are connecting to a peer which holds a dynamic address such as
a dial-in user or DHCP client.
Essentially, --float tells OpenVPN to accept authenticated packets from any address, not only the address which was
specified in the --remote option.
--ipchange cmd
Run command cmd when our remote ip-address is initially authenticated or changes.
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
When cmd is executed two arguments are appended after any arguments specified in cmd , as follows:
cmd ip_address port_number
Don't use --ipchange in --mode server mode. Use a --client-connect script instead.
See the "Environmental Variables" section below for additional parameters passed as environmental variables.
If you are running in a dynamic IP address environment where the IP addresses of either peer could change without no‐
tice, you can use this script, for example, to edit the /etc/hosts file with the current address of the peer. The
script will be run every time the remote peer changes its IP address.
Similarly if our IP address changes due to DHCP, we should configure our IP address change script (see man page for
dhcpcd(8) ) to deliver a SIGHUP or SIGUSR1 signal to OpenVPN. OpenVPN will then reestablish a connection with its
most recently authenticated peer on its new IP address.
--port port
TCP/UDP port number or port name for both local and remote (sets both --lport and --rport options to given port).
The current default of 1194 represents the official IANA port number assignment for OpenVPN and has been used since
version 2.0-beta17. Previous versions used port 5000 as the default.
--lport port
Set local TCP/UDP port number or name. Cannot be used together with --nobind option.
--rport port
Set TCP/UDP port number or name used by the --remote option. The port can also be set directly using the --remote op‐
tion.
--bind [ipv6only]
Bind to local address and port. This is the default unless any of --proto tcp-client , --http-proxy or --socks-proxy
are used.
If the ipv6only keyword is present OpenVPN will bind only to IPv6 (as oposed to IPv6 and IPv4) when a IPv6 socket is
opened.
--nobind
Do not bind to local address and port. The IP stack will allocate a dynamic port for returning packets. Since the
value of the dynamic port could not be known in advance by a peer, this option is only suitable for peers which will
be initiating connections by using the --remote option.
--dev tunX | tapX | null
TUN/TAP virtual network device ( X can be omitted for a dynamic device.)
See examples section below for an example on setting up a TUN device.
You must use either tun devices on both ends of the connection or tap devices on both ends. You cannot mix them, as
they represent different underlying network layers.
tun devices encapsulate IPv4 or IPv6 (OSI Layer 3) while tap devices encapsulate Ethernet 802.3 (OSI Layer 2).
--dev-type device-type
Which device type are we using? device-type should be tun (OSI Layer 3) or tap (OSI Layer 2). Use this option only
if the TUN/TAP device used with --dev does not begin with tun or tap.
--topology mode
Configure virtual addressing topology when running in --dev tun mode. This directive has no meaning in --dev tap
mode, which always uses a subnet topology.
If you set this directive on the server, the --server and --server-bridge directives will automatically push your
chosen topology setting to clients as well. This directive can also be manually pushed to clients. Like the --dev
directive, this directive must always be compatible between client and server.
mode can be one of:
net30 -- Use a point-to-point topology, by allocating one /30 subnet per client. This is designed to allow
point-to-point semantics when some or all of the connecting clients might be Windows systems. This is the default on
OpenVPN 2.0.
p2p -- Use a point-to-point topology where the remote endpoint of the client's tun interface always points to the lo‐
cal endpoint of the server's tun interface. This mode allocates a single IP address per connecting client. Only use
when none of the connecting clients are Windows systems. This mode is functionally equivalent to the --ifcon‐
fig-pool-linear directive which is available in OpenVPN 2.0, is deprecated and will be removed in OpenVPN 2.5
subnet -- Use a subnet rather than a point-to-point topology by configuring the tun interface with a local IP address
and subnet mask, similar to the topology used in --dev tap and ethernet bridging mode. This mode allocates a single
IP address per connecting client and works on Windows as well. Only available when server and clients are OpenVPN
2.1 or higher, or OpenVPN 2.0.x which has been manually patched with the --topology directive code. When used on
Windows, requires version 8.2 or higher of the TAP-Win32 driver. When used on *nix, requires that the tun driver
supports an ifconfig(8) command which sets a subnet instead of a remote endpoint IP address.
This option exists in OpenVPN 2.1 or higher.
Note: Using --topology subnet changes the interpretation of the arguments of --ifconfig to mean "address netmask", no
longer "local remote".
--dev-node node
Explicitly set the device node rather than using /dev/net/tun, /dev/tun, /dev/tap, etc. If OpenVPN cannot figure out
whether node is a TUN or TAP device based on the name, you should also specify --dev-type tun or --dev-type tap.
Under Mac OS X this option can be used to specify the default tun implementation. Using --dev-node utun forces usage
of the native Darwin tun kernel support. Use --dev-node utunN to select a specific utun instance. To force using the
tun.kext (/dev/tunX) use --dev-node tun. When not specifying a --dev-node option openvpn will first try to open
utun, and fall back to tun.kext.
On Windows systems, select the TAP-Win32 adapter which is named node in the Network Connections Control Panel or the
raw GUID of the adapter enclosed by braces. The --show-adapters option under Windows can also be used to enumerate
all available TAP-Win32 adapters and will show both the network connections control panel name and the GUID for each
TAP-Win32 adapter.
--lladdr address
Specify the link layer address, more commonly known as the MAC address. Only applied to TAP devices.
--iproute cmd
Set alternate command to execute instead of default iproute2 command. May be used in order to execute OpenVPN in un‐
privileged environment.
--ifconfig l rn
Set TUN/TAP adapter parameters. l is the IP address of the local VPN endpoint. For TUN devices in point-to-point
mode, rn is the IP address of the remote VPN endpoint. For TAP devices, or TUN devices used with --topology subnet,
rn is the subnet mask of the virtual network segment which is being created or connected to.
For TUN devices, which facilitate virtual point-to-point IP connections (when used in --topology net30 or p2p mode),
the proper usage of --ifconfig is to use two private IP addresses which are not a member of any existing subnet which
is in use. The IP addresses may be consecutive and should have their order reversed on the remote peer. After the
VPN is established, by pinging rn, you will be pinging across the VPN.
For TAP devices, which provide the ability to create virtual ethernet segments, or TUN devices in --topology subnet
mode (which create virtual "multipoint networks"), --ifconfig is used to set an IP address and subnet mask just as a
physical ethernet adapter would be similarly configured. If you are attempting to connect to a remote ethernet
bridge, the IP address and subnet should be set to values which would be valid on the the bridged ethernet segment
(note also that DHCP can be used for the same purpose).
This option, while primarily a proxy for the ifconfig(8) command, is designed to simplify TUN/TAP tunnel configura‐
tion by providing a standard interface to the different ifconfig implementations on different platforms.
--ifconfig parameters which are IP addresses can also be specified as a DNS or /etc/hosts file resolvable name.
For TAP devices, --ifconfig should not be used if the TAP interface will be getting an IP address lease from a DHCP
server.
--ifconfig-noexec
Don't actually execute ifconfig/netsh commands, instead pass --ifconfig parameters to scripts using environmental
variables.
--ifconfig-nowarn
Don't output an options consistency check warning if the --ifconfig option on this side of the connection doesn't
match the remote side. This is useful when you want to retain the overall benefits of the options consistency check
(also see --disable-occ option) while only disabling the ifconfig component of the check.
For example, if you have a configuration where the local host uses --ifconfig but the remote host does not, use --if‐
config-nowarn on the local host.
This option will also silence warnings about potential address conflicts which occasionally annoy more experienced
users by triggering "false positive" warnings.
--route network/IP [netmask] [gateway] [metric]
Add route to routing table after connection is established. Multiple routes can be specified. Routes will be auto‐
matically torn down in reverse order prior to TUN/TAP device close.
This option is intended as a convenience proxy for the route(8) shell command, while at the same time providing por‐
table semantics across OpenVPN's platform space.
netmask default -- 255.255.255.255
gateway default -- taken from --route-gateway or the second parameter to --ifconfig when --dev tun is specified.
metric default -- taken from --route-metric otherwise 0.
The default can be specified by leaving an option blank or setting it to "default".
The network and gateway parameters can also be specified as a DNS or /etc/hosts file resolvable name, or as one of
three special keywords:
vpn_gateway -- The remote VPN endpoint address (derived either from --route-gateway or the second parameter to --if‐
config when --dev tun is specified).
net_gateway -- The pre-existing IP default gateway, read from the routing table (not supported on all OSes).
remote_host -- The --remote address if OpenVPN is being run in client mode, and is undefined in server mode.
--route-gateway gw|'dhcp'
Specify a default gateway gw for use with --route.
If dhcp is specified as the parameter, the gateway address will be extracted from a DHCP negotiation with the OpenVPN
server-side LAN.
--route-metric m
Specify a default metric m for use with --route.
--route-delay [n] [w]
Delay n seconds (default=0) after connection establishment, before adding routes. If n is 0, routes will be added im‐
mediately upon connection establishment. If --route-delay is omitted, routes will be added immediately after TUN/TAP
device open and --up script execution, before any --user or --group privilege downgrade (or --chroot execution.)
This option is designed to be useful in scenarios where DHCP is used to set tap adapter addresses. The delay will
give the DHCP handshake time to complete before routes are added.
On Windows, --route-delay tries to be more intelligent by waiting w seconds (w=30 by default) for the TAP-Win32
adapter to come up before adding routes.
--route-up cmd
Run command cmd after routes are added, subject to --route-delay.
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
See the "Environmental Variables" section below for additional parameters passed as environmental variables.
--route-pre-down cmd
Run command cmd before routes are removed upon disconnection.
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
See the "Environmental Variables" section below for additional parameters passed as environmental variables.
--route-noexec
Don't add or remove routes automatically. Instead pass routes to --route-up script using environmental variables.
--route-nopull
When used with --client or --pull, accept options pushed by server EXCEPT for routes, block-outside-dns and dhcp op‐
tions like DNS servers.
When used on the client, this option effectively bars the server from adding routes to the client's routing table,
however note that this option still allows the server to set the TCP/IP properties of the client's TUN/TAP interface.
--allow-pull-fqdn
Allow client to pull DNS names from server (rather than being limited to IP address) for --ifconfig, --route, and
--route-gateway.
--client-nat snat|dnat network netmask alias
This pushable client option sets up a stateless one-to-one NAT rule on packet addresses (not ports), and is useful in
cases where routes or ifconfig settings pushed to the client would create an IP numbering conflict.
network/netmask (for example 192.168.0.0/255.255.0.0) defines the local view of a resource from the client perspec‐
tive, while alias/netmask (for example 10.64.0.0/255.255.0.0) defines the remote view from the server perspective.
Use snat (source NAT) for resources owned by the client and dnat (destination NAT) for remote resources.
Set --verb 6 for debugging info showing the transformation of src/dest addresses in packets.
--redirect-gateway flags...
Automatically execute routing commands to cause all outgoing IP traffic to be redirected over the VPN. This is a
client-side option.
This option performs three steps:
(1) Create a static route for the --remote address which forwards to the pre-existing default gateway. This is done
so that (3) will not create a routing loop.
(2) Delete the default gateway route.
(3) Set the new default gateway to be the VPN endpoint address (derived either from --route-gateway or the second pa‐
rameter to --ifconfig when --dev tun is specified).
When the tunnel is torn down, all of the above steps are reversed so that the original default route is restored.
Option flags:
local -- Add the local flag if both OpenVPN peers are directly connected via a common subnet, such as with wireless.
The local flag will cause step 1 above to be omitted.
autolocal -- Try to automatically determine whether to enable local flag above.
def1 -- Use this flag to override the default gateway by using 0.0.0.0/1 and 128.0.0.0/1 rather than 0.0.0.0/0. This
has the benefit of overriding but not wiping out the original default gateway.
bypass-dhcp -- Add a direct route to the DHCP server (if it is non-local) which bypasses the tunnel (Available on
Windows clients, may not be available on non-Windows clients).
bypass-dns -- Add a direct route to the DNS server(s) (if they are non-local) which bypasses the tunnel (Available on
Windows clients, may not be available on non-Windows clients).
block-local -- Block access to local LAN when the tunnel is active, except for the LAN gateway itself. This is ac‐
complished by routing the local LAN (except for the LAN gateway address) into the tunnel.
ipv6 -- Redirect IPv6 routing into the tunnel. This works similar to the def1 flag, that is, more specific IPv6
routes are added (2000::/4, 3000::/4), covering the whole IPv6 unicast space.
!ipv4 -- Do not redirect IPv4 traffic - typically used in the flag pair ipv6 !ipv4 to redirect IPv6-only.
--link-mtu n
Sets an upper bound on the size of UDP packets which are sent between OpenVPN peers. It's best not to set this pa‐
rameter unless you know what you're doing.
--redirect-private [flags]
Like --redirect-gateway, but omit actually changing the default gateway. Useful when pushing private subnets.
--tun-mtu n
Take the TUN device MTU to be n and derive the link MTU from it (default=1500). In most cases, you will probably
want to leave this parameter set to its default value.
The MTU (Maximum Transmission Units) is the maximum datagram size in bytes that can be sent unfragmented over a par‐
ticular network path. OpenVPN requires that packets on the control or data channels be sent unfragmented.
MTU problems often manifest themselves as connections which hang during periods of active usage.
It's best to use the --fragment and/or --mssfix options to deal with MTU sizing issues.
--tun-mtu-extra n
Assume that the TUN/TAP device might return as many as n bytes more than the --tun-mtu size on read. This parameter
defaults to 0, which is sufficient for most TUN devices. TAP devices may introduce additional overhead in excess of
the MTU size, and a setting of 32 is the default when TAP devices are used. This parameter only controls internal
OpenVPN buffer sizing, so there is no transmission overhead associated with using a larger value.
--mtu-disc type
Should we do Path MTU discovery on TCP/UDP channel? Only supported on OSes such as Linux that supports the necessary
system call to set.
'no' -- Never send DF (Don't Fragment) frames
'maybe' -- Use per-route hints
'yes' -- Always DF (Don't Fragment)
--mtu-test
To empirically measure MTU on connection startup, add the --mtu-test option to your configuration. OpenVPN will send
ping packets of various sizes to the remote peer and measure the largest packets which were successfully received.
The --mtu-test process normally takes about 3 minutes to complete.
--fragment max
Enable internal datagram fragmentation so that no UDP datagrams are sent which are larger than max bytes.
The max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP packet size after encapsu‐
lation overhead has been added in, but not including the UDP header itself.
The --fragment option only makes sense when you are using the UDP protocol ( --proto udp ).
--fragment adds 4 bytes of overhead per datagram.
See the --mssfix option below for an important related option to --fragment.
It should also be noted that this option is not meant to replace UDP fragmentation at the IP stack level. It is only
meant as a last resort when path MTU discovery is broken. Using this option is less efficient than fixing path MTU
discovery for your IP link and using native IP fragmentation instead.
Having said that, there are circumstances where using OpenVPN's internal fragmentation capability may be your only
option, such as tunneling a UDP multicast stream which requires fragmentation.
--mssfix max
Announce to TCP sessions running over the tunnel that they should limit their send packet sizes such that after Open‐
VPN has encapsulated them, the resulting UDP packet size that OpenVPN sends to its peer will not exceed max bytes.
The default value is 1450.
The max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP packet size after encapsu‐
lation overhead has been added in, but not including the UDP header itself. Resulting packet would be at most 28
bytes larger for IPv4 and 48 bytes for IPv6 (20/40 bytes for IP header and 8 bytes for UDP header). Default value of
1450 allows IPv4 packets to be transmitted over a link with MTU 1473 or higher without IP level fragmentation.
The --mssfix option only makes sense when you are using the UDP protocol for OpenVPN peer-to-peer communication, i.e.
--proto udp.
--mssfix and --fragment can be ideally used together, where --mssfix will try to keep TCP from needing packet frag‐
mentation in the first place, and if big packets come through anyhow (from protocols other than TCP), --fragment will
internally fragment them.
Both --fragment and --mssfix are designed to work around cases where Path MTU discovery is broken on the network path
between OpenVPN peers.
The usual symptom of such a breakdown is an OpenVPN connection which successfully starts, but then stalls during ac‐
tive usage.
If --fragment and --mssfix are used together, --mssfix will take its default max parameter from the --fragment max
option.
Therefore, one could lower the maximum UDP packet size to 1300 (a good first try for solving MTU-related connection
problems) with the following options:
--tun-mtu 1500 --fragment 1300 --mssfix
--sndbuf size
Set the TCP/UDP socket send buffer size. Defaults to operation system default.
--rcvbuf size
Set the TCP/UDP socket receive buffer size. Defaults to operation system default.
--mark value
Mark encrypted packets being sent with value. The mark value can be matched in policy routing and packetfilter rules.
This option is only supported in Linux and does nothing on other operating systems.
--socket-flags flags...
Apply the given flags to the OpenVPN transport socket. Currently, only TCP_NODELAY is supported.
The TCP_NODELAY socket flag is useful in TCP mode, and causes the kernel to send tunnel packets immediately over the
TCP connection without trying to group several smaller packets into a larger packet. This can result in a consider‐
ably improvement in latency.
This option is pushable from server to client, and should be used on both client and server for maximum effect.
--txqueuelen n
(Linux only) Set the TX queue length on the TUN/TAP interface. Currently defaults to 100.
--shaper n
Limit bandwidth of outgoing tunnel data to n bytes per second on the TCP/UDP port. Note that this will only work if
mode is set to p2p. If you want to limit the bandwidth in both directions, use this option on both peers.
OpenVPN uses the following algorithm to implement traffic shaping: Given a shaper rate of n bytes per second, after a
datagram write of b bytes is queued on the TCP/UDP port, wait a minimum of (b / n) seconds before queuing the next
write.
It should be noted that OpenVPN supports multiple tunnels between the same two peers, allowing you to construct
full-speed and reduced bandwidth tunnels at the same time, routing low-priority data such as off-site backups over
the reduced bandwidth tunnel, and other data over the full-speed tunnel.
Also note that for low bandwidth tunnels (under 1000 bytes per second), you should probably use lower MTU values as
well (see above), otherwise the packet latency will grow so large as to trigger timeouts in the TLS layer and TCP
connections running over the tunnel.
OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.
--inactive n [bytes]
Causes OpenVPN to exit after n seconds of inactivity on the TUN/TAP device. The time length of inactivity is measured
since the last incoming or outgoing tunnel packet. The default value is 0 seconds, which disables this feature.
If the optional bytes parameter is included, exit if less than bytes of combined in/out traffic are produced on the
tun/tap device in n seconds.
In any case, OpenVPN's internal ping packets (which are just keepalives) and TLS control packets are not considered
"activity", nor are they counted as traffic, as they are used internally by OpenVPN and are not an indication of ac‐
tual user activity.
--ping n
Ping remote over the TCP/UDP control channel if no packets have been sent for at least n seconds (specify --ping on
both peers to cause ping packets to be sent in both directions since OpenVPN ping packets are not echoed like IP ping
packets). When used in one of OpenVPN's secure modes (where --secret, --tls-server, or --tls-client is specified),
the ping packet will be cryptographically secure.
This option has two intended uses:
(1) Compatibility with stateful firewalls. The periodic ping will ensure that a stateful firewall rule which allows
OpenVPN UDP packets to pass will not time out.
(2) To provide a basis for the remote to test the existence of its peer using the --ping-exit option.
--ping-exit n
Causes OpenVPN to exit after n seconds pass without reception of a ping or other packet from remote. This option can
be combined with --inactive, --ping, and --ping-exit to create a two-tiered inactivity disconnect.
For example,
openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60
when used on both peers will cause OpenVPN to exit within 60 seconds if its peer disconnects, but will exit after one
hour if no actual tunnel data is exchanged.
--ping-restart n
Similar to --ping-exit, but trigger a SIGUSR1 restart after n seconds pass without reception of a ping or other
packet from remote.
This option is useful in cases where the remote peer has a dynamic IP address and a low-TTL DNS name is used to track
the IP address using a service such as http://dyndns.org/ + a dynamic DNS client such as ddclient.
If the peer cannot be reached, a restart will be triggered, causing the hostname used with --remote to be re-resolved
(if --resolv-retry is also specified).
In server mode, --ping-restart, --inactive, or any other type of internally generated signal will always be applied
to individual client instance objects, never to whole server itself. Note also in server mode that any internally
generated signal which would normally cause a restart, will cause the deletion of the client instance object instead.
In client mode, the --ping-restart parameter is set to 120 seconds by default. This default will hold until the
client pulls a replacement value from the server, based on the --keepalive setting in the server configuration. To
disable the 120 second default, set --ping-restart 0 on the client.
See the signals section below for more information on SIGUSR1.
Note that the behavior of SIGUSR1 can be modified by the --persist-tun, --persist-key, --persist-local-ip, and --per‐
sist-remote-ip options.
Also note that --ping-exit and --ping-restart are mutually exclusive and cannot be used together.
--keepalive interval timeout
A helper directive designed to simplify the expression of --ping and --ping-restart.
This option can be used on both client and server side, but it is enough to add this on the server side as it will
push appropriate --ping and --ping-restart options to the client. If used on both server and client, the values
pushed from server will override the client local values.
The timeout argument will be twice as long on the server side. This ensures that a timeout is detected on client
side before the server side drops the connection.
For example, --keepalive 10 60 expands as follows:
if mode server:
ping 10 # Argument: interval
ping-restart 120 # Argument: timeout*2
push "ping 10" # Argument: interval
push "ping-restart 60" # Argument: timeout
else
ping 10 # Argument: interval
ping-restart 60 # Argument: timeout
--ping-timer-rem
Run the --ping-exit / --ping-restart timer only if we have a remote address. Use this option if you are starting the
daemon in listen mode (i.e. without an explicit --remote peer), and you don't want to start clocking timeouts until a
remote peer connects.
--persist-tun
Don't close and reopen TUN/TAP device or run up/down scripts across SIGUSR1 or --ping-restart restarts.
SIGUSR1 is a restart signal similar to SIGHUP, but which offers finer-grained control over reset options.
--persist-key
Don't re-read key files across SIGUSR1 or --ping-restart.
This option can be combined with --user nobody to allow restarts triggered by the SIGUSR1 signal. Normally if you
drop root privileges in OpenVPN, the daemon cannot be restarted since it will now be unable to re-read protected key
files.
This option solves the problem by persisting keys across SIGUSR1 resets, so they don't need to be re-read.
--persist-local-ip
Preserve initially resolved local IP address and port number across SIGUSR1 or --ping-restart restarts.
--persist-remote-ip
Preserve most recently authenticated remote IP address and port number across SIGUSR1 or --ping-restart restarts.
--mlock
Disable paging by calling the POSIX mlockall function. Requires that OpenVPN be initially run as root (though Open‐
VPN can subsequently downgrade its UID using the --user option).
Using this option ensures that key material and tunnel data are never written to disk due to virtual memory paging
operations which occur under most modern operating systems. It ensures that even if an attacker was able to crack
the box running OpenVPN, he would not be able to scan the system swap file to recover previously used ephemeral keys,
which are used for a period of time governed by the --reneg options (see below), then are discarded.
The downside of using --mlock is that it will reduce the amount of physical memory available to other applications.
--up cmd
Run command cmd after successful TUN/TAP device open (pre --user UID change).
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
The up command is useful for specifying route commands which route IP traffic destined for private subnets which ex‐
ist at the other end of the VPN connection into the tunnel.
For --dev tun execute as:
cmd tun_dev tun_mtu link_mtu ifconfig_local_ip ifconfig_remote_ip [ init | restart ]
For --dev tap execute as:
cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask [ init | restart ]
See the "Environmental Variables" section below for additional parameters passed as environmental variables.
Note that if cmd includes arguments, all OpenVPN-generated arguments will be appended to them to build an argument
list with which the executable will be called.
Typically, cmd will run a script to add routes to the tunnel.
Normally the up script is called after the TUN/TAP device is opened. In this context, the last command line parame‐
ter passed to the script will be init. If the --up-restart option is also used, the up script will be called for
restarts as well. A restart is considered to be a partial reinitialization of OpenVPN where the TUN/TAP instance is
preserved (the --persist-tun option will enable such preservation). A restart can be generated by a SIGUSR1 signal,
a --ping-restart timeout, or a connection reset when the TCP protocol is enabled with the --proto option. If a
restart occurs, and --up-restart has been specified, the up script will be called with restart as the last parameter.
NOTE: on restart, OpenVPN will not pass the full set of environment variables to the script. Namely, everything re‐
lated to routing and gateways will not be passed, as nothing needs to be done anyway - all the routing setup is al‐
ready in place. Additionally, the up-restart script will run with the downgraded UID/GID settings (if configured).
The following standalone example shows how the --up script can be called in both an initialization and restart con‐
text. (NOTE: for security reasons, don't run the following example unless UDP port 9999 is blocked by your firewall.
Also, the example will run indefinitely, so you should abort with control-c).
openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 --up 'echo up' --down 'echo down' --persist-tun --up-restart
Note that OpenVPN also provides the --ifconfig option to automatically ifconfig the TUN device, eliminating the need
to define an --up script, unless you also want to configure routes in the --up script.
If --ifconfig is also specified, OpenVPN will pass the ifconfig local and remote endpoints on the command line to the
--up script so that they can be used to configure routes such as:
route add -net 10.0.0.0 netmask 255.255.255.0 gw $5
--up-delay
Delay TUN/TAP open and possible --up script execution until after TCP/UDP connection establishment with peer.
In --proto udp mode, this option normally requires the use of --ping to allow connection initiation to be sensed in
the absence of tunnel data, since UDP is a "connectionless" protocol.
On Windows, this option will delay the TAP-Win32 media state transitioning to "connected" until connection establish‐
ment, i.e. the receipt of the first authenticated packet from the peer.
--down cmd
Run command cmd after TUN/TAP device close (post --user UID change and/or --chroot ). cmd consists of a path to
script (or executable program), optionally followed by arguments. The path and arguments may be single- or dou‐
ble-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
Called with the same parameters and environmental variables as the --up option above.
Note that if you reduce privileges by using --user and/or --group, your --down script will also run at reduced privi‐
lege.
--down-pre
Call --down cmd/script before, rather than after, TUN/TAP close.
--up-restart
Enable the --up and --down scripts to be called for restarts as well as initial program start. This option is de‐
scribed more fully above in the --up option documentation.
--setenv name value
Set a custom environmental variable name=value to pass to script.
--setenv FORWARD_COMPATIBLE 1
Relax config file syntax checking so that unknown directives will trigger a warning but not a fatal error, on the as‐
sumption that a given unknown directive might be valid in future OpenVPN versions.
This option should be used with caution, as there are good security reasons for having OpenVPN fail if it detects
problems in a config file. Having said that, there are valid reasons for wanting new software features to gracefully
degrade when encountered by older software versions.
It is also possible to tag a single directive so as not to trigger a fatal error if the directive isn't recognized.
To do this, prepend the following before the directive: setenv opt
Versions prior to OpenVPN 2.3.3 will always ignore options set with the setenv opt directive.
See also --ignore-unknown-option
--setenv-safe name value
Set a custom environmental variable OPENVPN_name=value to pass to script.
This directive is designed to be pushed by the server to clients, and the prepending of "OPENVPN_" to the environmen‐
tal variable is a safety precaution to prevent a LD_PRELOAD style attack from a malicious or compromised server.
--ignore-unknown-option opt1 opt2 opt3 ... optN
When one of options opt1 ... optN is encountered in the configuration file the configuration file parsing does not
fail if this OpenVPN version does not support the option. Multiple --ignore-unknown-option options can be given to
support a larger number of options to ignore.
This option should be used with caution, as there are good security reasons for having OpenVPN fail if it detects
problems in a config file. Having said that, there are valid reasons for wanting new software features to gracefully
degrade when encountered by older software versions.
--ignore-unknown-option is available since OpenVPN 2.3.3.
--script-security level
This directive offers policy-level control over OpenVPN's usage of external programs and scripts. Lower level values
are more restrictive, higher values are more permissive. Settings for level:
0 -- Strictly no calling of external programs.
1 -- (Default) Only call built-in executables such as ifconfig, ip, route, or netsh.
2 -- Allow calling of built-in executables and user-defined scripts.
3 -- Allow passwords to be passed to scripts via environmental variables (potentially unsafe).
OpenVPN releases before v2.3 also supported a method flag which indicated how OpenVPN should call external commands
and scripts. This could be either execve or system. As of OpenVPN 2.3, this flag is no longer accepted. In most
*nix environments the execve() approach has been used without any issues.
Some directives such as --up allow options to be passed to the external script. In these cases make sure the script
name does not contain any spaces or the configuration parser will choke because it can't determine where the script
name ends and script options start.
To run scripts in Windows in earlier OpenVPN versions you needed to either add a full path to the script interpreter
which can parse the script or use the system flag to run these scripts. As of OpenVPN 2.3 it is now a strict re‐
quirement to have full path to the script interpreter when running non-executables files. This is not needed for ex‐
ecutable files, such as .exe, .com, .bat or .cmd files. For example, if you have a Visual Basic script, you must use
this syntax now:
--up 'C:\\Windows\\System32\\wscript.exe C:\\Program\ Files\\OpenVPN\\config\\my-up-script.vbs'
Please note the single quote marks and the escaping of the backslashes (\) and the space character.
The reason the support for the system flag was removed is due to the security implications with shell expansions when
executing scripts via the system() call.
--disable-occ
Don't output a warning message if option inconsistencies are detected between peers. An example of an option incon‐
sistency would be where one peer uses --dev tun while the other peer uses --dev tap.
Use of this option is discouraged, but is provided as a temporary fix in situations where a recent version of OpenVPN
must connect to an old version.
--user user
Change the user ID of the OpenVPN process to user after initialization, dropping privileges in the process. This op‐
tion is useful to protect the system in the event that some hostile party was able to gain control of an OpenVPN ses‐
sion. Though OpenVPN's security features make this unlikely, it is provided as a second line of defense.
By setting user to nobody or somebody similarly unprivileged, the hostile party would be limited in what damage they
could cause. Of course once you take away privileges, you cannot return them to an OpenVPN session. This means, for
example, that if you want to reset an OpenVPN daemon with a SIGUSR1 signal (for example in response to a DHCP reset),
you should make use of one or more of the --persist options to ensure that OpenVPN doesn't need to execute any privi‐
leged operations in order to restart (such as re-reading key files or running ifconfig on the TUN device).
--group group
Similar to the --user option, this option changes the group ID of the OpenVPN process to group after initialization.
--cd dir
Change directory to dir prior to reading any files such as configuration files, key files, scripts, etc. dir should
be an absolute path, with a leading "/", and without any references to the current directory such as "." or "..".
This option is useful when you are running OpenVPN in --daemon mode, and you want to consolidate all of your OpenVPN
control files in one location.
--chroot dir
Chroot to dir after initialization. --chroot essentially redefines dir as being the top level directory tree (/).
OpenVPN will therefore be unable to access any files outside this tree. This can be desirable from a security stand‐
point.
Since the chroot operation is delayed until after initialization, most OpenVPN options that reference files will op‐
erate in a pre-chroot context.
In many cases, the dir parameter can point to an empty directory, however complications can result when scripts or
restarts are executed after the chroot operation.
Note: The SSL library will probably need /dev/urandom to be available inside the chroot directory dir. This is be‐
cause SSL libraries occasionally need to collect fresh random. Newer linux kernels and some BSDs implement a getran‐
dom() or getentropy() syscall that removes the need for /dev/urandom to be available.
--setcon context
Apply SELinux context after initialization. This essentially provides the ability to restrict OpenVPN's rights to
only network I/O operations, thanks to SELinux. This goes further than --user and --chroot in that those two, while
being great security features, unfortunately do not protect against privilege escalation by exploitation of a vulner‐
able system call. You can of course combine all three, but please note that since setcon requires access to /proc you
will have to provide it inside the chroot directory (e.g. with mount --bind).
Since the setcon operation is delayed until after initialization, OpenVPN can be restricted to just network-related
system calls, whereas by applying the context before startup (such as the OpenVPN one provided in the SELinux Refer‐
ence Policies) you will have to allow many things required only during initialization.
Like with chroot, complications can result when scripts or restarts are executed after the setcon operation, which is
why you should really consider using the --persist-key and --persist-tun options.
--daemon [progname]
Become a daemon after all initialization functions are completed. This option will cause all message and error out‐
put to be sent to the syslog file (such as /var/log/messages), except for the output of scripts and ifconfig com‐
mands, which will go to /dev/null unless otherwise redirected. The syslog redirection occurs immediately at the
point that --daemon is parsed on the command line even though the daemonization point occurs later. If one of the
--log options is present, it will supersede syslog redirection.
The optional progname parameter will cause OpenVPN to report its program name to the system logger as progname. This
can be useful in linking OpenVPN messages in the syslog file with specific tunnels. When unspecified, progname de‐
faults to "openvpn".
When OpenVPN is run with the --daemon option, it will try to delay daemonization until the majority of initialization
functions which are capable of generating fatal errors are complete. This means that initialization scripts can test
the return status of the openvpn command for a fairly reliable indication of whether the command has correctly ini‐
tialized and entered the packet forwarding event loop.
In OpenVPN, the vast majority of errors which occur after initialization are non-fatal.
Note: as soon as OpenVPN has daemonized, it can not ask for usernames, passwords, or key pass phrases anymore. This
has certain consequences, namely that using a password-protected private key will fail unless the --askpass option is
used to tell OpenVPN to ask for the pass phrase (this requirement is new in v2.3.7, and is a consequence of calling
daemon() before initializing the crypto layer).
Further, using --daemon together with --auth-user-pass (entered on console) and --auth-nocache will fail as soon as
key renegotiation (and reauthentication) occurs.
--syslog [progname]
Direct log output to system logger, but do not become a daemon. See --daemon directive above for description of
progname parameter.
--errors-to-stderr
Output errors to stderr instead of stdout unless log output is redirected by one of the --log options.
--passtos
Set the TOS field of the tunnel packet to what the payload's TOS is.
--inetd [wait|nowait] [progname]
Use this option when OpenVPN is being run from the inetd or xinetd(8) server.
The wait/nowait option must match what is specified in the inetd/xinetd config file. The nowait mode can only be
used with --proto tcp-server. The default is wait. The nowait mode can be used to instantiate the OpenVPN daemon as
a classic TCP server, where client connection requests are serviced on a single port number. For additional informa‐
tion on this kind of configuration, see the OpenVPN FAQ: http://openvpn.net/faq.html#oneport
This option precludes the use of --daemon, --local, or --remote. Note that this option causes message and error out‐
put to be handled in the same way as the --daemon option. The optional progname parameter is also handled exactly as
in --daemon.
Also note that in wait mode, each OpenVPN tunnel requires a separate TCP/UDP port and a separate inetd or xinetd en‐
try. See the OpenVPN 1.x HOWTO for an example on using OpenVPN with xinetd: http://openvpn.net/1xhowto.html
--log file
Output logging messages to file, including output to stdout/stderr which is generated by called scripts. If file al‐
ready exists it will be truncated. This option takes effect immediately when it is parsed in the command line and
will supersede syslog output if --daemon or --inetd is also specified. This option is persistent over the entire
course of an OpenVPN instantiation and will not be reset by SIGHUP, SIGUSR1, or --ping-restart.
Note that on Windows, when OpenVPN is started as a service, logging occurs by default without the need to specify
this option.
--log-append file
Append logging messages to file. If file does not exist, it will be created. This option behaves exactly like --log
except that it appends to rather than truncating the log file.
--suppress-timestamps
Avoid writing timestamps to log messages, even when they otherwise would be prepended. In particular, this applies to
log messages sent to stdout.
--machine-readable-output
Always write timestamps and message flags to log messages, even when they otherwise would not be prefixed. In partic‐
ular, this applies to log messages sent to stdout.
--writepid file
Write OpenVPN's main process ID to file.
--nice n
Change process priority after initialization ( n greater than 0 is lower priority, n less than zero is higher prior‐
ity).
--fast-io
(Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a call to poll/epoll/select prior to the write operation.
The purpose of such a call would normally be to block until the device or socket is ready to accept the write. Such
blocking is unnecessary on some platforms which don't support write blocking on UDP sockets or TUN/TAP devices. In
such cases, one can optimize the event loop by avoiding the poll/epoll/select call, improving CPU efficiency by 5% to
10%.
This option can only be used on non-Windows systems, when --proto udp is specified, and when --shaper is NOT speci‐
fied.
--multihome
Configure a multi-homed UDP server. This option needs to be used when a server has more than one IP address (e.g.
multiple interfaces, or secondary IP addresses), and is not using --local to force binding to one specific address
only. This option will add some extra lookups to the packet path to ensure that the UDP reply packets are always
sent from the address that the client is talking to. This is not supported on all platforms, and it adds more pro‐
cessing, so it's not enabled by default.
Note: this option is only relevant for UDP servers.
Note 2: if you do an IPv6+IPv4 dual-stack bind on a Linux machine with multiple IPv4 address, connections to IPv4 ad‐
dresses will not work right on kernels before 3.15, due to missing kernel support for the IPv4-mapped case (some dis‐
tributions have ported this to earlier kernel versions, though).
--echo [parms...]
Echo parms to log output.
Designed to be used to send messages to a controlling application which is receiving the OpenVPN log output.
--remap-usr1 signal
Control whether internally or externally generated SIGUSR1 signals are remapped to SIGHUP (restart without persisting
state) or SIGTERM (exit).
signal can be set to "SIGHUP" or "SIGTERM". By default, no remapping occurs.
--verb n
Set output verbosity to n (default=1). Each level shows all info from the previous levels. Level 3 is recommended
if you want a good summary of what's happening without being swamped by output.
0 -- No output except fatal errors.
1 to 4 -- Normal usage range.
5 -- Output R and W characters to the console for each packet read and write, uppercase is used for TCP/UDP packets
and lowercase is used for TUN/TAP packets.
6 to 11 -- Debug info range (see errlevel.h for additional information on debug levels).
--status file [n]
Write operational status to file every n seconds.
Status can also be written to the syslog by sending a SIGUSR2 signal.
With multi-client capability enabled on a server, the status file includes a list of clients and a routing table. The
output format can be controlled by the --status-version option in that case.
For clients or instances running in point-to-point mode, it will contain the traffic statistics.
--status-version [n]
Set the status file format version number to n.
This only affects the status file on servers with multi-client capability enabled.
1 -- traditional format (default). The client list contains the following fields comma-separated: Common Name, Real
Address, Bytes Received, Bytes Sent, Connected Since.
2 -- a more reliable format for external processing. Compared to version 1, the client list contains some additional
fields: Virtual Address, Virtual IPv6 Address, Username, Client ID, Peer ID. Future versions may extend the number
of fields.
3 -- identical to 2, but fields are tab-separated.
--mute n
Log at most n consecutive messages in the same category. This is useful to limit repetitive logging of similar mes‐
sage types.
--compress [algorithm]
Enable a compression algorithm.
The algorithm parameter may be "lzo", "lz4", or empty. LZO and LZ4 are different compression algorithms, with LZ4
generally offering the best performance with least CPU usage. For backwards compatibility with OpenVPN versions be‐
fore v2.4, use "lzo" (which is identical to the older option "--comp-lzo yes").
If the algorithm parameter is empty, compression will be turned off, but the packet framing for compression will
still be enabled, allowing a different setting to be pushed later.
Security Considerations
Compression and encryption is a tricky combination. If an attacker knows or is able to control (parts of) the plain‐
text of packets that contain secrets, the attacker might be able to extract the secret if compression is enabled.
See e.g. the CRIME and BREACH attacks on TLS which also leverage compression to break encryption. If you are not en‐
tirely sure that the above does not apply to your traffic, you are advised to *not* enable compression.
--comp-lzo [mode]
DEPRECATED This option will be removed in a future OpenVPN release. Use the newer --compress instead.
Use LZO compression -- may add up to 1 byte per packet for incompressible data. mode may be "yes", "no", or "adap‐
tive" (default).
In a server mode setup, it is possible to selectively turn compression on or off for individual clients.
First, make sure the client-side config file enables selective compression by having at least one --comp-lzo direc‐
tive, such as --comp-lzo no. This will turn off compression by default, but allow a future directive push from the
server to dynamically change the on/off/adaptive setting.
Next in a --client-config-dir file, specify the compression setting for the client, for example:
comp-lzo yes
push "comp-lzo yes"
The first line sets the comp-lzo setting for the server side of the link, the second sets the client side.
--comp-noadapt
When used in conjunction with --comp-lzo, this option will disable OpenVPN's adaptive compression algorithm. Nor‐
mally, adaptive compression is enabled with --comp-lzo.
Adaptive compression tries to optimize the case where you have compression enabled, but you are sending predominantly
incompressible (or pre-compressed) packets over the tunnel, such as an FTP or rsync transfer of a large, compressed
file. With adaptive compression, OpenVPN will periodically sample the compression process to measure its efficiency.
If the data being sent over the tunnel is already compressed, the compression efficiency will be very low, triggering
openvpn to disable compression for a period of time until the next re-sample test.
--management socket-name unix [pw-file] (recommended)
--management IP port [pw-file]
Enable a management server on a socket-name Unix socket on those platforms supporting it, or on a designated TCP
port.
pw-file , if specified, is a password file where the password must be on first line. Instead of a filename it can
use the keyword stdin which will prompt the user for a password to use when OpenVPN is starting.
For unix sockets, the default behaviour is to create a unix domain socket that may be connected to by any process.
Use the --management-client-user and --management-client-group directives to restrict access.
The management interface provides a special mode where the TCP management link can operate over the tunnel itself.
To enable this mode, set IP to tunnel. Tunnel mode will cause the management interface to listen for a TCP connec‐
tion on the local VPN address of the TUN/TAP interface.
BEWARE of enabling the management interface over TCP. In these cases you should ALWAYS make use of pw-file to pass‐
word protect the management interface. Any user who can connect to this TCP IP:port will be able to manage and con‐
trol (and interfere with) the OpenVPN process. It is also strongly recommended to set IP to 127.0.0.1 (localhost) to
restrict accessibility of the management server to local clients.
While the management port is designed for programmatic control of OpenVPN by other applications, it is possible to
telnet to the port, using a telnet client in "raw" mode. Once connected, type "help" for a list of commands.
For detailed documentation on the management interface, see the management-notes.txt file in the management folder of
the OpenVPN source distribution.
--management-client
Management interface will connect as a TCP/unix domain client to IP:port specified by --management rather than listen
as a TCP server or on a unix domain socket.
If the client connection fails to connect or is disconnected, a SIGTERM signal will be generated causing OpenVPN to
quit.
--management-query-passwords
Query management channel for private key password and --auth-user-pass username/password. Only query the management
channel for inputs which ordinarily would have been queried from the console.
--management-query-proxy
Query management channel for proxy server information for a specific --remote (client-only).
--management-query-remote
Allow management interface to override --remote directives (client-only).
--management-external-key
Allows usage for external private key file instead of --key option (client-only).
--management-external-cert certificate-hint
Allows usage for external certificate instead of --cert option (client-only). certificate-hint is an arbitrary
string which is passed to a management interface client as an argument of NEED-CERTIFICATE notification. Requires
--management-external-key.
--management-forget-disconnect
Make OpenVPN forget passwords when management session disconnects.
This directive does not affect the --http-proxy username/password. It is always cached.
--management-hold
Start OpenVPN in a hibernating state, until a client of the management interface explicitly starts it with the hold
release command.
--management-signal
Send SIGUSR1 signal to OpenVPN if management session disconnects. This is useful when you wish to disconnect an
OpenVPN session on user logoff. For --management-client this option is not needed since a disconnect will always gen‐
erate a SIGTERM.
--management-log-cache n
Cache the most recent n lines of log file history for usage by the management channel.
--management-up-down
Report tunnel up/down events to management interface.
--management-client-auth
Gives management interface client the responsibility to authenticate clients after their client certificate has been
verified. See management-notes.txt in OpenVPN distribution for detailed notes.
--management-client-pf
Management interface clients must specify a packet filter file for each connecting client. See management-notes.txt
in OpenVPN distribution for detailed notes.
--management-client-user u
When the management interface is listening on a unix domain socket, only allow connections from user u.
--management-client-group g
When the management interface is listening on a unix domain socket, only allow connections from group g.
--plugin module-pathname [init-string]
Load plug-in module from the file module-pathname, passing init-string as an argument to the module initialization
function. Multiple plugin modules may be loaded into one OpenVPN process.
The module-pathname argument can be just a filename or a filename with a relative or absolute path. The format of
the filename and path defines if the plug-in will be loaded from a default plug-in directory or outside this direc‐
tory.
--plugin path Effective directory used
====================================================
myplug.so DEFAULT_DIR/myplug.so
subdir/myplug.so DEFAULT_DIR/subdir/myplug.so
./subdir/myplug.so CWD/subdir/myplug.so
/usr/lib/my/plug.so /usr/lib/my/plug.so
DEFAULT_DIR is replaced by the default plug-in directory, which is configured at the build time of OpenVPN. CWD is
the current directory where OpenVPN was started or the directory OpenVPN have swithed into via the --cd option before
the --plugin option.
For more information and examples on how to build OpenVPN plug-in modules, see the README file in the plugin folder
of the OpenVPN source distribution.
If you are using an RPM install of OpenVPN, see /usr/share/openvpn/plugin. The documentation is in doc and the ac‐
tual plugin modules are in lib.
Multiple plugin modules can be cascaded, and modules can be used in tandem with scripts. The modules will be called
by OpenVPN in the order that they are declared in the config file. If both a plugin and script are configured for
the same callback, the script will be called last. If the return code of the module/script controls an authentica‐
tion function (such as tls-verify, auth-user-pass-verify, or client-connect), then every module and script must re‐
turn success (0) in order for the connection to be authenticated.
--keying-material-exporter label len
Save Exported Keying Material [RFC5705] of len bytes (must be between 16 and 4095 bytes) using label in environment
(exported_keying_material) for use by plugins in OPENVPN_PLUGIN_TLS_FINAL callback.
Note that exporter labels have the potential to collide with existing PRF labels. In order to prevent this, labels
MUST begin with "EXPORTER".
This option requires OpenSSL 1.0.1 or newer.
Server Mode
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and can be enabled with the --mode server op‐
tion. In server mode, OpenVPN will listen on a single port for incoming client connections. All client connections will be
routed through a single tun or tap interface. This mode is designed for scalability and should be able to support hundreds
or even thousands of clients on sufficiently fast hardware. SSL/TLS authentication must be used in this mode.
--server network netmask ['nopool']
A helper directive designed to simplify the configuration of OpenVPN's server mode. This directive will set up an
OpenVPN server which will allocate addresses to clients out of the given network/netmask. The server itself will
take the ".1" address of the given network for use as the server-side endpoint of the local TUN/TAP interface.
For example, --server 10.8.0.0 255.255.255.0 expands as follows:
mode server
tls-server
push "topology [topology]"
if dev tun AND (topology == net30 OR topology == p2p):
ifconfig 10.8.0.1 10.8.0.2
if !nopool:
ifconfig-pool 10.8.0.4 10.8.0.251
route 10.8.0.0 255.255.255.0
if client-to-client:
push "route 10.8.0.0 255.255.255.0"
else if topology == net30:
push "route 10.8.0.1"
if dev tap OR (dev tun AND topology == subnet):
ifconfig 10.8.0.1 255.255.255.0
if !nopool:
ifconfig-pool 10.8.0.2 10.8.0.253 255.255.255.0
push "route-gateway 10.8.0.1"
if route-gateway unset:
route-gateway 10.8.0.2
Don't use --server if you are ethernet bridging. Use --server-bridge instead.
--server-bridge gateway netmask pool-start-IP pool-end-IP
--server-bridge ['nogw']
A helper directive similar to --server which is designed to simplify the configuration of OpenVPN's server mode in
ethernet bridging configurations.
If --server-bridge is used without any parameters, it will enable a DHCP-proxy mode, where connecting OpenVPN clients
will receive an IP address for their TAP adapter from the DHCP server running on the OpenVPN server-side LAN. Note
that only clients that support the binding of a DHCP client with the TAP adapter (such as Windows) can support this
mode. The optional nogw flag (advanced) indicates that gateway information should not be pushed to the client.
To configure ethernet bridging, you must first use your OS's bridging capability to bridge the TAP interface with the
ethernet NIC interface. For example, on Linux this is done with the brctl tool, and with Windows XP it is done in
the Network Connections Panel by selecting the ethernet and TAP adapters and right-clicking on "Bridge Connections".
Next you you must manually set the IP/netmask on the bridge interface. The gateway and netmask parameters to
--server-bridge can be set to either the IP/netmask of the bridge interface, or the IP/netmask of the default gate‐
way/router on the bridged subnet.
Finally, set aside a IP range in the bridged subnet, denoted by pool-start-IP and pool-end-IP, for OpenVPN to allo‐
cate to connecting clients.
For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128 10.8.0.254 expands as follows:
mode server
tls-server
ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.4"
In another example, --server-bridge (without parameters) expands as follows:
mode server
tls-server
push "route-gateway dhcp"
Or --server-bridge nogw expands as follows:
mode server
tls-server
--push option
Push a config file option back to the client for remote execution. Note that option must be enclosed in double
quotes (""). The client must specify --pull in its config file. The set of options which can be pushed is limited
by both feasibility and security. Some options such as those which would execute scripts are banned, since they
would effectively allow a compromised server to execute arbitrary code on the client. Other options such as TLS or
MTU parameters cannot be pushed because the client needs to know them before the connection to the server can be ini‐
tiated.
This is a partial list of options which can currently be pushed: --route, --route-gateway, --route-delay, --redi‐
rect-gateway, --ip-win32, --dhcp-option, --inactive, --ping, --ping-exit, --ping-restart, --setenv, --auth-token,
--persist-key, --persist-tun, --echo, --comp-lzo, --socket-flags, --sndbuf, --rcvbuf
--push-reset
Don't inherit the global push list for a specific client instance. Specify this option in a client-specific context
such as with a --client-config-dir configuration file. This option will ignore --push options at the global config
file level.
--push-remove opt
selectively remove all --push options matching "opt" from the option list for a client. "opt" is matched as a sub‐
string against the whole option string to-be-pushed to the client, so --push-remove route would remove all --push
route ... and --push route-ipv6 ... statements, while --push-remove 'route-ipv6 2001:' would only remove IPv6
routes for 2001:... networks.
--push-remove can only be used in a client-specific context, like in a --client-config-dir file, or --client-connect
script or plugin -- similar to --push-reset, just more selective.
NOTE: to change an option, --push-remove can be used to first remove the old value, and then add a new --push option
with the new value.
--push-peer-info
Push additional information about the client to server. The following data is always pushed to the server:
IV_VER=<version> -- the client OpenVPN version
IV_PLAT=[linux|solaris|openbsd|mac|netbsd|freebsd|win] -- the client OS platform
IV_LZO_STUB=1 -- if client was built with LZO stub capability
IV_LZ4=1 -- if the client supports LZ4 compressions.
IV_PROTO=2 -- if the client supports peer-id floating mechansim
IV_NCP=2 -- negotiable ciphers, client supports --cipher pushed by the server, a value of 2 or greater indicates
client supports AES-GCM-128 and AES-GCM-256.
IV_GUI_VER=<gui_id> <version> -- the UI version of a UI if one is running, for example "de.blinkt.openvpn 0.5.47" for
the Android app.
When --push-peer-info is enabled the additional information consists of the following data:
IV_HWADDR=<mac address> -- the MAC address of clients default gateway
IV_SSL=<version string> -- the ssl version used by the client, e.g. "OpenSSL 1.0.2f 28 Jan 2016".
IV_PLAT_VER=x.y - the version of the operating system, e.g. 6.1 for Windows 7.
UV_<name>=<value> -- client environment variables whose names start with "UV_"
--disable
Disable a particular client (based on the common name) from connecting. Don't use this option to disable a client
due to key or password compromise. Use a CRL (certificate revocation list) instead (see the --crl-verify option).
This option must be associated with a specific client instance, which means that it must be specified either in a
client instance config file using --client-config-dir or dynamically generated using a --client-connect script.
--ifconfig-pool start-IP end-IP [netmask]
Set aside a pool of subnets to be dynamically allocated to connecting clients, similar to a DHCP server. For
tun-style tunnels, each client will be given a /30 subnet (for interoperability with Windows clients). For tap-style
tunnels, individual addresses will be allocated, and the optional netmask parameter will also be pushed to clients.
--ifconfig-pool-persist file [seconds]
Persist/unpersist ifconfig-pool data to file, at seconds intervals (default=600), as well as on program startup and
shutdown.
The goal of this option is to provide a long-term association between clients (denoted by their common name) and the
virtual IP address assigned to them from the ifconfig-pool. Maintaining a long-term association is good for clients
because it allows them to effectively use the --persist-tun option.
file is a comma-delimited ASCII file, formatted as <Common-Name>,<IP-address>.
If seconds = 0, file will be treated as read-only. This is useful if you would like to treat file as a configuration
file.
Note that the entries in this file are treated by OpenVPN as suggestions only, based on past associations between a
common name and IP address. They do not guarantee that the given common name will always receive the given IP ad‐
dress. If you want guaranteed assignment, use --ifconfig-push
--ifconfig-pool-linear
DEPRECATED This option will be removed in OpenVPN 2.5
Modifies the --ifconfig-pool directive to allocate individual TUN interface addresses for clients rather than /30
subnets. NOTE: This option is incompatible with Windows clients.
This option is deprecated, and should be replaced with --topology p2p which is functionally equivalent.
--ifconfig-push local remote-netmask [alias]
Push virtual IP endpoints for client tunnel, overriding the --ifconfig-pool dynamic allocation.
The parameters local and remote-netmask are set according to the --ifconfig directive which you want to execute on
the client machine to configure the remote end of the tunnel. Note that the parameters local and remote-netmask are
from the perspective of the client, not the server. They may be DNS names rather than IP addresses, in which case
they will be resolved on the server at the time of client connection.
The optional alias parameter may be used in cases where NAT causes the client view of its local endpoint to differ
from the server view. In this case local/remote-netmask will refer to the server view while alias/remote-netmask
will refer to the client view.
This option must be associated with a specific client instance, which means that it must be specified either in a
client instance config file using --client-config-dir or dynamically generated using a --client-connect script.
Remember also to include a --route directive in the main OpenVPN config file which encloses local, so that the kernel
will know to route it to the server's TUN/TAP interface.
OpenVPN's internal client IP address selection algorithm works as follows:
1 -- Use --client-connect script generated file for static IP (first choice).
2 -- Use --client-config-dir file for static IP (next choice).
3 -- Use --ifconfig-pool allocation for dynamic IP (last choice).
--iroute network [netmask]
Generate an internal route to a specific client. The netmask parameter, if omitted, defaults to 255.255.255.255.
This directive can be used to route a fixed subnet from the server to a particular client, regardless of where the
client is connecting from. Remember that you must also add the route to the system routing table as well (such as by
using the --route directive). The reason why two routes are needed is that the --route directive routes the packet
from the kernel to OpenVPN. Once in OpenVPN, the --iroute directive routes to the specific client.
This option must be specified either in a client instance config file using --client-config-dir or dynamically gener‐
ated using a --client-connect script.
The --iroute directive also has an important interaction with --push "route ...". --iroute essentially defines a
subnet which is owned by a particular client (we will call this client A). If you would like other clients to be
able to reach A's subnet, you can use --push "route ..." together with --client-to-client to effect this. In order
for all clients to see A's subnet, OpenVPN must push this route to all clients EXCEPT for A, since the subnet is al‐
ready owned by A. OpenVPN accomplishes this by not not pushing a route to a client if it matches one of the client's
iroutes.
--client-to-client
Because the OpenVPN server mode handles multiple clients through a single tun or tap interface, it is effectively a
router. The --client-to-client flag tells OpenVPN to internally route client-to-client traffic rather than pushing
all client-originating traffic to the TUN/TAP interface.
When this option is used, each client will "see" the other clients which are currently connected. Otherwise, each
client will only see the server. Don't use this option if you want to firewall tunnel traffic using custom,
per-client rules.
--duplicate-cn
Allow multiple clients with the same common name to concurrently connect. In the absence of this option, OpenVPN
will disconnect a client instance upon connection of a new client having the same common name.
--client-connect cmd
Run command cmd on client connection.
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
The command is passed the common name and IP address of the just-authenticated client as environmental variables (see
environmental variable section below). The command is also passed the pathname of a freshly created temporary file
as the last argument (after any arguments specified in cmd ), to be used by the command to pass dynamically generated
config file directives back to OpenVPN.
If the script wants to generate a dynamic config file to be applied on the server when the client connects, it should
write it to the file named by the last argument.
See the --client-config-dir option below for options which can be legally used in a dynamically generated config
file.
Note that the return value of script is significant. If script returns a non-zero error status, it will cause the
client to be disconnected.
--client-disconnect cmd
Like --client-connect but called on client instance shutdown. Will not be called unless the --client-connect script
and plugins (if defined) were previously called on this instance with successful (0) status returns.
The exception to this rule is if the --client-disconnect command or plugins are cascaded, and at least one
client-connect function succeeded, then ALL of the client-disconnect functions for scripts and plugins will be called
on client instance object deletion, even in cases where some of the related client-connect functions returned an er‐
ror status.
The --client-disconnect command is passed the same pathname as the corresponding --client-connect command as its last
argument. (after any arguments specified in cmd ).
--client-config-dir dir
Specify a directory dir for custom client config files. After a connecting client has been authenticated, OpenVPN
will look in this directory for a file having the same name as the client's X509 common name. If a matching file ex‐
ists, it will be opened and parsed for client-specific configuration options. If no matching file is found, OpenVPN
will instead try to open and parse a default file called "DEFAULT", which may be provided but is not required. Note
that the configuration files must be readable by the OpenVPN process after it has dropped it's root privileges.
This file can specify a fixed IP address for a given client using --ifconfig-push, as well as fixed subnets owned by
the client using --iroute.
One of the useful properties of this option is that it allows client configuration files to be conveniently created,
edited, or removed while the server is live, without needing to restart the server.
The following options are legal in a client-specific context: --push, --push-reset, --push-remove, --iroute, --ifcon‐
fig-push, and --config.
--ccd-exclusive
Require, as a condition of authentication, that a connecting client has a --client-config-dir file.
--tmp-dir dir
Specify a directory dir for temporary files. This directory will be used by openvpn processes and script to communi‐
cate temporary data with openvpn main process. Note that the directory must be writable by the OpenVPN process after
it has dropped it's root privileges.
This directory will be used by in the following cases:
* --client-connect scripts to dynamically generate client-specific configuration files.
* OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin hook to return success/failure via auth_control_file when using de‐
ferred auth method
* OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules via pf_file
--hash-size r v
Set the size of the real address hash table to r and the virtual address table to v. By default, both tables are
sized at 256 buckets.
--bcast-buffers n
Allocate n buffers for broadcast datagrams (default=256).
--tcp-queue-limit n
Maximum number of output packets queued before TCP (default=64).
When OpenVPN is tunneling data from a TUN/TAP device to a remote client over a TCP connection, it is possible that
the TUN/TAP device might produce data at a faster rate than the TCP connection can support. When the number of out‐
put packets queued before sending to the TCP socket reaches this limit for a given client connection, OpenVPN will
start to drop outgoing packets directed at this client.
--tcp-nodelay
This macro sets the TCP_NODELAY socket flag on the server as well as pushes it to connecting clients. The TCP_NODE‐
LAY flag disables the Nagle algorithm on TCP sockets causing packets to be transmitted immediately with low latency,
rather than waiting a short period of time in order to aggregate several packets into a larger containing packet. In
VPN applications over TCP, TCP_NODELAY is generally a good latency optimization.
The macro expands as follows:
if mode server:
socket-flags TCP_NODELAY
push "socket-flags TCP_NODELAY"
--max-clients n
Limit server to a maximum of n concurrent clients.
--max-routes-per-client n
Allow a maximum of n internal routes per client (default=256). This is designed to help contain DoS attacks where an
authenticated client floods the server with packets appearing to come from many unique MAC addresses, forcing the
server to deplete virtual memory as its internal routing table expands. This directive can be used in a
--client-config-dir file or auto-generated by a --client-connect script to override the global value for a particular
client.
Note that this directive affects OpenVPN's internal routing table, not the kernel routing table.
--stale-routes-check n [t]
Remove routes haven't had activity for n seconds (i.e. the ageing time).
This check is ran every t seconds (i.e. check interval).
If t is not present it defaults to n
This option helps to keep the dynamic routing table small. See also --max-routes-per-client
--connect-freq n sec
Allow a maximum of n new connections per sec seconds from clients. This is designed to contain DoS attacks which
flood the server with connection requests using certificates which will ultimately fail to authenticate.
This is an imperfect solution however, because in a real DoS scenario, legitimate connections might also be refused.
For the best protection against DoS attacks in server mode, use --proto udp and either --tls-auth or --tls-crypt.
--learn-address cmd
Run command cmd to validate client virtual addresses or routes.
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
Three arguments will be appended to any arguments in cmd as follows:
[1] operation -- "add", "update", or "delete" based on whether or not the address is being added to, modified, or
deleted from OpenVPN's internal routing table.
[2] address -- The address being learned or unlearned. This can be an IPv4 address such as "198.162.10.14", an IPv4
subnet such as "198.162.10.0/24", or an ethernet MAC address (when --dev tap is being used) such as
"00:FF:01:02:03:04".
[3] common name -- The common name on the certificate associated with the client linked to this address. Only
present for "add" or "update" operations, not "delete".
On "add" or "update" methods, if the script returns a failure code (non-zero), OpenVPN will reject the address and
will not modify its internal routing table.
Normally, the cmd script will use the information provided above to set appropriate firewall entries on the VPN
TUN/TAP interface. Since OpenVPN provides the association between virtual IP or MAC address and the client's authen‐
ticated common name, it allows a user-defined script to configure firewall access policies with regard to the
client's high-level common name, rather than the low level client virtual addresses.
--auth-user-pass-verify cmd method
Require the client to provide a username/password (possibly in addition to a client certificate) for authentication.
OpenVPN will run command cmd to validate the username/password provided by the client.
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
If method is set to "via-env", OpenVPN will call script with the environmental variables username and password set to
the username/password strings provided by the client. Be aware that this method is insecure on some platforms which
make the environment of a process publicly visible to other unprivileged processes.
If method is set to "via-file", OpenVPN will write the username and password to the first two lines of a temporary
file. The filename will be passed as an argument to script, and the file will be automatically deleted by OpenVPN
after the script returns. The location of the temporary file is controlled by the --tmp-dir option, and will default
to the current directory if unspecified. For security, consider setting --tmp-dir to a volatile storage medium such
as /dev/shm (if available) to prevent the username/password file from touching the hard drive.
The script should examine the username and password, returning a success exit code (0) if the client's authentication
request is to be accepted, or a failure code (1) to reject the client.
This directive is designed to enable a plugin-style interface for extending OpenVPN's authentication capabilities.
To protect against a client passing a maliciously formed username or password string, the username string must con‐
sist only of these characters: alphanumeric, underbar ('_'), dash ('-'), dot ('.'), or at ('@'). The password string
can consist of any printable characters except for CR or LF. Any illegal characters in either the username or pass‐
word string will be converted to underbar ('_').
Care must be taken by any user-defined scripts to avoid creating a security vulnerability in the way that these
strings are handled. Never use these strings in such a way that they might be escaped or evaluated by a shell inter‐
preter.
For a sample script that performs PAM authentication, see sample-scripts/auth-pam.pl in the OpenVPN source distribu‐
tion.
--auth-gen-token [lifetime]
After successful user/password authentication, the OpenVPN server will with this option generate a temporary authen‐
tication token and push that to client. On the following renegotiations, the OpenVPN client will pass this token in‐
stead of the users password. On the server side the server will do the token authentication internally and it will
NOT do any additional authentications against configured external user/password authentication mechanisms.
The lifetime argument defines how long the generated token is valid. The lifetime is defined in seconds. If life‐
time is not set or it is set to 0, the token will never expire.
This feature is useful for environments which is configured to use One Time Passwords (OTP) as part of the user/pass‐
word authentications and that authentication mechanism does not implement any auth-token support.
--opt-verify
Clients that connect with options that are incompatible with those of the server will be disconnected.
Options that will be compared for compatibility include dev-type, link-mtu, tun-mtu, proto, ifconfig, comp-lzo, frag‐
ment, keydir, cipher, auth, keysize, secret, no-replay, no-iv, tls-auth, key-method, tls-server, and tls-client.
This option requires that --disable-occ NOT be used.
--auth-user-pass-optional
Allow connections by clients that do not specify a username/password. Normally, when --auth-user-pass-verify or
--management-client-auth is specified (or an authentication plugin module), the OpenVPN server daemon will require
connecting clients to specify a username and password. This option makes the submission of a username/password by
clients optional, passing the responsibility to the user-defined authentication module/script to accept or deny the
client based on other factors (such as the setting of X509 certificate fields). When this option is used, and a con‐
necting client does not submit a username/password, the user-defined authentication module/script will see the user‐
name and password as being set to empty strings (""). The authentication module/script MUST have logic to detect
this condition and respond accordingly.
--client-cert-not-required
DEPRECATED This option will be removed in OpenVPN 2.5
Don't require client certificate, client will authenticate using username/password only. Be aware that using this
directive is less secure than requiring certificates from all clients.
Please note: This is replaced by --verify-client-cert which allows for more flexibility. The option --ver‐
ify-client-cert none is functionally equivalent to --client-cert-not-required
--verify-client-cert none|optional|require
Specify whether the client is required to supply a valid certificate.
Possible options are
none : a client certificate is not required. the client need to authenticate using username/password only. Be aware
that using this directive is less secure than requiring certificates from all clients.
If you use this directive, the entire responsibility of authentication will rest on your --auth-user-pass-verify
script, so keep in mind that bugs in your script could potentially compromise the security of your VPN.
--verify-client-cert none is functionally equivalent to --client-cert-not-required.
optional : a client may present a certificate but it is not required to do so. When using this directive, you should
also use a --auth-user-pass-verify script to ensure that clients are authenticated using a certificate, a username
and password, or possibly even both.
Again, the entire responsibility of authentication will rest on your --auth-user-pass-verify script, so keep in mind
that bugs in your script could potentially compromise the security of your VPN.
require : this is the default option. A client is required to present a certificate, otherwise VPN access is refused.
If you don't use this directive (or use --verify-client-cert require ) but you also specify an --auth-user-pass-ver‐
ify script, then OpenVPN will perform double authentication. The client certificate verification AND the
--auth-user-pass-verify script will need to succeed in order for a client to be authenticated and accepted onto the
VPN.
--username-as-common-name
For --auth-user-pass-verify authentication, use the authenticated username as the common name, rather than the common
name from the client cert.
--compat-names [no-remapping]
DEPRECATED This option will be removed in OpenVPN 2.5
Until OpenVPN v2.3 the format of the X.509 Subject fields was formatted like this:
/C=US/L=Somewhere/CN=John Doe/emailAddress=john@example.com
In addition the old behaviour was to remap any character other than alphanumeric, underscore ('_'), dash ('-'), dot
('.'), and slash ('/') to underscore ('_'). The X.509 Subject string as returned by the tls_id environmental vari‐
able, could additionally contain colon (':') or equal ('=').
When using the --compat-names option, this old formatting and remapping will be re-enabled again. This is purely im‐
plemented for compatibility reasons when using older plug-ins or scripts which does not handle the new formatting or
UTF-8 characters.
In OpenVPN 2.3 the formatting of these fields changed into a more standardised format. It now looks like:
C=US, L=Somewhere, CN=John Doe, emailAddress=john@example.com
The new default format in OpenVPN 2.3 also does not do the character remapping which happened earlier. This new for‐
mat enables proper support for UTF-8 characters in the usernames, X.509 Subject fields and Common Name variables and
it complies to the RFC 2253, UTF-8 String Representation of Distinguished Names.
The no-remapping mode flag can be used with the --compat-names option to be compatible with the now deprecated
--no-name-remapping option. It is only available at the server. When this mode flag is used, the Common Name, Sub‐
ject, and username strings are allowed to include any printable character including space, but excluding control
characters such as tab, newline, and carriage-return. no-remapping is only available on the server side.
Please note: This option is immediately deprecated. It is only implemented to make the transition to the new format‐
ting less intrusive. It will be removed in OpenVPN 2.5. So please update your scripts/plug-ins where necessary.
--no-name-remapping
DEPRECATED This option will be removed in OpenVPN 2.5
The --no-name-remapping option is an alias for --compat-names no-remapping. It ensures compatibility with server
configurations using the --no-name-remapping option.
Please note: This option is now deprecated. It will be removed in OpenVPN 2.5. So please make sure you support the
new X.509 name formatting described with the --compat-names option as soon as possible.
--port-share host port [dir]
When run in TCP server mode, share the OpenVPN port with another application, such as an HTTPS server. If OpenVPN
senses a connection to its port which is using a non-OpenVPN protocol, it will proxy the connection to the server at
host:port. Currently only designed to work with HTTP/HTTPS, though it would be theoretically possible to extend to
other protocols such as ssh.
dir specifies an optional directory where a temporary file with name N containing content C will be dynamically gen‐
erated for each proxy connection, where N is the source IP:port of the client connection and C is the source IP:port
of the connection to the proxy receiver. This directory can be used as a dictionary by the proxy receiver to deter‐
mine the origin of the connection. Each generated file will be automatically deleted when the proxied connection is
torn down.
Not implemented on Windows.
Client Mode
Use client mode when connecting to an OpenVPN server which has --server, --server-bridge, or --mode server in it's configu‐
ration.
--client
A helper directive designed to simplify the configuration of OpenVPN's client mode. This directive is equivalent to:
pull
tls-client
--pull This option must be used on a client which is connecting to a multi-client server. It indicates to OpenVPN that it
should accept options pushed by the server, provided they are part of the legal set of pushable options (note that
the --pull option is implied by --client ).
In particular, --pull allows the server to push routes to the client, so you should not use --pull or --client in
situations where you don't trust the server to have control over the client's routing table.
--pull-filter accept|ignore|reject text
Filter options received from the server if the option starts with text. Runs on client. The action flag accept al‐
lows the option, ignore removes it and reject flags an error and triggers a SIGUSR1 restart. The filters may be
specified multiple times, and each filter is applied in the order it is specified. The filtering of each option stops
as soon as a match is found. Unmatched options are accepted by default.
Prefix comparison is used to match text against the received option so that
--pull-filter ignore "route"
would remove all pushed options starting with route which would include, for example, route-gateway. Enclose text in
quotes to embed spaces.
--pull-filter accept "route 192.168.1."
--pull-filter ignore "route "
would remove all routes that do not start with 192.168.1.
This option may be used only on clients. Note that reject may result in a repeated cycle of failure and reconnect,
unless multiple remotes are specified and connection to the next remote succeeds. To silently ignore an option pushed
by the server, use ignore.
--auth-user-pass [up]
Authenticate with server using username/password. up is a file containing username/password on 2 lines. If the pass‐
word line is missing, OpenVPN will prompt for one.
If up is omitted, username/password will be prompted from the console.
The server configuration must specify an --auth-user-pass-verify script to verify the username/password provided by
the client.
--auth-retry type
Controls how OpenVPN responds to username/password verification errors such as the client-side response to an
AUTH_FAILED message from the server or verification failure of the private key password.
Normally used to prevent auth errors from being fatal on the client side, and to permit username/password requeries
in case of error.
An AUTH_FAILED message is generated by the server if the client fails --auth-user-pass authentication, or if the
server-side --client-connect script returns an error status when the client tries to connect.
type can be one of:
none -- Client will exit with a fatal error (this is the default).
nointeract -- Client will retry the connection without requerying for an --auth-user-pass username/password. Use
this option for unattended clients.
interact -- Client will requery for an --auth-user-pass username/password and/or private key password before attempt‐
ing a reconnection.
Note that while this option cannot be pushed, it can be controlled from the management interface.
--static-challenge t e
Enable static challenge/response protocol using challenge text t, with echo flag given by e (0|1).
The echo flag indicates whether or not the user's response to the challenge should be echoed.
See management-notes.txt in the OpenVPN distribution for a description of the OpenVPN challenge/response protocol.
--server-poll-timeout n, --connect-timeout n
When connecting to a remote server do not wait for more than n seconds waiting for a response before trying the next
server. The default value is 120s. This timeout includes proxy and TCP connect timeouts.
--explicit-exit-notify [n]
In UDP client mode or point-to-point mode, send server/peer an exit notification if tunnel is restarted or OpenVPN
process is exited. In client mode, on exit/restart, this option will tell the server to immediately close its client
instance object rather than waiting for a timeout. The n parameter (default=1) controls the maximum number of at‐
tempts that the client will try to resend the exit notification message.
In UDP server mode, send RESTART control channel command to connected clients. The n parameter (default=1) controls
client behavior. With n = 1 client will attempt to reconnect to the same server, with n = 2 client will advance to
the next server.
OpenVPN will not send any exit notifications unless this option is enabled.
--allow-recursive-routing
When this option is set, OpenVPN will not drop incoming tun packets with same destination as host.
Data Channel Encryption Options:
These options are meaningful for both Static & TLS-negotiated key modes (must be compatible between peers).
--secret file [direction]
Enable Static Key encryption mode (non-TLS). Use pre-shared secret file which was generated with --genkey.
The optional direction parameter enables the use of 4 distinct keys (HMAC-send, cipher-encrypt, HMAC-receive, ci‐
pher-decrypt), so that each data flow direction has a different set of HMAC and cipher keys. This has a number of
desirable security properties including eliminating certain kinds of DoS and message replay attacks.
When the direction parameter is omitted, 2 keys are used bidirectionally, one for HMAC and the other for encryp‐
tion/decryption.
The direction parameter should always be complementary on either side of the connection, i.e. one side should use "0"
and the other should use "1", or both sides should omit it altogether.
The direction parameter requires that file contains a 2048 bit key. While pre-1.5 versions of OpenVPN generate 1024
bit key files, any version of OpenVPN which supports the direction parameter, will also support 2048 bit key file
generation using the --genkey option.
Static key encryption mode has certain advantages, the primary being ease of configuration.
There are no certificates or certificate authorities or complicated negotiation handshakes and protocols. The only
requirement is that you have a pre-existing secure channel with your peer (such as ssh ) to initially copy the key.
This requirement, along with the fact that your key never changes unless you manually generate a new one, makes it
somewhat less secure than TLS mode (see below). If an attacker manages to steal your key, everything that was ever
encrypted with it is compromised. Contrast that to the perfect forward secrecy features of TLS mode (using Diffie
Hellman key exchange), where even if an attacker was able to steal your private key, he would gain no information to
help him decrypt past sessions.
Another advantageous aspect of Static Key encryption mode is that it is a handshake-free protocol without any distin‐
guishing signature or feature (such as a header or protocol handshake sequence) that would mark the ciphertext pack‐
ets as being generated by OpenVPN. Anyone eavesdropping on the wire would see nothing but random-looking data.
--key-direction
Alternative way of specifying the optional direction parameter for the --tls-auth and --secret options. Useful when
using inline files (See section on inline files).
--auth alg
Authenticate data channel packets and (if enabled) tls-auth control channel packets with HMAC using message digest
algorithm alg. (The default is SHA1 ). HMAC is a commonly used message authentication algorithm (MAC) that uses a
data string, a secure hash algorithm, and a key, to produce a digital signature.
The OpenVPN data channel protocol uses encrypt-then-mac (i.e. first encrypt a packet, then HMAC the resulting cipher‐
text), which prevents padding oracle attacks.
If an AEAD cipher mode (e.g. GCM) is chosen, the specified --auth algorithm is ignored for the data channel, and the
authentication method of the AEAD cipher is used instead. Note that alg still specifies the digest used for
tls-auth.
In static-key encryption mode, the HMAC key is included in the key file generated by --genkey. In TLS mode, the HMAC
key is dynamically generated and shared between peers via the TLS control channel. If OpenVPN receives a packet with
a bad HMAC it will drop the packet. HMAC usually adds 16 or 20 bytes per packet. Set alg=none to disable authenti‐
cation.
For more information on HMAC see http://www.cs.ucsd.edu/users/mihir/papers/hmac.html
--cipher alg
Encrypt data channel packets with cipher algorithm alg.
The default is BF-CBC, an abbreviation for Blowfish in Cipher Block Chaining mode. When cipher negotiation (NCP) is
allowed, OpenVPN 2.4 and newer on both client and server side will automatically upgrade to AES-256-GCM. See
--ncp-ciphers and --ncp-disable for more details on NCP.
Using BF-CBC is no longer recommended, because of its 64-bit block size. This small block size allows attacks based
on collisions, as demonstrated by SWEET32. See https://community.openvpn.net/openvpn/wiki/SWEET32 for details. Due
to this, support for BF-CBC, DES, CAST5, IDEA and RC2 ciphers will be removed in OpenVPN 2.6.
To see other ciphers that are available with OpenVPN, use the --show-ciphers option.
Set alg=none to disable encryption.
--ncp-ciphers cipher_list
Restrict the allowed ciphers to be negotiated to the ciphers in cipher_list. cipher_list is a colon-separated list
of ciphers, and defaults to "AES-256-GCM:AES-128-GCM".
For servers, the first cipher from cipher_list will be pushed to clients that support cipher negotiation.
Cipher negotiation is enabled in client-server mode only. I.e. if --mode is set to 'server' (server-side, implied by
setting --server ), or if --pull is specified (client-side, implied by setting --client).
If both peers support and do not disable NCP, the negotiated cipher will override the cipher specified by --cipher.
Additionally, to allow for more smooth transition, if NCP is enabled, OpenVPN will inherit the cipher of the peer if
that cipher is different from the local --cipher setting, but the peer cipher is one of the ciphers specified in
--ncp-ciphers. E.g. a non-NCP client (<=v2.3, or with --ncp-disabled set) connecting to a NCP server (v2.4+) with
"--cipher BF-CBC" and "--ncp-ciphers AES-256-GCM:AES-256-CBC" set can either specify "--cipher BF-CBC" or "--cipher
AES-256-CBC" and both will work.
--ncp-disable
Disable "negotiable crypto parameters". This completely disables cipher negotiation.
--keysize n
DEPRECATED This option will be removed in OpenVPN 2.6.
Size of cipher key in bits (optional). If unspecified, defaults to cipher-specific default. The --show-ciphers op‐
tion (see below) shows all available OpenSSL ciphers, their default key sizes, and whether the key size can be
changed. Use care in changing a cipher's default key size. Many ciphers have not been extensively cryptanalyzed
with non-standard key lengths, and a larger key may offer no real guarantee of greater security, or may even reduce
security.
--prng alg [nsl]
(Advanced) For PRNG (Pseudo-random number generator), use digest algorithm alg (default=sha1), and set nsl (de‐
fault=16) to the size in bytes of the nonce secret length (between 16 and 64).
Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes function instead for all of OpenVPN's pseudo-random
number needs.
--engine [engine-name]
Enable OpenSSL hardware-based crypto engine functionality.
If engine-name is specified, use a specific crypto engine. Use the --show-engines standalone option to list the
crypto engines which are supported by OpenSSL.
--no-replay
DEPRECATED This option will be removed in OpenVPN 2.5.
(Advanced) Disable OpenVPN's protection against replay attacks. Don't use this option unless you are prepared to
make a tradeoff of greater efficiency in exchange for less security.
OpenVPN provides datagram replay protection by default.
Replay protection is accomplished by tagging each outgoing datagram with an identifier that is guaranteed to be
unique for the key being used. The peer that receives the datagram will check for the uniqueness of the identifier.
If the identifier was already received in a previous datagram, OpenVPN will drop the packet. Replay protection is
important to defeat attacks such as a SYN flood attack, where the attacker listens in the wire, intercepts a TCP SYN
packet (identifying it by the context in which it occurs in relation to other packets), then floods the receiving
peer with copies of this packet.
OpenVPN's replay protection is implemented in slightly different ways, depending on the key management mode you have
selected.
In Static Key mode or when using an CFB or OFB mode cipher, OpenVPN uses a 64 bit unique identifier that combines a
time stamp with an incrementing sequence number.
When using TLS mode for key exchange and a CBC cipher mode, OpenVPN uses only a 32 bit sequence number without a time
stamp, since OpenVPN can guarantee the uniqueness of this value for each key. As in IPSec, if the sequence number is
close to wrapping back to zero, OpenVPN will trigger a new key exchange.
To check for replays, OpenVPN uses the sliding window algorithm used by IPSec.
--replay-window n [t]
Use a replay protection sliding-window of size n and a time window of t seconds.
By default n is 64 (the IPSec default) and t is 15 seconds.
This option is only relevant in UDP mode, i.e. when either --proto udp is specified, or no --proto option is speci‐
fied.
When OpenVPN tunnels IP packets over UDP, there is the possibility that packets might be dropped or delivered out of
order. Because OpenVPN, like IPSec, is emulating the physical network layer, it will accept an out-of-order packet
sequence, and will deliver such packets in the same order they were received to the TCP/IP protocol stack, provided
they satisfy several constraints.
(a) The packet cannot be a replay (unless --no-replay is specified, which disables replay protection altogether).
(b) If a packet arrives out of order, it will only be accepted if the difference between its sequence number and the
highest sequence number received so far is less than n.
(c) If a packet arrives out of order, it will only be accepted if it arrives no later than t seconds after any packet
containing a higher sequence number.
If you are using a network link with a large pipeline (meaning that the product of bandwidth and latency is high),
you may want to use a larger value for n. Satellite links in particular often require this.
If you run OpenVPN at --verb 4, you will see the message "Replay-window backtrack occurred [x]" every time the maxi‐
mum sequence number backtrack seen thus far increases. This can be used to calibrate n.
There is some controversy on the appropriate method of handling packet reordering at the security layer.
Namely, to what extent should the security layer protect the encapsulated protocol from attacks which masquerade as
the kinds of normal packet loss and reordering that occur over IP networks?
The IPSec and OpenVPN approach is to allow packet reordering within a certain fixed sequence number window.
OpenVPN adds to the IPSec model by limiting the window size in time as well as sequence space.
OpenVPN also adds TCP transport as an option (not offered by IPSec) in which case OpenVPN can adopt a very strict at‐
titude towards message deletion and reordering: Don't allow it. Since TCP guarantees reliability, any packet loss
or reordering event can be assumed to be an attack.
In this sense, it could be argued that TCP tunnel transport is preferred when tunneling non-IP or UDP application
protocols which might be vulnerable to a message deletion or reordering attack which falls within the normal opera‐
tional parameters of IP networks.
So I would make the statement that one should never tunnel a non-IP protocol or UDP application protocol over UDP, if
the protocol might be vulnerable to a message deletion or reordering attack that falls within the normal operating
parameters of what is to be expected from the physical IP layer. The problem is easily fixed by simply using TCP as
the VPN transport layer.
--mute-replay-warnings
Silence the output of replay warnings, which are a common false alarm on WiFi networks. This option preserves the
security of the replay protection code without the verbosity associated with warnings about duplicate packets.
--replay-persist file
Persist replay-protection state across sessions using file to save and reload the state.
This option will strengthen protection against replay attacks, especially when you are using OpenVPN in a dynamic
context (such as with --inetd) when OpenVPN sessions are frequently started and stopped.
This option will keep a disk copy of the current replay protection state (i.e. the most recent packet timestamp and
sequence number received from the remote peer), so that if an OpenVPN session is stopped and restarted, it will re‐
ject any replays of packets which were already received by the prior session.
This option only makes sense when replay protection is enabled (the default) and you are using either --secret
(shared-secret key mode) or TLS mode with --tls-auth.
--no-iv
DEPRECATED This option will be removed in OpenVPN 2.5.
(Advanced) Disable OpenVPN's use of IV (cipher initialization vector). Don't use this option unless you are prepared
to make a tradeoff of greater efficiency in exchange for less security.
OpenVPN uses an IV by default, and requires it for CFB and OFB cipher modes (which are totally insecure without it).
Using an IV is important for security when multiple messages are being encrypted/decrypted with the same key.
IV is implemented differently depending on the cipher mode used.
In CBC mode, OpenVPN uses a pseudo-random IV for each packet.
In CFB/OFB mode, OpenVPN uses a unique sequence number and time stamp as the IV. In fact, in CFB/OFB mode, OpenVPN
uses a datagram space-saving optimization that uses the unique identifier for datagram replay protection as the IV.
--use-prediction-resistance
Enable prediction resistance on mbed TLS's RNG.
Enabling prediction resistance causes the RNG to reseed in each call for random. Reseeding this often can quickly de‐
plete the kernel entropy pool.
If you need this option, please consider running a daemon that adds entropy to the kernel pool.
--test-crypto
Do a self-test of OpenVPN's crypto options by encrypting and decrypting test packets using the data channel encryp‐
tion options specified above. This option does not require a peer to function, and therefore can be specified with‐
out --dev or --remote.
The typical usage of --test-crypto would be something like this:
openvpn --test-crypto --secret key
or
openvpn --test-crypto --secret key --verb 9
This option is very useful to test OpenVPN after it has been ported to a new platform, or to isolate problems in the
compiler, OpenSSL crypto library, or OpenVPN's crypto code. Since it is a self-test mode, problems with encryption
and authentication can be debugged independently of network and tunnel issues.
TLS Mode Options:
TLS mode is the most powerful crypto mode of OpenVPN in both security and flexibility. TLS mode works by establishing con‐
trol and data channels which are multiplexed over a single TCP/UDP port. OpenVPN initiates a TLS session over the control
channel and uses it to exchange cipher and HMAC keys to protect the data channel. TLS mode uses a robust reliability layer
over the UDP connection for all control channel communication, while the data channel, over which encrypted tunnel data
passes, is forwarded without any mediation. The result is the best of both worlds: a fast data channel that forwards over
UDP with only the overhead of encrypt, decrypt, and HMAC functions, and a control channel that provides all of the security
features of TLS, including certificate-based authentication and Diffie Hellman forward secrecy.
To use TLS mode, each peer that runs OpenVPN should have its own local certificate/key pair ( --cert and --key ), signed by
the root certificate which is specified in --ca.
When two OpenVPN peers connect, each presents its local certificate to the other. Each peer will then check that its part‐
ner peer presented a certificate which was signed by the master root certificate as specified in --ca.
If that check on both peers succeeds, then the TLS negotiation will succeed, both OpenVPN peers will exchange temporary ses‐
sion keys, and the tunnel will begin passing data.
The OpenVPN project provides a set of scripts for managing RSA certificates & keys: https://github.com/OpenVPN/easy-rsa
--tls-server
Enable TLS and assume server role during TLS handshake. Note that OpenVPN is designed as a peer-to-peer application.
The designation of client or server is only for the purpose of negotiating the TLS control channel.
--tls-client
Enable TLS and assume client role during TLS handshake.
--ca file
Certificate authority (CA) file in .pem format, also referred to as the root certificate. This file can have multi‐
ple certificates in .pem format, concatenated together. You can construct your own certificate authority certificate
and private key by using a command such as:
openssl req -nodes -new -x509 -keyout ca.key -out ca.crt
Then edit your openssl.cnf file and edit the certificate variable to point to your new root certificate ca.crt.
For testing purposes only, the OpenVPN distribution includes a sample CA certificate (ca.crt). Of course you should
never use the test certificates and test keys distributed with OpenVPN in a production environment, since by virtue
of the fact that they are distributed with OpenVPN, they are totally insecure.
--capath dir
Directory containing trusted certificates (CAs and CRLs). Not available with mbed TLS.
When using the --capath option, you are required to supply valid CRLs for the CAs too. CAs in the capath directory
are expected to be named <hash>.<n>. CRLs are expected to be named <hash>.r<n>. See the -CApath option of openssl
verify , and the -hash option of openssl x509 and openssl crl for more information.
--dh file
File containing Diffie Hellman parameters in .pem format (required for --tls-server only).
Set file=none to disable Diffie Hellman key exchange (and use ECDH only). Note that this requires peers to be using
an SSL library that supports ECDH TLS cipher suites (e.g. OpenSSL 1.0.1+, or mbed TLS 2.0+).
Use openssl dhparam -out dh2048.pem 2048 to generate 2048-bit DH parameters. Diffie Hellman parameters may be consid‐
ered public.
--ecdh-curve name
Specify the curve to use for elliptic curve Diffie Hellman. Available curves can be listed with --show-curves. The
specified curve will only be used for ECDH TLS-ciphers.
This option is not supported in mbed TLS builds of OpenVPN.
--cert file
Local peer's signed certificate in .pem format -- must be signed by a certificate authority whose certificate is in
--ca file. Each peer in an OpenVPN link running in TLS mode should have its own certificate and private key file.
In addition, each certificate should have been signed by the key of a certificate authority whose public key resides
in the --ca certificate authority file. You can easily make your own certificate authority (see above) or pay money
to use a commercial service such as thawte.com (in which case you will be helping to finance the world's second space
tourist :). To generate a certificate, you can use a command such as:
openssl req -nodes -new -keyout mycert.key -out mycert.csr
If your certificate authority private key lives on another machine, copy the certificate signing request (mycert.csr)
to this other machine (this can be done over an insecure channel such as email). Now sign the certificate with a
command such as:
openssl ca -out mycert.crt -in mycert.csr
Now copy the certificate (mycert.crt) back to the peer which initially generated the .csr file (this can be over a
public medium). Note that the openssl ca command reads the location of the certificate authority key from its con‐
figuration file such as /usr/share/ssl/openssl.cnf -- note also that for certificate authority functions, you must
set up the files index.txt (may be empty) and serial (initialize to 01 ).
--extra-certs file
Specify a file containing one or more PEM certs (concatenated together) that complete the local certificate chain.
This option is useful for "split" CAs, where the CA for server certs is different than the CA for client certs.
Putting certs in this file allows them to be used to complete the local certificate chain without trusting them to
verify the peer-submitted certificate, as would be the case if the certs were placed in the ca file.
--key file
Local peer's private key in .pem format. Use the private key which was generated when you built your peer's certifi‐
cate (see --cert file above).
--tls-version-min version ['or-highest']
Sets the minimum TLS version we will accept from the peer (default is "1.0"). Examples for version include "1.0",
"1.1", or "1.2". If 'or-highest' is specified and version is not recognized, we will only accept the highest TLS
version supported by the local SSL implementation.
--tls-version-max version
Set the maximum TLS version we will use (default is the highest version supported). Examples for version include
"1.0", "1.1", or "1.2".
--pkcs12 file
Specify a PKCS #12 file containing local private key, local certificate, and root CA certificate. This option can be
used instead of --ca, --cert, and --key. Not available with mbed TLS.
--verify-hash hash [algo]
Specify SHA1 or SHA256 fingerprint for level-1 cert. The level-1 cert is the CA (or intermediate cert) that signs
the leaf certificate, and is one removed from the leaf certificate in the direction of the root. When accepting a
connection from a peer, the level-1 cert fingerprint must match hash or certificate verification will fail. Hash is
specified as XX:XX:... For example:
AD:B0:95:D8:09:C8:36:45:12:A9:89:C8:90:09:CB:13:72:A6:AD:16
The algo flag can be either SHA1 or SHA256. If not provided, it defaults to SHA1.
--pkcs11-cert-private [0|1]...
Set if access to certificate object should be performed after login. Every provider has its own setting.
--pkcs11-id name
Specify the serialized certificate id to be used. The id can be gotten by the standalone --show-pkcs11-ids option.
--pkcs11-id-management
Acquire PKCS#11 id from management interface. In this case a NEED-STR 'pkcs11-id-request' real-time message will be
triggered, application may use pkcs11-id-count command to retrieve available number of certificates, and
pkcs11-id-get command to retrieve certificate id and certificate body.
--pkcs11-pin-cache seconds
Specify how many seconds the PIN can be cached, the default is until the token is removed.
--pkcs11-protected-authentication [0|1]...
Use PKCS#11 protected authentication path, useful for biometric and external keypad devices. Every provider has its
own setting.
--pkcs11-providers provider...
Specify a RSA Security Inc. PKCS #11 Cryptographic Token Interface (Cryptoki) providers to load. This option can be
used instead of --cert, --key, and --pkcs12.
If p11-kit is present on the system, its p11-kit-proxy.so module will be loaded by default if either the --pkcs11-id
or --pkcs11-id-management options are specified without --pkcs11-provider being given.
--pkcs11-private-mode mode...
Specify which method to use in order to perform private key operations. A different mode can be specified for each
provider. Mode is encoded as hex number, and can be a mask one of the following:
0 (default) -- Try to determine automatically.
1 -- Use sign.
2 -- Use sign recover.
4 -- Use decrypt.
8 -- Use unwrap.
--cryptoapicert select-string
Load the certificate and private key from the Windows Certificate System Store (Windows/OpenSSL Only).
Use this option instead of --cert and --key.
This makes it possible to use any smart card, supported by Windows, but also any kind of certificate, residing in the
Cert Store, where you have access to the private key. This option has been tested with a couple of different smart
cards (GemSAFE, Cryptoflex, and Swedish Post Office eID) on the client side, and also an imported PKCS12 software
certificate on the server side.
To select a certificate, based on a substring search in the certificate's subject:
cryptoapicert "SUBJ:Peter Runestig"
To select a certificate, based on certificate's thumbprint:
cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."
The thumbprint hex string can easily be copy-and-pasted from the Windows Certificate Store GUI.
--key-method m
DEPRECATED This option will be removed in OpenVPN 2.5
Use data channel key negotiation method m. The key method must match on both sides of the connection.
After OpenVPN negotiates a TLS session, a new set of keys for protecting the tunnel data channel is generated and ex‐
changed over the TLS session.
In method 1 (the default for OpenVPN 1.x), both sides generate random encrypt and HMAC-send keys which are forwarded
to the other host over the TLS channel. Method 1 is deprecated in OpenVPN 2.4 , and will be removed in OpenVPN 2.5.
In method 2, (the default for OpenVPN 2.0) the client generates a random key. Both client and server also generate
some random seed material. All key source material is exchanged over the TLS channel. The actual keys are generated
using the TLS PRF function, taking source entropy from both client and server. Method 2 is designed to closely par‐
allel the key generation process used by TLS 1.0.
Note that in TLS mode, two separate levels of keying occur:
(1) The TLS connection is initially negotiated, with both sides of the connection producing certificates and verify‐
ing the certificate (or other authentication info provided) of the other side. The --key-method parameter has no ef‐
fect on this process.
(2) After the TLS connection is established, the tunnel session keys are separately negotiated over the existing se‐
cure TLS channel. Here, --key-method determines the derivation of the tunnel session keys.
--tls-cipher l
--tls-ciphersuites l
A list l of allowable TLS ciphers delimited by a colon (":").
These setting can be used to ensure that certain cipher suites are used (or not used) for the TLS connection. Open‐
VPN uses TLS to secure the control channel, over which the keys that are used to protect the actual VPN traffic are
exchanged.
The supplied list of ciphers is (after potential OpenSSL/IANA name translation) simply supplied to the crypto li‐
brary. Please see the OpenSSL and/or mbed TLS documentation for details on the cipher list interpretation.
For OpenSSL, the --tls-cipher is used for TLS 1.2 and below. For TLS 1.3 and up, the --tls-ciphersuites setting is
used. mbed TLS has no TLS 1.3 support yet and only the --tls-cipher setting is used.
Use --show-tls to see a list of TLS ciphers supported by your crypto library.
Warning! --tls-cipher and --tls-ciphersuites are expert features, which - if used correcly - can improve the secu‐
rity of your VPN connection. But it is also easy to unwittingly use them to carefully align a gun with your foot, or
just break your connection. Use with care!
The default for --tls-cipher is to use mbed TLS's default cipher list when using mbed TLS or "DE‐
FAULT:!EXP:!LOW:!MEDIUM:!kDH:!kECDH:!DSS:!PSK:!SRP:!kRSA" when using OpenSSL.
The default for --tls-ciphersuites is to use the crypto library's default.
--tls-cert-profile profile
Set the allowed cryptographic algorithms for certificates according to profile.
The following profiles are supported:
legacy (default): SHA1 and newer, RSA 2048-bit+, any elliptic curve.
preferred : SHA2 and newer, RSA 2048-bit+, any elliptic curve.
suiteb : SHA256/SHA384, ECDSA with P-256 or P-384.
This option is only fully supported for mbed TLS builds. OpenSSL builds use the following approximation:
legacy (default): sets "security level 1"
preferred : sets "security level 2"
suiteb : sets "security level 3" and --tls-cipher "SUITEB128".
OpenVPN will migrate to 'preferred' as default in the future. Please ensure that your keys already comply.
--tls-timeout n
Packet retransmit timeout on TLS control channel if no acknowledgment from remote within n seconds (default=2). When
OpenVPN sends a control packet to its peer, it will expect to receive an acknowledgement within n seconds or it will
retransmit the packet, subject to a TCP-like exponential backoff algorithm. This parameter only applies to control
channel packets. Data channel packets (which carry encrypted tunnel data) are never acknowledged, sequenced, or re‐
transmitted by OpenVPN because the higher level network protocols running on top of the tunnel such as TCP expect
this role to be left to them.
--reneg-bytes n
Renegotiate data channel key after n bytes sent or received (disabled by default with an exception, see below).
OpenVPN allows the lifetime of a key to be expressed as a number of bytes encrypted/decrypted, a number of packets,
or a number of seconds. A key renegotiation will be forced if any of these three criteria are met by either peer.
If using ciphers with cipher block sizes less than 128-bits, --reneg-bytes is set to 64MB by default, unless it is
explicitly disabled by setting the value to 0, but this is HIGHLY DISCOURAGED as this is designed to add some protec‐
tion against the SWEET32 attack vector. For more information see the --cipher option.
--reneg-pkts n
Renegotiate data channel key after n packets sent and received (disabled by default).
--reneg-sec n
Renegotiate data channel key after n seconds (default=3600).
When using dual-factor authentication, note that this default value may cause the end user to be challenged to reau‐
thorize once per hour.
Also, keep in mind that this option can be used on both the client and server, and whichever uses the lower value
will be the one to trigger the renegotiation. A common mistake is to set --reneg-sec to a higher value on either the
client or server, while the other side of the connection is still using the default value of 3600 seconds, meaning
that the renegotiation will still occur once per 3600 seconds. The solution is to increase --reneg-sec on both the
client and server, or set it to 0 on one side of the connection (to disable), and to your chosen value on the other
side.
--hand-window n
Handshake Window -- the TLS-based key exchange must finalize within n seconds of handshake initiation by any peer
(default = 60 seconds). If the handshake fails we will attempt to reset our connection with our peer and try again.
Even in the event of handshake failure we will still use our expiring key for up to --tran-window seconds to maintain
continuity of transmission of tunnel data.
--tran-window n
Transition window -- our old key can live this many seconds after a new a key renegotiation begins (default = 3600
seconds). This feature allows for a graceful transition from old to new key, and removes the key renegotiation se‐
quence from the critical path of tunnel data forwarding.
--single-session
After initially connecting to a remote peer, disallow any new connections. Using this option means that a remote
peer cannot connect, disconnect, and then reconnect.
If the daemon is reset by a signal or --ping-restart, it will allow one new connection.
--single-session can be used with --ping-exit or --inactive to create a single dynamic session that will exit when
finished.
--tls-exit
Exit on TLS negotiation failure.
--tls-auth file [direction]
Add an additional layer of HMAC authentication on top of the TLS control channel to mitigate DoS attacks and attacks
on the TLS stack.
In a nutshell, --tls-auth enables a kind of "HMAC firewall" on OpenVPN's TCP/UDP port, where TLS control channel
packets bearing an incorrect HMAC signature can be dropped immediately without response.
file (required) is a file in OpenVPN static key format which can be generated by --genkey
Older versions (up to OpenVPN 2.3) supported a freeform passphrase file. This is no longer supported in newer ver‐
sions (v2.4+).
See the --secret option for more information on the optional direction parameter.
--tls-auth is recommended when you are running OpenVPN in a mode where it is listening for packets from any IP ad‐
dress, such as when --remote is not specified, or --remote is specified with --float.
The rationale for this feature is as follows. TLS requires a multi-packet exchange before it is able to authenticate
a peer. During this time before authentication, OpenVPN is allocating resources (memory and CPU) to this potential
peer. The potential peer is also exposing many parts of OpenVPN and the OpenSSL library to the packets it is send‐
ing. Most successful network attacks today seek to either exploit bugs in programs (such as buffer overflow attacks)
or force a program to consume so many resources that it becomes unusable. Of course the first line of defense is al‐
ways to produce clean, well-audited code. OpenVPN has been written with buffer overflow attack prevention as a top
priority. But as history has shown, many of the most widely used network applications have, from time to time,
fallen to buffer overflow attacks.
So as a second line of defense, OpenVPN offers this special layer of authentication on top of the TLS control channel
so that every packet on the control channel is authenticated by an HMAC signature and a unique ID for replay protec‐
tion. This signature will also help protect against DoS (Denial of Service) attacks. An important rule of thumb in
reducing vulnerability to DoS attacks is to minimize the amount of resources a potential, but as yet unauthenticated,
client is able to consume.
--tls-auth does this by signing every TLS control channel packet with an HMAC signature, including packets which are
sent before the TLS level has had a chance to authenticate the peer. The result is that packets without the correct
signature can be dropped immediately upon reception, before they have a chance to consume additional system resources
such as by initiating a TLS handshake. --tls-auth can be strengthened by adding the --replay-persist option which
will keep OpenVPN's replay protection state in a file so that it is not lost across restarts.
It should be emphasized that this feature is optional and that the key file used with --tls-auth gives a peer nothing
more than the power to initiate a TLS handshake. It is not used to encrypt or authenticate any tunnel data.
Use --tls-crypt instead if you want to use the key file to not only authenticate, but also encrypt the TLS control
channel.
--tls-crypt keyfile
Encrypt and authenticate all control channel packets with the key from keyfile. (See --tls-auth for more back‐
ground.)
Encrypting (and authenticating) control channel packets:
• provides more privacy by hiding the certificate used for the TLS connection,
• makes it harder to identify OpenVPN traffic as such,
• provides "poor-man's" post-quantum security, against attackers who will never know the pre-shared key (i.e. no for‐
ward secrecy).
In contrast to --tls-auth, --tls-crypt does *not* require the user to set --key-direction.
Security Considerations
All peers use the same --tls-crypt pre-shared group key to authenticate and encrypt control channel messages. To en‐
sure that IV collisions remain unlikely, this key should not be used to encrypt more than 2^48 client-to-server or
2^48 server-to-client control channel messages. A typical initial negotiation is about 10 packets in each direction.
Assuming both initial negotiation and renegotiations are at most 2^16 (65536) packets (to be conservative), and
(re)negotiations happen each minute for each user (24/7), this limits the tls-crypt key lifetime to 8171 years di‐
vided by the number of users. So a setup with 1000 users should rotate the key at least once each eight years. (And
a setup with 8000 users each year.)
If IV collisions were to occur, this could result in the security of --tls-crypt degrading to the same security as
using --tls-auth. That is, the control channel still benefits from the extra protection against active
man-in-the-middle-attacks and DoS attacks, but may no longer offer extra privacy and post-quantum security on top of
what TLS itself offers.
--askpass [file]
Get certificate password from console or file before we daemonize.
For the extremely security conscious, it is possible to protect your private key with a password. Of course this
means that every time the OpenVPN daemon is started you must be there to type the password. The --askpass option al‐
lows you to start OpenVPN from the command line. It will query you for a password before it daemonizes. To protect
a private key with a password you should omit the -nodes option when you use the openssl command line tool to manage
certificates and private keys.
If file is specified, read the password from the first line of file. Keep in mind that storing your password in a
file to a certain extent invalidates the extra security provided by using an encrypted key.
--auth-nocache
Don't cache --askpass or --auth-user-pass username/passwords in virtual memory.
If specified, this directive will cause OpenVPN to immediately forget username/password inputs after they are used.
As a result, when OpenVPN needs a username/password, it will prompt for input from stdin, which may be multiple times
during the duration of an OpenVPN session.
When using --auth-nocache in combination with a user/password file and --chroot or --daemon, make sure to use an ab‐
solute path.
This directive does not affect the --http-proxy username/password. It is always cached.
--auth-token token
This is not an option to be used directly in any configuration files, but rather push this option from a
--client-connect script or a --plugin which hooks into the OPENVPN_PLUGIN_CLIENT_CONNECT or OPEN‐
VPN_PLUGIN_CLIENT_CONNECT_V2 calls. This option provides a possibility to replace the clients password with an au‐
thentication token during the lifetime of the OpenVPN client.
Whenever the connection is renegotiated and the --auth-user-pass-verify script or --plugin making use of the OPEN‐
VPN_PLUGIN_AUTH_USER_PASS_VERIFY hook is triggered, it will pass over this token as the password instead of the pass‐
word the user provided. The authentication token can only be reset by a full reconnect where the server can push new
options to the client. The password the user entered is never preserved once an authentication token have been set.
If the OpenVPN server side rejects the authentication token, the client will receive an AUTH_FAIL and disconnect.
The purpose of this is to enable two factor authentication methods, such as HOTP or TOTP, to be used without needing
to retrieve a new OTP code each time the connection is renegotiated. Another use case is to cache authentication
data on the client without needing to have the users password cached in memory during the life time of the session.
To make use of this feature, the --client-connect script or --plugin needs to put
push "auth-token UNIQUE_TOKEN_VALUE"
into the file/buffer for dynamic configuration data. This will then make the OpenVPN server to push this value to
the client, which replaces the local password with the UNIQUE_TOKEN_VALUE.
Newer clients (2.4.7+) will fall back to the original password method after a failed auth. Older clients will keep
using the token value and react acording to --auth-retry
--tls-verify cmd
Run command cmd to verify the X509 name of a pending TLS connection that has otherwise passed all other tests of cer‐
tification (except for revocation via --crl-verify directive; the revocation test occurs after the --tls-verify
test).
cmd should return 0 to allow the TLS handshake to proceed, or 1 to fail.
cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments
may be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
When cmd is executed two arguments are appended after any arguments specified in cmd , as follows:
cmd certificate_depth subject
These arguments are, respectively, the current certificate depth and the X509 common name (cn) of the peer.
This feature is useful if the peer you want to trust has a certificate which was signed by a certificate authority
who also signed many other certificates, where you don't necessarily want to trust all of them, but rather be selec‐
tive about which peer certificate you will accept. This feature allows you to write a script which will test the
X509 name on a certificate and decide whether or not it should be accepted. For a simple perl script which will test
the common name field on the certificate, see the file verify-cn in the OpenVPN distribution.
See the "Environmental Variables" section below for additional parameters passed as environmental variables.
--tls-export-cert directory
Store the certificates the clients uses upon connection to this directory. This will be done before --tls-verify is
called. The certificates will use a temporary name and will be deleted when the tls-verify script returns. The file
name used for the certificate is available via the peer_cert environment variable.
--x509-username-field [ext:]fieldname
Field in the X.509 certificate subject to be used as the username (default=CN). Typically, this option is specified
with fieldname as either of the following:
--x509-username-field emailAddress
--x509-username-field ext:subjectAltName
The first example uses the value of the "emailAddress" attribute in the certificate's Subject field as the username.
The second example uses the ext: prefix to signify that the X.509 extension fieldname "subjectAltName" be searched
for an rfc822Name (email) field to be used as the username. In cases where there are multiple email addresses in
ext:fieldname, the last occurrence is chosen.
When this option is used, the --verify-x509-name option will match against the chosen fieldname instead of the Common
Name.
Only the subjectAltName and issuerAltName X.509 extensions are supported.
Please note: This option has a feature which will convert an all-lowercase fieldname to uppercase characters, e.g.,
ou -> OU. A mixed-case fieldname or one having the ext: prefix will be left as-is. This automatic upcasing feature
is deprecated and will be removed in a future release.
--verify-x509-name name type
Accept connections only if a host's X.509 name is equal to name. The remote host must also pass all other tests of
verification.
Which X.509 name is compared to name depends on the setting of type. type can be "subject" to match the complete
subject DN (default), "name" to match a subject RDN or "name-prefix" to match a subject RDN prefix. Which RDN is
verified as name depends on the --x509-username-field option. But it defaults to the common name (CN), e.g. a cer‐
tificate with a subject DN "C=KG, ST=NA, L=Bishkek, CN=Server-1" would be matched by:
--verify-x509-name 'C=KG, ST=NA, L=Bishkek, CN=Server-1' and --verify-x509-name Server-1 name or you could use --ver‐
ify-x509-name Server- name-prefix if you want a client to only accept connections to "Server-1", "Server-2", etc.
--verify-x509-name is a useful replacement for the --tls-verify option to verify the remote host, because --ver‐
ify-x509-name works in a --chroot environment without any dependencies.
Using a name prefix is a useful alternative to managing a CRL (Certificate Revocation List) on the client, since it
allows the client to refuse all certificates except for those associated with designated servers.
NOTE: Test against a name prefix only when you are using OpenVPN with a custom CA certificate that is under your con‐
trol. Never use this option with type "name-prefix" when your client certificates are signed by a third party, such
as a commercial web CA.
--x509-track attribute
Save peer X509 attribute value in environment for use by plugins and management interface. Prepend a '+' to attri‐
bute to save values from full cert chain. Values will be encoded as X509_<depth>_<attribute>=<value>. Multiple
--x509-track options can be defined to track multiple attributes.
--ns-cert-type client|server
DEPRECATED This option will be removed in OpenVPN 2.5. Use the more modern equivalent --remote-cert-tls instead.
This option will be removed in OpenVPN 2.5.
Require that peer certificate was signed with an explicit nsCertType designation of "client" or "server".
This is a useful security option for clients, to ensure that the host they connect with is a designated server.
See the easy-rsa/build-key-server script for an example of how to generate a certificate with the nsCertType field
set to "server".
If the server certificate's nsCertType field is set to "server", then the clients can verify this with --ns-cert-type
server.
This is an important security precaution to protect against a man-in-the-middle attack where an authorized client at‐
tempts to connect to another client by impersonating the server. The attack is easily prevented by having clients
verify the server certificate using any one of --ns-cert-type, --verify-x509-name, or --tls-verify.
--remote-cert-ku [v...]
Require that peer certificate was signed with an explicit key usage.
If present in the certificate, the keyUsage value is validated by the TLS library during the TLS handshake. Specify‐
ing this option without arguments requires this extension to be present (so the TLS library will verify it).
If the list v... is also supplied, the keyUsage field must have at least the same bits set as the bits in one of the
values supplied in the list v...
The key usage values in the list must be encoded in hex, e.g. "--remote-cert-ku a0"
--remote-cert-eku oid
Require that peer certificate was signed with an explicit extended key usage.
This is a useful security option for clients, to ensure that the host they connect to is a designated server.
The extended key usage should be encoded in oid notation, or OpenSSL symbolic representation.
--remote-cert-tls client|server
Require that peer certificate was signed with an explicit key usage and extended key usage based on RFC3280 TLS
rules.
This is a useful security option for clients, to ensure that the host they connect to is a designated server. Or the
other way around; for a server to verify that only hosts with a client certificate can connect.
The --remote-cert-tls client option is equivalent to --remote-cert-ku --remote-cert-eku "TLS Web Client Authentica‐
tion"
The --remote-cert-tls server option is equivalent to --remote-cert-ku --remote-cert-eku "TLS Web Server Authentica‐
tion"
This is an important security precaution to protect against a man-in-the-middle attack where an authorized client at‐
tempts to connect to another client by impersonating the server. The attack is easily prevented by having clients
verify the server certificate using any one of --remote-cert-tls, --verify-x509-name, or --tls-verify.
--crl-verify crl ['dir']
Check peer certificate against the file crl in PEM format.
A CRL (certificate revocation list) is used when a particular key is compromised but when the overall PKI is still
intact.
Suppose you had a PKI consisting of a CA, root certificate, and a number of client certificates. Suppose a laptop
computer containing a client key and certificate was stolen. By adding the stolen certificate to the CRL file, you
could reject any connection which attempts to use it, while preserving the overall integrity of the PKI.
The only time when it would be necessary to rebuild the entire PKI from scratch would be if the root certificate key
itself was compromised.
If the optional dir flag is specified, enable a different mode where crl is a directory containing files named as re‐
voked serial numbers (the files may be empty, the contents are never read). If a client requests a connection, where
the client certificate serial number (decimal string) is the name of a file present in the directory, it will be re‐
jected.
Note: As the crl file (or directory) is read every time a peer connects, if you are dropping root privileges with
--user, make sure that this user has sufficient privileges to read the file.
SSL Library information:
--show-ciphers
(Standalone) Show all cipher algorithms to use with the --cipher option.
--show-digests
(Standalone) Show all message digest algorithms to use with the --auth option.
--show-tls
(Standalone) Show all TLS ciphers supported by the crypto library. OpenVPN uses TLS to secure the control channel,
over which the keys that are used to protect the actual VPN traffic are exchanged. The TLS ciphers will be sorted
from highest preference (most secure) to lowest.
Be aware that whether a cipher suite in this list can actually work depends on the specific setup of both peers (e.g.
both peers must support the cipher, and an ECDSA cipher suite will not work if you are using an RSA certificate,
etc.).
--show-engines
(Standalone) Show currently available hardware-based crypto acceleration engines supported by the OpenSSL library.
--show-curves
(Standalone) Show all available elliptic curves to use with the --ecdh-curve option.
Generate a random key:
Used only for non-TLS static key encryption mode.
--genkey
(Standalone) Generate a random key to be used as a shared secret, for use with the --secret option. This file must
be shared with the peer over a pre-existing secure channel such as scp(1)
--secret file
Write key to file.
TUN/TAP persistent tunnel config mode:
Available with Linux 2.4.7+. These options comprise a standalone mode of OpenVPN which can be used to create and delete
persistent tunnels.
--mktun
(Standalone) Create a persistent tunnel on platforms which support them such as Linux. Normally TUN/TAP tunnels ex‐
ist only for the period of time that an application has them open. This option takes advantage of the TUN/TAP
driver's ability to build persistent tunnels that live through multiple instantiations of OpenVPN and die only when
they are deleted or the machine is rebooted.
One of the advantages of persistent tunnels is that they eliminate the need for separate --up and --down scripts to
run the appropriate ifconfig(8) and route(8) commands. These commands can be placed in the the same shell script
which starts or terminates an OpenVPN session.
Another advantage is that open connections through the TUN/TAP-based tunnel will not be reset if the OpenVPN peer
restarts. This can be useful to provide uninterrupted connectivity through the tunnel in the event of a DHCP reset
of the peer's public IP address (see the --ipchange option above).
One disadvantage of persistent tunnels is that it is harder to automatically configure their MTU value (see
--link-mtu and --tun-mtu above).
On some platforms such as Windows, TAP-Win32 tunnels are persistent by default.
--rmtun
(Standalone) Remove a persistent tunnel.
--dev tunX | tapX
TUN/TAP device
--user user
Optional user to be owner of this tunnel.
--group group
Optional group to be owner of this tunnel.
Windows-Specific Options:
--win-sys path
Set the Windows system directory pathname to use when looking for system executables such as route.exe and netsh.exe.
By default, if this directive is not specified, OpenVPN will use the SystemRoot environment variable.
This option have changed behaviour in OpenVPN 2.3. Earlier you had to define --win-sys env to use the SystemRoot en‐
vironment variable, otherwise it defaulted to C:\WINDOWS. It is not needed to use the env keyword any more, and it
will just be ignored. A warning is logged when this is found in the configuration file.
--ip-win32 method
When using --ifconfig on Windows, set the TAP-Win32 adapter IP address and netmask using method. Don't use this op‐
tion unless you are also using --ifconfig.
manual -- Don't set the IP address or netmask automatically. Instead output a message to the console telling the
user to configure the adapter manually and indicating the IP/netmask which OpenVPN expects the adapter to be set to.
dynamic [offset] [lease-time] -- Automatically set the IP address and netmask by replying to DHCP query messages gen‐
erated by the kernel. This mode is probably the "cleanest" solution for setting the TCP/IP properties since it uses
the well-known DHCP protocol. There are, however, two prerequisites for using this mode: (1) The TCP/IP properties
for the TAP-Win32 adapter must be set to "Obtain an IP address automatically," and (2) OpenVPN needs to claim an IP
address in the subnet for use as the virtual DHCP server address. By default in --dev tap mode, OpenVPN will take
the normally unused first address in the subnet. For example, if your subnet is 192.168.4.0 netmask 255.255.255.0,
then OpenVPN will take the IP address 192.168.4.0 to use as the virtual DHCP server address. In --dev tun mode,
OpenVPN will cause the DHCP server to masquerade as if it were coming from the remote endpoint. The optional offset
parameter is an integer which is > -256 and < 256 and which defaults to -1. If offset is positive, the DHCP server
will masquerade as the IP address at network address + offset. If offset is negative, the DHCP server will masquer‐
ade as the IP address at broadcast address + offset. The Windows ipconfig /all command can be used to show what Win‐
dows thinks the DHCP server address is. OpenVPN will "claim" this address, so make sure to use a free address. Hav‐
ing said that, different OpenVPN instantiations, including different ends of the same connection, can share the same
virtual DHCP server address. The lease-time parameter controls the lease time of the DHCP assignment given to the
TAP-Win32 adapter, and is denoted in seconds. Normally a very long lease time is preferred because it prevents
routes involving the TAP-Win32 adapter from being lost when the system goes to sleep. The default lease time is one
year.
netsh -- Automatically set the IP address and netmask using the Windows command-line "netsh" command. This method
appears to work correctly on Windows XP but not Windows 2000.
ipapi -- Automatically set the IP address and netmask using the Windows IP Helper API. This approach does not have
ideal semantics, though testing has indicated that it works okay in practice. If you use this option, it is best to
leave the TCP/IP properties for the TAP-Win32 adapter in their default state, i.e. "Obtain an IP address automati‐
cally."
adaptive -- (Default) Try dynamic method initially and fail over to netsh if the DHCP negotiation with the TAP-Win32
adapter does not succeed in 20 seconds. Such failures have been known to occur when certain third-party firewall
packages installed on the client machine block the DHCP negotiation used by the TAP-Win32 adapter. Note that if the
netsh failover occurs, the TAP-Win32 adapter TCP/IP properties will be reset from DHCP to static, and this will cause
future OpenVPN startups using the adaptive mode to use netsh immediately, rather than trying dynamic first. To "un‐
stick" the adaptive mode from using netsh, run OpenVPN at least once using the dynamic mode to restore the TAP-Win32
adapter TCP/IP properties to a DHCP configuration.
--route-method m
Which method m to use for adding routes on Windows?
adaptive (default) -- Try IP helper API first. If that fails, fall back to the route.exe shell command.
ipapi -- Use IP helper API.
exe -- Call the route.exe shell command.
--dhcp-option type [parm]
Set extended TAP-Win32 TCP/IP properties, must be used with --ip-win32 dynamic or --ip-win32 adaptive. This option
can be used to set additional TCP/IP properties on the TAP-Win32 adapter, and is particularly useful for configuring
an OpenVPN client to access a Samba server across the VPN.
DOMAIN name -- Set Connection-specific DNS Suffix.
DNS addr -- Set primary domain name server IPv4 or IPv6 address. Repeat this option to set secondary DNS server ad‐
dresses.
Note: DNS IPv6 servers are currently set using netsh (the existing DHCP code can only do IPv4 DHCP, and that protocol
only permits IPv4 addresses anywhere). The option will be put into the environment, so an --up script could act upon
it if needed.
WINS addr -- Set primary WINS server address (NetBIOS over TCP/IP Name Server). Repeat this option to set secondary
WINS server addresses.
NBDD addr -- Set primary NBDD server address (NetBIOS over TCP/IP Datagram Distribution Server) Repeat this option to
set secondary NBDD server addresses.
NTP addr -- Set primary NTP server address (Network Time Protocol). Repeat this option to set secondary NTP server
addresses.
NBT type -- Set NetBIOS over TCP/IP Node type. Possible options: 1 = b-node (broadcasts), 2 = p-node (point-to-point
name queries to a WINS server), 4 = m-node (broadcast then query name server), and 8 = h-node (query name server,
then broadcast).
NBS scope-id -- Set NetBIOS over TCP/IP Scope. A NetBIOS Scope ID provides an extended naming service for the NetBIOS
over TCP/IP (Known as NBT) module. The primary purpose of a NetBIOS scope ID is to isolate NetBIOS traffic on a sin‐
gle network to only those nodes with the same NetBIOS scope ID. The NetBIOS scope ID is a character string that is
appended to the NetBIOS name. The NetBIOS scope ID on two hosts must match, or the two hosts will not be able to com‐
municate. The NetBIOS Scope ID also allows computers to use the same computer name, as they have different scope IDs.
The Scope ID becomes a part of the NetBIOS name, making the name unique. (This description of NetBIOS scopes cour‐
tesy of NeonSurge@abyss.com)
DISABLE-NBT -- Disable Netbios-over-TCP/IP.
Note that if --dhcp-option is pushed via --push to a non-windows client, the option will be saved in the client's en‐
vironment before the up script is called, under the name "foreign_option_{n}".
--tap-sleep n
Cause OpenVPN to sleep for n seconds immediately after the TAP-Win32 adapter state is set to "connected".
This option is intended to be used to troubleshoot problems with the --ifconfig and --ip-win32 options, and is used
to give the TAP-Win32 adapter time to come up before Windows IP Helper API operations are applied to it.
--show-net-up
Output OpenVPN's view of the system routing table and network adapter list to the syslog or log file after the
TUN/TAP adapter has been brought up and any routes have been added.
--block-outside-dns
Block DNS servers on other network adapters to prevent DNS leaks. This option prevents any application from accessing
TCP or UDP port 53 except one inside the tunnel. It uses Windows Filtering Platform (WFP) and works on Windows Vista
or later.
This option is considered unknown on non-Windows platforms and unsupported on Windows XP, resulting in fatal error.
You may want to use --setenv opt or --ignore-unknown-option (not suitable for Windows XP) to ignore said error. Note
that pushing unknown options from server does not trigger fatal errors.
--dhcp-renew
Ask Windows to renew the TAP adapter lease on startup. This option is normally unnecessary, as Windows automatically
triggers a DHCP renegotiation on the TAP adapter when it comes up, however if you set the TAP-Win32 adapter Media
Status property to "Always Connected", you may need this flag.
--dhcp-release
Ask Windows to release the TAP adapter lease on shutdown. This option has no effect now, as it is enabled by default
starting with OpenVPN 2.4.1.
--register-dns
Run ipconfig /flushdns and ipconfig /registerdns on connection initiation. This is known to kick Windows into recog‐
nizing pushed DNS servers.
--pause-exit
Put up a "press any key to continue" message on the console prior to OpenVPN program exit. This option is automati‐
cally used by the Windows explorer when OpenVPN is run on a configuration file using the right-click explorer menu.
--service exit-event [0|1]
Should be used when OpenVPN is being automatically executed by another program in such a context that no interaction
with the user via display or keyboard is possible. In general, end-users should never need to explicitly use this
option, as it is automatically added by the OpenVPN service wrapper when a given OpenVPN configuration is being run
as a service.
exit-event is the name of a Windows global event object, and OpenVPN will continuously monitor the state of this
event object and exit when it becomes signaled.
The second parameter indicates the initial state of exit-event and normally defaults to 0.
Multiple OpenVPN processes can be simultaneously executed with the same exit-event parameter. In any case, the con‐
trolling process can signal exit-event, causing all such OpenVPN processes to exit.
When executing an OpenVPN process using the --service directive, OpenVPN will probably not have a console window to
output status/error messages, therefore it is useful to use --log or --log-append to write these messages to a file.
--show-adapters
(Standalone) Show available TAP-Win32 adapters which can be selected using the --dev-node option. On non-Windows
systems, the ifconfig(8) command provides similar functionality.
--allow-nonadmin [TAP-adapter]
(Standalone) Set TAP-adapter to allow access from non-administrative accounts. If TAP-adapter is omitted, all TAP
adapters on the system will be configured to allow non-admin access. The non-admin access setting will only persist
for the length of time that the TAP-Win32 device object and driver remain loaded, and will need to be re-enabled af‐
ter a reboot, or if the driver is unloaded and reloaded. This directive can only be used by an administrator.
--show-valid-subnets
(Standalone) Show valid subnets for --dev tun emulation. Since the TAP-Win32 driver exports an ethernet interface to
Windows, and since TUN devices are point-to-point in nature, it is necessary for the TAP-Win32 driver to impose cer‐
tain constraints on TUN endpoint address selection.
Namely, the point-to-point endpoints used in TUN device emulation must be the middle two addresses of a /30 subnet
(netmask 255.255.255.252).
--show-net
(Standalone) Show OpenVPN's view of the system routing table and network adapter list.
PKCS#11 Standalone Options:
--show-pkcs11-ids [provider] [cert_private]
(Standalone) Show PKCS#11 token object list. Specify cert_private as 1 if certificates are stored as private objects.
If p11-kit is present on the system, the provider argument is optional; if omitted the default p11-kit-proxy.so mod‐
ule will be queried.
--verb option can be used BEFORE this option to produce debugging information.
Standalone Debug Options:
--show-gateway [v6target]
(Standalone) Show current IPv4 and IPv6 default gateway and interface towards the gateway (if the protocol in ques‐
tion is enabled). If an IPv6 address is passed as argument, the IPv6 route for this host is reported.
IPv6 Related Options
The following options exist to support IPv6 tunneling in peer-to-peer and client-server mode. All options are modeled after
their IPv4 counterparts, so more detailed explanations given there apply here as well (except for --topology , which has no
effect on IPv6).
--ifconfig-ipv6 ipv6addr/bits ipv6remote
configure IPv6 address ipv6addr/bits on the ``tun'' device. The second parameter is used as route target for
--route-ipv6 if no gateway is specified.
--route-ipv6 ipv6addr/bits [gateway] [metric]
setup IPv6 routing in the system to send the specified IPv6 network into OpenVPN's ``tun''. The gateway parameter is
only used for IPv6 routes across ``tap'' devices, and if missing, the ``ipv6remote'' field from --ifconfig-ipv6 is
used.
--server-ipv6 ipv6addr/bits
convenience-function to enable a number of IPv6 related options at once, namely --ifconfig-ipv6, --ifconfig-ipv6-pool
and --push tun-ipv6 Is only accepted if ``--mode server'' or ``--server'' is set. Pushing of the --tun-ipv6 directive
is done for older clients which require an explicit ``--tun-ipv6'' in their configuration.
--ifconfig-ipv6-pool ipv6addr/bits
Specify an IPv6 address pool for dynamic assignment to clients. The pool starts at ipv6addr and matches the offset
determined from the start of the IPv4 pool.
--ifconfig-ipv6-push ipv6addr/bits ipv6remote
for ccd/ per-client static IPv6 interface configuration, see --client-config-dir and --ifconfig-push for more de‐
tails.
--iroute-ipv6 ipv6addr/bits
for ccd/ per-client static IPv6 route configuration, see --iroute for more details how to setup and use this, and how
--iroute and --route interact.
SCRIPTING AND ENVIRONMENTAL VARIABLES
OpenVPN exports a series of environmental variables for use by user-defined scripts.
Script Order of Execution
--up Executed after TCP/UDP socket bind and TUN/TAP open.
--tls-verify
Executed when we have a still untrusted remote peer.
--ipchange
Executed after connection authentication, or remote IP address change.
--client-connect
Executed in --mode server mode immediately after client authentication.
--route-up
Executed after connection authentication, either immediately after, or some number of seconds after as defined by the
--route-delay option.
--route-pre-down
Executed right before the routes are removed.
--client-disconnect
Executed in --mode server mode on client instance shutdown.
--down Executed after TCP/UDP and TUN/TAP close.
--learn-address
Executed in --mode server mode whenever an IPv4 address/route or MAC address is added to OpenVPN's internal routing
table.
--auth-user-pass-verify
Executed in --mode server mode on new client connections, when the client is still untrusted.
String Types and Remapping
In certain cases, OpenVPN will perform remapping of characters in strings. Essentially, any characters outside the set of
permitted characters for each string type will be converted to underbar ('_').
Q: Why is string remapping necessary?
A: It's an important security feature to prevent the malicious coding of strings from untrusted sources to be passed as pa‐
rameters to scripts, saved in the environment, used as a common name, translated to a filename, etc.
Q: Can string remapping be disabled?
A: Yes, by using the --no-name-remapping option, however this should be considered an advanced option.
Here is a brief rundown of OpenVPN's current string types and the permitted character class for each string:
X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at ('@'), colon (':'), slash ('/'), and equal ('='). Al‐
phanumeric is defined as a character which will cause the C library isalnum() function to return true.
Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at ('@').
--auth-user-pass username: Same as Common Name, with one exception: starting with OpenVPN 2.0.1, the username is passed to
the OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw form, without string remapping.
--auth-user-pass password: Any "printable" character except CR or LF. Printable is defined to be a character which will
cause the C library isprint() function to return true.
--client-config-dir filename as derived from common name or username: Alphanumeric, underbar ('_'), dash ('-'), and dot
('.') except for "." or ".." as standalone strings. As of v2.0.1-rc6, the at ('@') character has been added as well for
compatibility with the common name character class.
Environmental variable names: Alphanumeric or underbar ('_').
Environmental variable values: Any printable character.
For all cases, characters in a string which are not members of the legal character class for that string type will be
remapped to underbar ('_').
Environmental Variables
Once set, a variable is persisted indefinitely until it is reset by a new value or a restart,
As of OpenVPN 2.0-beta12, in server mode, environmental variables set by OpenVPN are scoped according to the client objects
they are associated with, so there should not be any issues with scripts having access to stale, previously set variables
which refer to different client instances.
bytes_received
Total number of bytes received from client during VPN session. Set prior to execution of the --client-disconnect
script.
bytes_sent
Total number of bytes sent to client during VPN session. Set prior to execution of the --client-disconnect script.
common_name
The X509 common name of an authenticated client. Set prior to execution of --client-connect, --client-disconnect,
and --auth-user-pass-verify scripts.
config Name of first --config file. Set on program initiation and reset on SIGHUP.
daemon Set to "1" if the --daemon directive is specified, or "0" otherwise. Set on program initiation and reset on SIGHUP.
daemon_log_redirect
Set to "1" if the --log or --log-append directives are specified, or "0" otherwise. Set on program initiation and
reset on SIGHUP.
dev The actual name of the TUN/TAP device, including a unit number if it exists. Set prior to --up or --down script exe‐
cution.
dev_idx
On Windows, the device index of the TUN/TAP adapter (to be used in netsh.exe calls which sometimes just do not work
right with interface names). Set prior to --up or --down script execution.
foreign_option_{n}
An option pushed via --push to a client which does not natively support it, such as --dhcp-option on a non-Windows
system, will be recorded to this environmental variable sequence prior to --up script execution.
ifconfig_broadcast
The broadcast address for the virtual ethernet segment which is derived from the --ifconfig option when --dev tap is
used. Set prior to OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which normally oc‐
curs prior to --up script execution.
ifconfig_ipv6_local
The local VPN endpoint IPv6 address specified in the --ifconfig-ipv6 option (first parameter). Set prior to OpenVPN
calling the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to --up script exe‐
cution.
ifconfig_ipv6_netbits
The prefix length of the IPv6 network on the VPN interface. Derived from the /nnn parameter of the IPv6 address in
the --ifconfig-ipv6 option (first parameter). Set prior to OpenVPN calling the ifconfig or netsh (windows version of
ifconfig) commands which normally occurs prior to --up script execution.
ifconfig_ipv6_remote
The remote VPN endpoint IPv6 address specified in the --ifconfig-ipv6 option (second parameter). Set prior to Open‐
VPN calling the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to --up script
execution.
ifconfig_local
The local VPN endpoint IP address specified in the --ifconfig option (first parameter). Set prior to OpenVPN calling
the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to --up script execution.
ifconfig_remote
The remote VPN endpoint IP address specified in the --ifconfig option (second parameter) when --dev tun is used. Set
prior to OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to
--up script execution.
ifconfig_netmask
The subnet mask of the virtual ethernet segment that is specified as the second parameter to --ifconfig when --dev
tap is being used. Set prior to OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which
normally occurs prior to --up script execution.
ifconfig_pool_local_ip
The local virtual IP address for the TUN/TAP tunnel taken from an --ifconfig-push directive if specified, or other‐
wise from the ifconfig pool (controlled by the --ifconfig-pool config file directive). Only set for --dev tun tun‐
nels. This option is set on the server prior to execution of the --client-connect and --client-disconnect scripts.
ifconfig_pool_netmask
The virtual IP netmask for the TUN/TAP tunnel taken from an --ifconfig-push directive if specified, or otherwise from
the ifconfig pool (controlled by the --ifconfig-pool config file directive). Only set for --dev tap tunnels. This
option is set on the server prior to execution of the --client-connect and --client-disconnect scripts.
ifconfig_pool_remote_ip
The remote virtual IP address for the TUN/TAP tunnel taken from an --ifconfig-push directive if specified, or other‐
wise from the ifconfig pool (controlled by the --ifconfig-pool config file directive). This option is set on the
server prior to execution of the --client-connect and --client-disconnect scripts.
link_mtu
The maximum packet size (not including the IP header) of tunnel data in UDP tunnel transport mode. Set prior to --up
or --down script execution.
local The --local parameter. Set on program initiation and reset on SIGHUP.
local_port
The local port number or name, specified by --port or --lport. Set on program initiation and reset on SIGHUP.
password
The password provided by a connecting client. Set prior to --auth-user-pass-verify script execution only when the
via-env modifier is specified, and deleted from the environment after the script returns.
proto The --proto parameter. Set on program initiation and reset on SIGHUP.
remote_{n}
The --remote parameter. Set on program initiation and reset on SIGHUP.
remote_port_{n}
The remote port number, specified by --port or --rport. Set on program initiation and reset on SIGHUP.
route_net_gateway
The pre-existing default IP gateway in the system routing table. Set prior to --up script execution.
route_vpn_gateway
The default gateway used by --route options, as specified in either the --route-gateway option or the second parame‐
ter to --ifconfig when --dev tun is specified. Set prior to --up script execution.
route_{parm}_{n}
A set of variables which define each route to be added, and are set prior to --up script execution.
parm will be one of "network", "netmask", "gateway", or "metric".
n is the OpenVPN route number, starting from 1.
If the network or gateway are resolvable DNS names, their IP address translations will be recorded rather than their
names as denoted on the command line or configuration file.
route_ipv6_{parm}_{n}
A set of variables which define each IPv6 route to be added, and are set prior to --up script execution.
parm will be one of "network" or "gateway" ("netmask" is contained as "/nnn" in the route_ipv6_network_{n}, unlike
IPv4 where it is passed in a separate environment variable).
n is the OpenVPN route number, starting from 1.
If the network or gateway are resolvable DNS names, their IP address translations will be recorded rather than their
names as denoted on the command line or configuration file.
peer_cert
Temporary file name containing the client certificate upon connection. Useful in conjunction with --tls-verify
script_context
Set to "init" or "restart" prior to up/down script execution. For more information, see documentation for --up.
script_type
Prior to execution of any script, this variable is set to the type of script being run. It can be one of the follow‐
ing: up, down, ipchange, route-up, tls-verify, auth-user-pass-verify, client-connect, client-disconnect, or learn-ad‐
dress. Set prior to execution of any script.
signal The reason for exit or restart. Can be one of sigusr1, sighup, sigterm, sigint, inactive (controlled by --inactive
option), ping-exit (controlled by --ping-exit option), ping-restart (controlled by --ping-restart option), connec‐
tion-reset (triggered on TCP connection reset), error, or unknown (unknown signal). This variable is set just prior
to down script execution.
time_ascii
Client connection timestamp, formatted as a human-readable time string. Set prior to execution of the --client-con‐
nect script.
time_duration
The duration (in seconds) of the client session which is now disconnecting. Set prior to execution of the
--client-disconnect script.
time_unix
Client connection timestamp, formatted as a unix integer date/time value. Set prior to execution of the
--client-connect script.
tls_digest_{n} / tls_digest_sha256_{n}
Contains the certificate SHA1 / SHA256 fingerprint, where n is the verification level. Only set for TLS connections.
Set prior to execution of --tls-verify script.
tls_id_{n}
A series of certificate fields from the remote peer, where n is the verification level. Only set for TLS connec‐
tions. Set prior to execution of --tls-verify script.
tls_serial_{n}
The serial number of the certificate from the remote peer, where n is the verification level. Only set for TLS con‐
nections. Set prior to execution of --tls-verify script. This is in the form of a decimal string like "933971680",
which is suitable for doing serial-based OCSP queries (with OpenSSL, do not prepend "0x" to the string) If something
goes wrong while reading the value from the certificate it will be an empty string, so your code should check that.
See the contrib/OCSP_check/OCSP_check.sh script for an example.
tls_serial_hex_{n}
Like tls_serial_{n}, but in hex form (e.g. "12:34:56:78:9A").
tun_mtu
The MTU of the TUN/TAP device. Set prior to --up or --down script execution.
trusted_ip (or trusted_ip6)
Actual IP address of connecting client or peer which has been authenticated. Set prior to execution of --ipchange,
--client-connect, and --client-disconnect scripts. If using ipv6 endpoints (udp6, tcp6), trusted_ip6 will be set in‐
stead.
trusted_port
Actual port number of connecting client or peer which has been authenticated. Set prior to execution of --ipchange,
--client-connect, and --client-disconnect scripts.
untrusted_ip (or untrusted_ip6)
Actual IP address of connecting client or peer which has not been authenticated yet. Sometimes used to nmap the con‐
necting host in a --tls-verify script to ensure it is firewalled properly. Set prior to execution of --tls-verify
and --auth-user-pass-verify scripts. If using ipv6 endpoints (udp6, tcp6), untrusted_ip6 will be set instead.
untrusted_port
Actual port number of connecting client or peer which has not been authenticated yet. Set prior to execution of
--tls-verify and --auth-user-pass-verify scripts.
username
The username provided by a connecting client. Set prior to --auth-user-pass-verify script execution only when the
via-env modifier is specified.
X509_{n}_{subject_field}
An X509 subject field from the remote peer certificate, where n is the verification level. Only set for TLS connec‐
tions. Set prior to execution of --tls-verify script. This variable is similar to tls_id_{n} except the component
X509 subject fields are broken out, and no string remapping occurs on these field values (except for remapping of
control characters to "_"). For example, the following variables would be set on the OpenVPN server using the sample
client certificate in sample-keys (client.crt). Note that the verification level is 0 for the client certificate and
1 for the CA certificate.
X509_0_emailAddress=me@myhost.mydomain
X509_0_CN=Test-Client
X509_0_O=OpenVPN-TEST
X509_0_ST=NA
X509_0_C=KG
X509_1_emailAddress=me@myhost.mydomain
X509_1_O=OpenVPN-TEST
X509_1_L=BISHKEK
X509_1_ST=NA
X509_1_C=KG
INLINE FILE SUPPORT
OpenVPN allows including files in the main configuration for the --ca, --cert, --dh, --extra-certs, --key, --pkcs12, --se‐
cret, --crl-verify, --http-proxy-user-pass, --tls-auth and --tls-crypt options.
Each inline file started by the line <option> and ended by the line </option>
Here is an example of an inline file usage
<cert>
-----BEGIN CERTIFICATE-----
[...]
-----END CERTIFICATE-----
</cert>
When using the inline file feature with --pkcs12 the inline file has to be base64 encoded. Encoding of a .p12 file into
base64 can be done for example with OpenSSL by running openssl base64 -in input.p12
SIGNALS
SIGHUP Cause OpenVPN to close all TUN/TAP and network connections, restart, re-read the configuration file (if any), and re‐
open TUN/TAP and network connections.
SIGUSR1
Like SIGHUP, except don't re-read configuration file, and possibly don't close and reopen TUN/TAP device, re-read key
files, preserve local IP address/port, or preserve most recently authenticated remote IP address/port based on --per‐
sist-tun, --persist-key, --persist-local-ip, and --persist-remote-ip options respectively (see above).
This signal may also be internally generated by a timeout condition, governed by the --ping-restart option.
This signal, when combined with --persist-remote-ip, may be sent when the underlying parameters of the host's network
interface change such as when the host is a DHCP client and is assigned a new IP address. See --ipchange above for
more information.
SIGUSR2
Causes OpenVPN to display its current statistics (to the syslog file if --daemon is used, or stdout otherwise).
SIGINT, SIGTERM
Causes OpenVPN to exit gracefully.
TUN/TAP DRIVER SETUP
If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP driver already installed. If so, there are still a
few things you need to do:
Make device: mknod /dev/net/tun c 10 200
Load driver: modprobe tun
EXAMPLES
Prior to running these examples, you should have OpenVPN installed on two machines with network connectivity between them.
If you have not yet installed OpenVPN, consult the INSTALL file included in the OpenVPN distribution.
TUN/TAP Setup:
If you are using Linux 2.4 or higher, make the tun device node and load the tun module:
mknod /dev/net/tun c 10 200
modprobe tun
If you installed from RPM, the mknod step may be omitted, because the RPM install does that for you.
Only Linux 2.4 and newer are supported.
For other platforms, consult the INSTALL file at http://openvpn.net/install.html for more information.
Firewall Setup:
If firewalls exist between the two machines, they should be set to forward UDP port 1194 in both directions. If you do not
have control over the firewalls between the two machines, you may still be able to use OpenVPN by adding --ping 15 to each
of the openvpn commands used below in the examples (this will cause each peer to send out a UDP ping to its remote peer once
every 15 seconds which will cause many stateful firewalls to forward packets in both directions without an explicit firewall
rule).
If you are using a Linux iptables-based firewall, you may need to enter the following command to allow incoming packets on
the TUN device:
iptables -A INPUT -i tun+ -j ACCEPT
See the firewalls section below for more information on configuring firewalls for use with OpenVPN.
VPN Address Setup:
For purposes of our example, our two machines will be called bob.example.com and alice.example.com. If you are constructing
a VPN over the internet, then replace bob.example.com and alice.example.com with the internet hostname or IP address that
each machine will use to contact the other over the internet.
Now we will choose the tunnel endpoints. Tunnel endpoints are private IP addresses that only have meaning in the context of
the VPN. Each machine will use the tunnel endpoint of the other machine to access it over the VPN. In our example, the
tunnel endpoint for bob.example.com will be 10.4.0.1 and for alice.example.com, 10.4.0.2.
Once the VPN is established, you have essentially created a secure alternate path between the two hosts which is addressed
by using the tunnel endpoints. You can control which network traffic passes between the hosts (a) over the VPN or (b) inde‐
pendently of the VPN, by choosing whether to use (a) the VPN endpoint address or (b) the public internet address, to access
the remote host. For example if you are on bob.example.com and you wish to connect to alice.example.com via ssh without us‐
ing the VPN (since ssh has its own built-in security) you would use the command ssh alice.example.com. However in the same
scenario, you could also use the command telnet 10.4.0.2 to create a telnet session with alice.example.com over the VPN,
that would use the VPN to secure the session rather than ssh.
You can use any address you wish for the tunnel endpoints but make sure that they are private addresses (such as those that
begin with 10 or 192.168) and that they are not part of any existing subnet on the networks of either peer, unless you are
bridging. If you use an address that is part of your local subnet for either of the tunnel endpoints, you will get a weird
feedback loop.
Example 1: A simple tunnel without security
On bob:
openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 9
On alice:
openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 9
Now verify the tunnel is working by pinging across the tunnel.
On bob:
ping 10.4.0.2
On alice:
ping 10.4.0.1
The --verb 9 option will produce verbose output, similar to the tcpdump(8) program. Omit the --verb 9 option to have Open‐
VPN run quietly.
Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)
First build a static key on bob.
openvpn --genkey --secret key
This command will build a random key file called key (in ascii format). Now copy key to alice over a secure medium such as
by using the scp(1) program.
On bob:
openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 5 --secret key
On alice:
openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 5 --secret key
Now verify the tunnel is working by pinging across the tunnel.
On bob:
ping 10.4.0.2
On alice:
ping 10.4.0.1
Example 3: A tunnel with full TLS-based security
For this test, we will designate bob as the TLS client and alice as the TLS server. Note that client or server designation
only has meaning for the TLS subsystem. It has no bearing on OpenVPN's peer-to-peer, UDP-based communication model.
First, build a separate certificate/key pair for both bob and alice (see above where --cert is discussed for more info).
Then construct Diffie Hellman parameters (see above where --dh is discussed for more info). You can also use the included
test files client.crt, client.key, server.crt, server.key and ca.crt. The .crt files are certificates/public-keys, the .key
files are private keys, and ca.crt is a certification authority who has signed both client.crt and server.crt. For Diffie
Hellman parameters you can use the included file dh1024.pem. Note that all client, server, and certificate authority cer‐
tificates and keys included in the OpenVPN distribution are totally insecure and should be used for testing only.
On bob:
openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --tls-client --ca ca.crt --cert client.crt
--key client.key --reneg-sec 60 --verb 5
On alice:
openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --tls-server --dh dh1024.pem --ca ca.crt
--cert server.crt --key server.key --reneg-sec 60 --verb 5
Now verify the tunnel is working by pinging across the tunnel.
On bob:
ping 10.4.0.2
On alice:
ping 10.4.0.1
Notice the --reneg-sec 60 option we used above. That tells OpenVPN to renegotiate the data channel keys every minute.
Since we used --verb 5 above, you will see status information on each new key negotiation.
For production operations, a key renegotiation interval of 60 seconds is probably too frequent. Omit the --reneg-sec 60 op‐
tion to use OpenVPN's default key renegotiation interval of one hour.
Routing:
Assuming you can ping across the tunnel, the next step is to route a real subnet over the secure tunnel. Suppose that bob
and alice have two network interfaces each, one connected to the internet, and the other to a private network. Our goal is
to securely connect both private networks. We will assume that bob's private subnet is 10.0.0.0/24 and alice's is
10.0.1.0/24.
First, ensure that IP forwarding is enabled on both peers. On Linux, enable routing:
echo 1 > /proc/sys/net/ipv4/ip_forward
and enable TUN packet forwarding through the firewall:
iptables -A FORWARD -i tun+ -j ACCEPT
On bob:
route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2
On alice:
route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1
Now any machine on the 10.0.0.0/24 subnet can access any machine on the 10.0.1.0/24 subnet over the secure tunnel (or vice
versa).
In a production environment, you could put the route command(s) in a script and execute with the --up option.
FIREWALLS
OpenVPN's usage of a single UDP port makes it fairly firewall-friendly. You should add an entry to your firewall rules to
allow incoming OpenVPN packets. On Linux 2.4+:
iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT
This will allow incoming packets on UDP port 1194 (OpenVPN's default UDP port) from an OpenVPN peer at 1.2.3.4.
If you are using HMAC-based packet authentication (the default in any of OpenVPN's secure modes), having the firewall filter
on source address can be considered optional, since HMAC packet authentication is a much more secure method of verifying the
authenticity of a packet source. In that case:
iptables -A INPUT -p udp --dport 1194 -j ACCEPT
would be adequate and would not render the host inflexible with respect to its peer having a dynamic IP address.
OpenVPN also works well on stateful firewalls. In some cases, you may not need to add any static rules to the firewall list
if you are using a stateful firewall that knows how to track UDP connections. If you specify --ping n, OpenVPN will be
guaranteed to send a packet to its peer at least once every n seconds. If n is less than the stateful firewall connection
timeout, you can maintain an OpenVPN connection indefinitely without explicit firewall rules.
You should also add firewall rules to allow incoming IP traffic on TUN or TAP devices such as:
iptables -A INPUT -i tun+ -j ACCEPT
to allow input packets from tun devices,
iptables -A FORWARD -i tun+ -j ACCEPT
to allow input packets from tun devices to be forwarded to other hosts on the local network,
iptables -A INPUT -i tap+ -j ACCEPT
to allow input packets from tap devices, and
iptables -A FORWARD -i tap+ -j ACCEPT
to allow input packets from tap devices to be forwarded to other hosts on the local network.
These rules are secure if you use packet authentication, since no incoming packets will arrive on a TUN or TAP virtual de‐
vice unless they first pass an HMAC authentication test.
FAQ
http://openvpn.net/faq.html
HOWTO
For a more comprehensive guide to setting up OpenVPN in a production setting, see the OpenVPN HOWTO at http://open‐
vpn.net/howto.html
PROTOCOL
For a description of OpenVPN's underlying protocol, see http://openvpn.net/security.html
WEB
OpenVPN's web site is at http://openvpn.net/
Go here to download the latest version of OpenVPN, subscribe to the mailing lists, read the mailing list archives, or browse
the SVN repository.
BUGS
Report all bugs to the OpenVPN team <info@openvpn.net>.
SEE ALSO
dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)
NOTES
This product includes software developed by the OpenSSL Project ( http://www.openssl.org/ )
For more information on the TLS protocol, see http://www.ietf.org/rfc/rfc2246.txt
For more information on the LZO real-time compression library see http://www.oberhumer.com/opensource/lzo/
COPYRIGHT
Copyright (C) 2002-2018 OpenVPN Inc This program is free software; you can redistribute it and/or modify it under the terms
of the GNU General Public License version 2 as published by the Free Software Foundation.
AUTHORS
James Yonan <jim@yonan.net>
28 February 2018 openvpn(8)
Help output
sudo openvpn --helpOpenVPN 2.4.7 x86_64-pc-linux-gnu [SSL (OpenSSL)] [LZO] [LZ4] [EPOLL] [PKCS11] [MH/PKTINFO] [AEAD] built on Apr 28 2021
General Options:
--config file : Read configuration options from file.
--help : Show options.
--version : Show copyright and version information.
Tunnel Options:
--local host : Local host name or ip address. Implies --bind.
--remote host [port] : Remote host name or ip address.
--remote-random : If multiple --remote options specified, choose one randomly.
--remote-random-hostname : Add a random string to remote DNS name.
--mode m : Major mode, m = 'p2p' (default, point-to-point) or 'server'.
--proto p : Use protocol p for communicating with peer.
p = udp (default), tcp-server, or tcp-client
--proto-force p : only consider protocol p in list of connection profiles.
p = udp6, tcp6-server, or tcp6-client (ipv6)
--connect-retry n [m] : For client, number of seconds to wait between
connection retries (default=5). On repeated retries
the wait time is exponentially increased to a maximum of m
(default=300).
--connect-retry-max n : Maximum connection attempt retries, default infinite.
--http-proxy s p [up] [auth] : Connect to remote host
through an HTTP proxy at address s and port p.
If proxy authentication is required,
up is a file containing username/password on 2 lines, or
'stdin' to prompt from console. Add auth='ntlm' if
the proxy requires NTLM authentication.
--http-proxy s p 'auto[-nct]' : Like the above directive, but automatically
determine auth method and query for username/password
if needed. auto-nct disables weak proxy auth methods.
--http-proxy-option type [parm] : Set extended HTTP proxy options.
Repeat to set multiple options.
VERSION version (default=1.0)
AGENT user-agent
--socks-proxy s [p] [up] : Connect to remote host through a Socks5 proxy at
address s and port p (default port = 1080).
If proxy authentication is required,
up is a file containing username/password on 2 lines, or
'stdin' to prompt for console.
--socks-proxy-retry : Retry indefinitely on Socks proxy errors.
--resolv-retry n: If hostname resolve fails for --remote, retry
resolve for n seconds before failing (disabled by default).
Set n="infinite" to retry indefinitely.
--float : Allow remote to change its IP address/port, such as through
DHCP (this is the default if --remote is not used).
--ipchange cmd : Run command cmd on remote ip address initial
setting or change -- execute as: cmd ip-address port#
--port port : TCP/UDP port # for both local and remote.
--lport port : TCP/UDP port # for local (default=1194). Implies --bind.
--rport port : TCP/UDP port # for remote (default=1194).
--bind : Bind to local address and port. (This is the default unless
--proto tcp-client or --http-proxy or --socks-proxy is used).
--nobind : Do not bind to local address and port.
--dev tunX|tapX : tun/tap device (X can be omitted for dynamic device.
--dev-type dt : Which device type are we using? (dt = tun or tap) Use
this option only if the tun/tap device used with --dev
does not begin with "tun" or "tap".
--dev-node node : Explicitly set the device node rather than using
/dev/net/tun, /dev/tun, /dev/tap, etc.
--lladdr hw : Set the link layer address of the tap device.
--topology t : Set --dev tun topology: 'net30', 'p2p', or 'subnet'.
--iproute cmd : Use this command instead of default /sbin/ip.
--ifconfig l rn : TUN: configure device to use IP address l as a local
endpoint and rn as a remote endpoint. l & rn should be
swapped on the other peer. l & rn must be private
addresses outside of the subnets used by either peer.
TAP: configure device to use IP address l as a local
endpoint and rn as a subnet mask.
--ifconfig-ipv6 l r : configure device to use IPv6 address l as local
endpoint (as a /64) and r as remote endpoint
--ifconfig-noexec : Don't actually execute ifconfig/netsh command, instead
pass --ifconfig parms by environment to scripts.
--ifconfig-nowarn : Don't warn if the --ifconfig option on this side of the
connection doesn't match the remote side.
--route network [netmask] [gateway] [metric] :
Add route to routing table after connection
is established. Multiple routes can be specified.
netmask default: 255.255.255.255
gateway default: taken from --route-gateway or --ifconfig
Specify default by leaving blank or setting to "default".
--route-ipv6 network/bits [gateway] [metric] :
Add IPv6 route to routing table after connection
is established. Multiple routes can be specified.
gateway default: taken from 'remote' in --ifconfig-ipv6
--route-gateway gw|'dhcp' : Specify a default gateway for use with --route.
--route-metric m : Specify a default metric for use with --route.
--route-delay n [w] : Delay n seconds after connection initiation before
adding routes (may be 0). If not specified, routes will
be added immediately after tun/tap open. On Windows, wait
up to w seconds for TUN/TAP adapter to come up.
--route-up cmd : Run command cmd after routes are added.
--route-pre-down cmd : Run command cmd before routes are removed.
--route-noexec : Don't add routes automatically. Instead pass routes to
--route-up script using environmental variables.
--route-nopull : When used with --client or --pull, accept options pushed
by server EXCEPT for routes and dhcp options.
--allow-pull-fqdn : Allow client to pull DNS names from server for
--ifconfig, --route, and --route-gateway.
--redirect-gateway [flags]: Automatically execute routing
commands to redirect all outgoing IP traffic through the
VPN. Add 'local' flag if both OpenVPN servers are directly
connected via a common subnet, such as with WiFi.
Add 'def1' flag to set default route using using 0.0.0.0/1
and 128.0.0.0/1 rather than 0.0.0.0/0. Add 'bypass-dhcp'
flag to add a direct route to DHCP server, bypassing tunnel.
Add 'bypass-dns' flag to similarly bypass tunnel for DNS.
--redirect-private [flags]: Like --redirect-gateway, but omit actually changing
the default gateway. Useful when pushing private subnets.
--client-nat snat|dnat network netmask alias : on client add 1-to-1 NAT rule.
--push-peer-info : (client only) push client info to server.
--setenv name value : Set a custom environmental variable to pass to script.
--setenv FORWARD_COMPATIBLE 1 : Relax config file syntax checking to allow
directives for future OpenVPN versions to be ignored.
--ignore-unkown-option opt1 opt2 ...: Relax config file syntax. Allow
these options to be ignored when unknown
--script-security level: Where level can be:
0 -- strictly no calling of external programs
1 -- (default) only call built-ins such as ifconfig
2 -- allow calling of built-ins and scripts
3 -- allow password to be passed to scripts via env
--shaper n : Restrict output to peer to n bytes per second.
--keepalive n m : Helper option for setting timeouts in server mode. Send
ping once every n seconds, restart if ping not received
for m seconds.
--inactive n [bytes] : Exit after n seconds of activity on tun/tap device
produces a combined in/out byte count < bytes.
--ping-exit n : Exit if n seconds pass without reception of remote ping.
--ping-restart n: Restart if n seconds pass without reception of remote ping.
--ping-timer-rem: Run the --ping-exit/--ping-restart timer only if we have a
remote address.
--ping n : Ping remote once every n seconds over TCP/UDP port.
--multihome : Configure a multi-homed UDP server.
--fast-io : (experimental) Optimize TUN/TAP/UDP writes.
--remap-usr1 s : On SIGUSR1 signals, remap signal (s='SIGHUP' or 'SIGTERM').
--persist-tun : Keep tun/tap device open across SIGUSR1 or --ping-restart.
--persist-remote-ip : Keep remote IP address across SIGUSR1 or --ping-restart.
--persist-local-ip : Keep local IP address across SIGUSR1 or --ping-restart.
--persist-key : Don't re-read key files across SIGUSR1 or --ping-restart.
--passtos : TOS passthrough (applies to IPv4 only).
--tun-mtu n : Take the tun/tap device MTU to be n and derive the
TCP/UDP MTU from it (default=1500).
--tun-mtu-extra n : Assume that tun/tap device might return as many
as n bytes more than the tun-mtu size on read
(default TUN=0 TAP=32).
--link-mtu n : Take the TCP/UDP device MTU to be n and derive the tun MTU
from it.
--mtu-disc type : Should we do Path MTU discovery on TCP/UDP channel?
'no' -- Never send DF (Don't Fragment) frames
'maybe' -- Use per-route hints
'yes' -- Always DF (Don't Fragment)
--mtu-test : Empirically measure and report MTU.
--fragment max : Enable internal datagram fragmentation so that no UDP
datagrams are sent which are larger than max bytes.
Adds 4 bytes of overhead per datagram.
--mssfix [n] : Set upper bound on TCP MSS, default = tun-mtu size
or --fragment max value, whichever is lower.
--sndbuf size : Set the TCP/UDP send buffer size.
--rcvbuf size : Set the TCP/UDP receive buffer size.
--mark value : Mark encrypted packets being sent with value. The mark value
can be matched in policy routing and packetfilter rules.
--txqueuelen n : Set the tun/tap TX queue length to n (Linux only).
--memstats file : Write live usage stats to memory mapped binary file.
--mlock : Disable Paging -- ensures key material and tunnel
data will never be written to disk.
--up cmd : Run command cmd after successful tun device open.
Execute as: cmd tun/tap-dev tun-mtu link-mtu \
ifconfig-local-ip ifconfig-remote-ip
(pre --user or --group UID/GID change)
--up-delay : Delay tun/tap open and possible --up script execution
until after TCP/UDP connection establishment with peer.
--down cmd : Run command cmd after tun device close.
(post --user/--group UID/GID change and/or --chroot)
(command parameters are same as --up option)
--down-pre : Run --down command before TUN/TAP close.
--up-restart : Run up/down commands for all restarts including those
caused by --ping-restart or SIGUSR1
--user user : Set UID to user after initialization.
--group group : Set GID to group after initialization.
--chroot dir : Chroot to this directory after initialization.
--cd dir : Change to this directory before initialization.
--daemon [name] : Become a daemon after initialization.
The optional 'name' parameter will be passed
as the program name to the system logger.
--syslog [name] : Output to syslog, but do not become a daemon.
See --daemon above for a description of the 'name' parm.
--inetd [name] ['wait'|'nowait'] : Run as an inetd or xinetd server.
See --daemon above for a description of the 'name' parm.
--log file : Output log to file which is created/truncated on open.
--log-append file : Append log to file, or create file if nonexistent.
--suppress-timestamps : Don't log timestamps to stdout/stderr.
--machine-readable-output : Always log timestamp, message flags to stdout/stderr.
--writepid file : Write main process ID to file.
--nice n : Change process priority (>0 = lower, <0 = higher).
--echo [parms ...] : Echo parameters to log output.
--verb n : Set output verbosity to n (default=1):
(Level 3 is recommended if you want a good summary
of what's happening without being swamped by output).
: 0 -- no output except fatal errors
: 1 -- startup info + connection initiated messages +
non-fatal encryption & net errors
: 2,3 -- show TLS negotiations & route info
: 4 -- show parameters
: 5 -- show 'RrWw' chars on console for each packet sent
and received from TCP/UDP (caps) or tun/tap (lc)
: 6 to 11 -- debug messages of increasing verbosity
--mute n : Log at most n consecutive messages in the same category.
--status file n : Write operational status to file every n seconds.
--status-version [n] : Choose the status file format version number.
Currently, n can be 1, 2, or 3 (default=1).
--disable-occ : Disable options consistency check between peers.
--gremlin mask : Special stress testing mode (for debugging only).
--compress alg : Use compression algorithm alg
--comp-lzo : Use LZO compression -- may add up to 1 byte per
packet for uncompressible data.
--comp-noadapt : Don't use adaptive compression when --comp-lzo
is specified.
--management ip port [pass] : Enable a TCP server on ip:port to handle
management functions. pass is a password file
or 'stdin' to prompt from console.
To listen on a unix domain socket, specific the pathname
in place of ip and use 'unix' as the port number.
--management-client : Management interface will connect as a TCP client to
ip/port rather than listen as a TCP server.
--management-query-passwords : Query management channel for private key
and auth-user-pass passwords.
--management-query-proxy : Query management channel for proxy information.
--management-query-remote : Query management channel for --remote directive.
--management-hold : Start OpenVPN in a hibernating state, until a client
of the management interface explicitly starts it.
--management-signal : Issue SIGUSR1 when management disconnect event occurs.
--management-forget-disconnect : Forget passwords when management disconnect
event occurs.
--management-up-down : Report tunnel up/down events to management interface.
--management-log-cache n : Cache n lines of log file history for usage
by the management channel.
--management-client-user u : When management interface is a unix socket, only
allow connections from user u.
--management-client-group g : When management interface is a unix socket, only
allow connections from group g.
--management-client-auth : gives management interface client the responsibility
to authenticate clients after their client certificate
has been verified.
--management-client-pf : management interface clients must specify a packet
filter file for each connecting client.
--plugin m [str]: Load plug-in module m passing str as an argument
to its initialization function.
Multi-Client Server options (when --mode server is used):
--server network netmask : Helper option to easily configure server mode.
--server-ipv6 network/bits : Configure IPv6 server mode.
--server-bridge [IP netmask pool-start-IP pool-end-IP] : Helper option to
easily configure ethernet bridging server mode.
--push "option" : Push a config file option back to the peer for remote
execution. Peer must specify --pull in its config file.
--push-reset : Don't inherit global push list for specific
client instance.
--ifconfig-pool start-IP end-IP [netmask] : Set aside a pool of subnets
to be dynamically allocated to connecting clients.
--ifconfig-pool-linear : (DEPRECATED) Use individual addresses rather
than /30 subnets
in tun mode. Not compatible with
Windows clients.
--ifconfig-pool-persist file [seconds] : Persist/unpersist ifconfig-pool
data to file, at seconds intervals (default=600).
If seconds=0, file will be treated as read-only.
--ifconfig-ipv6-pool base-IP/bits : set aside an IPv6 network block
to be dynamically allocated to connecting clients.
--ifconfig-push local remote-netmask : Push an ifconfig option to remote,
overrides --ifconfig-pool dynamic allocation.
Only valid in a client-specific config file.
--ifconfig-ipv6-push local/bits remote : Push an ifconfig-ipv6 option to
remote, overrides --ifconfig-ipv6-pool allocation.
Only valid in a client-specific config file.
--iroute network [netmask] : Route subnet to client.
--iroute-ipv6 network/bits : Route IPv6 subnet to client.
Sets up internal routes only.
Only valid in a client-specific config file.
--disable : Client is disabled.
Only valid in a client-specific config file.
--client-cert-not-required : (DEPRECATED) Don't require client certificate, client
will authenticate using username/password.
--verify-client-cert [none|optional|require] : perform no, optional or
mandatory client certificate verification.
Default is to require the client to supply a certificate.
--username-as-common-name : For auth-user-pass authentication, use
the authenticated username as the common name,
rather than the common name from the client cert.
--auth-user-pass-verify cmd method: Query client for username/password and
run command cmd to verify. If method='via-env', pass
user/pass via environment, if method='via-file', pass
user/pass via temporary file.
--auth-gen-token [lifetime] Generate a random authentication token which is pushed
to each client, replacing the password. Useful when
OTP based two-factor auth mechanisms are in use and
--reneg-* options are enabled. Optionally a lifetime in seconds
for generated tokens can be set.
--opt-verify : Clients that connect with options that are incompatible
with those of the server will be disconnected.
--auth-user-pass-optional : Allow connections by clients that don't
specify a username/password.
--no-name-remapping : (DEPRECATED) Allow Common Name and X509 Subject to include
any printable character.
--client-to-client : Internally route client-to-client traffic.
--duplicate-cn : Allow multiple clients with the same common name to
concurrently connect.
--client-connect cmd : Run command cmd on client connection.
--client-disconnect cmd : Run command cmd on client disconnection.
--client-config-dir dir : Directory for custom client config files.
--ccd-exclusive : Refuse connection unless custom client config is found.
--tmp-dir dir : Temporary directory, used for --client-connect return file and plugin communication.
--hash-size r v : Set the size of the real address hash table to r and the
virtual address table to v.
--bcast-buffers n : Allocate n broadcast buffers.
--tcp-queue-limit n : Maximum number of queued TCP output packets.
--tcp-nodelay : Macro that sets TCP_NODELAY socket flag on the server
as well as pushes it to connecting clients.
--learn-address cmd : Run command cmd to validate client virtual addresses.
--connect-freq n s : Allow a maximum of n new connections per s seconds.
--max-clients n : Allow a maximum of n simultaneously connected clients.
--max-routes-per-client n : Allow a maximum of n internal routes per client.
--stale-routes-check n [t] : Remove routes with a last activity timestamp
older than n seconds. Run this check every t
seconds (defaults to n).
--explicit-exit-notify [n] : In UDP server mode send [RESTART] command on exit/restart to connected
clients. n = 1 - reconnect to same server,
2 - advance to next server, default=1.
--port-share host port [dir] : When run in TCP mode, proxy incoming HTTPS
sessions to a web server at host:port. dir specifies an
optional directory to write origin IP:port data.
Client options (when connecting to a multi-client server):
--client : Helper option to easily configure client mode.
--auth-user-pass [up] : Authenticate with server using username/password.
up is a file containing the username on the first line,
and a password on the second. If either the password or both
the username and the password are omitted OpenVPN will prompt
for them from console.
--pull : Accept certain config file options from the peer as if they
were part of the local config file. Must be specified
when connecting to a '--mode server' remote host.
--pull-filter accept|ignore|reject t : Filter each option received from the
server if it starts with the text t. The action flag accept,
ignore or reject causes the option to be allowed, removed or
rejected with error. May be specified multiple times, and
each filter is applied in the order of appearance.
--auth-retry t : How to handle auth failures. Set t to
none (default), interact, or nointeract.
--static-challenge t e : Enable static challenge/response protocol using
challenge text t, with e indicating echo flag (0|1)
--connect-timeout n : when polling possible remote servers to connect to
in a round-robin fashion, spend no more than n seconds
waiting for a response before trying the next server.
--allow-recursive-routing : When this option is set, OpenVPN will not drop
incoming tun packets with same destination as host.
--explicit-exit-notify [n] : On exit/restart, send exit signal to
server/remote. n = # of retries, default=1.
Data Channel Encryption Options (must be compatible between peers):
(These options are meaningful for both Static Key & TLS-mode)
--secret f [d] : Enable Static Key encryption mode (non-TLS).
Use shared secret file f, generate with --genkey.
The optional d parameter controls key directionality.
If d is specified, use separate keys for each
direction, set d=0 on one side of the connection,
and d=1 on the other side.
--auth alg : Authenticate packets with HMAC using message
digest algorithm alg (default=SHA1).
(usually adds 16 or 20 bytes per packet)
Set alg=none to disable authentication.
--cipher alg : Encrypt packets with cipher algorithm alg
(default=BF-CBC).
Set alg=none to disable encryption.
--ncp-ciphers list : List of ciphers that are allowed to be negotiated.
--ncp-disable : Disable cipher negotiation.
--prng alg [nsl] : For PRNG, use digest algorithm alg, and
nonce_secret_len=nsl. Set alg=none to disable PRNG.
--keysize n : (DEPRECATED) Size of cipher key in bits (optional).
If unspecified, defaults to cipher-specific default.
--engine [name] : Enable OpenSSL hardware crypto engine functionality.
--no-replay : (DEPRECATED) Disable replay protection.
--mute-replay-warnings : Silence the output of replay warnings to log file.
--replay-window n [t] : Use a replay protection sliding window of size n
and a time window of t seconds.
Default n=64 t=15
--no-iv : Disable cipher IV -- only allowed with CBC mode ciphers.
--replay-persist file : Persist replay-protection state across sessions
using file.
--test-crypto : Run a self-test of crypto features enabled.
For debugging only.
TLS Key Negotiation Options:
(These options are meaningful only for TLS-mode)
--tls-server : Enable TLS and assume server role during TLS handshake.
--tls-client : Enable TLS and assume client role during TLS handshake.
--key-method m : (DEPRECATED) Data channel key exchange method. m should be a method
number, such as 1 (default), 2, etc.
--ca file : Certificate authority file in .pem format containing
root certificate.
--capath dir : A directory of trusted certificates (CAs and CRLs).
--dh file : File containing Diffie Hellman parameters
in .pem format (for --tls-server only).
Use "openssl dhparam -out dh1024.pem 1024" to generate.
--cert file : Local certificate in .pem format -- must be signed
by a Certificate Authority in --ca file.
--extra-certs file : one or more PEM certs that complete the cert chain.
--key file : Local private key in .pem format.
--tls-version-min <version> ['or-highest'] : sets the minimum TLS version we
will accept from the peer. If version is unrecognized and 'or-highest'
is specified, require max TLS version supported by SSL implementation.
--tls-version-max <version> : sets the maximum TLS version we will use.
--pkcs12 file : PKCS#12 file containing local private key, local certificate
and optionally the root CA certificate.
--x509-username-field : Field in x509 certificate containing the username.
Default is CN in the Subject field.
--verify-hash hash [algo] : Specify fingerprint for level-1 certificate.
Valid algo flags are SHA1 and SHA256.
--tls-cipher l : A list l of allowable TLS ciphers separated by : (optional).
: Use --show-tls to see a list of supported TLS ciphers.
--tls-cert-profile p : Set the allowed certificate crypto algorithm profile
(default=legacy).
--tls-timeout n : Packet retransmit timeout on TLS control channel
if no ACK from remote within n seconds (default=2).
--reneg-bytes n : Renegotiate data chan. key after n bytes sent and recvd.
--reneg-pkts n : Renegotiate data chan. key after n packets sent and recvd.
--reneg-sec n : Renegotiate data chan. key after n seconds (default=3600).
--hand-window n : Data channel key exchange must finalize within n seconds
of handshake initiation by any peer (default=60).
--tran-window n : Transition window -- old key can live this many seconds
after new key renegotiation begins (default=3600).
--single-session: Allow only one session (reset state on restart).
--tls-exit : Exit on TLS negotiation failure.
--tls-auth f [d]: Add an additional layer of authentication on top of the TLS
control channel to protect against attacks on the TLS stack
and DoS attacks.
f (required) is a shared-secret key file.
The optional d parameter controls key directionality,
see --secret option for more info.
--tls-crypt key : Add an additional layer of authenticated encryption on top
of the TLS control channel to hide the TLS certificate,
provide basic post-quantum security and protect against
attacks on the TLS stack and DoS attacks.
key (required) provides the pre-shared key file.
see --secret option for more info.
--askpass [file]: Get PEM password from controlling tty before we daemonize.
--auth-nocache : Don't cache --askpass or --auth-user-pass passwords.
--crl-verify crl ['dir']: Check peer certificate against a CRL.
--tls-verify cmd: Run command cmd to verify the X509 name of a
pending TLS connection that has otherwise passed all other
tests of certification. cmd should return 0 to allow
TLS handshake to proceed, or 1 to fail. (cmd is
executed as 'cmd certificate_depth subject')
--tls-export-cert [directory] : Get peer cert in PEM format and store it
in an openvpn temporary file in [directory]. Peer cert is
stored before tls-verify script execution and deleted after.
--verify-x509-name name: Accept connections only from a host with X509 subject
DN name. The remote host must also pass all other tests
of verification.
--ns-cert-type t: (DEPRECATED) Require that peer certificate was signed with
an explicit nsCertType designation t = 'client' | 'server'.
--x509-track x : Save peer X509 attribute x in environment for use by
plugins and management interface.
--keying-material-exporter label len : Save Exported Keying Material (RFC5705)
of len bytes (min. 16 bytes) using label in environment for use by plugins.
--remote-cert-ku v ... : Require that the peer certificate was signed with
explicit key usage, you can specify more than one value.
value should be given in hex format.
--remote-cert-eku oid : Require that the peer certificate was signed with
explicit extended key usage. Extended key usage can be encoded
as an object identifier or OpenSSL string representation.
--remote-cert-tls t: Require that peer certificate was signed with explicit
key usage and extended key usage based on RFC3280 TLS rules.
t = 'client' | 'server'.
PKCS#11 Options:
--pkcs11-providers provider ... : PKCS#11 provider to load.
--pkcs11-protected-authentication [0|1] ... : Use PKCS#11 protected authentication
path. Set for each provider.
--pkcs11-private-mode hex ... : PKCS#11 private key mode mask.
0 : Try to determind automatically (default).
1 : Use Sign.
2 : Use SignRecover.
4 : Use Decrypt.
8 : Use Unwrap.
--pkcs11-cert-private [0|1] ... : Set if login should be performed before
certificate can be accessed. Set for each provider.
--pkcs11-pin-cache seconds : Number of seconds to cache PIN. The default is -1
cache until token is removed.
--pkcs11-id-management : Acquire identity from management interface.
--pkcs11-id serialized-id 'id' : Identity to use, get using standalone --show-pkcs11-ids
SSL Library information:
--show-ciphers : Show cipher algorithms to use with --cipher option.
--show-digests : Show message digest algorithms to use with --auth option.
--show-engines : Show hardware crypto accelerator engines (if available).
--show-tls : Show all TLS ciphers (TLS used only as a control channel).
Generate a random key (only for non-TLS static key encryption mode):
--genkey : Generate a random key to be used as a shared secret,
for use with the --secret option.
--secret file : Write key to file.
Tun/tap config mode (available with linux 2.4+):
--mktun : Create a persistent tunnel.
--rmtun : Remove a persistent tunnel.
--dev tunX|tapX : tun/tap device
--dev-type dt : Device type. See tunnel options above for details.
--user user : User to set privilege to.
--group group : Group to set privilege to.
PKCS#11 standalone options:
--show-pkcs11-ids provider [cert_private] : Show PKCS#11 available ids.
--verb option can be added *BEFORE* this.
General Standalone Options:
--show-gateway : Show info about default gateway.
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