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Version 4 (modified by Samuli Seppänen, 12 years ago) (diff)
Updated man-page for openvpn-2.3_beta1

openvpn(8) 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 configuration 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 running openvpn without any parameters.

DESCRIPTION
OpenVPN is a robust and highly flexible VPN daemon. OpenVPN supports SSL/TLS security, ethernet bridg‐
ing, 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 foot‐
print.

OPTIONS
OpenVPN allows any option to be placed either on the command line or in a configuration file. Though
all command line options 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 whitespace, and "#" or ";" characters in the first column can be used to denote com‐
ments.

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, OpenVPN 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 referring 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 <con‐
nection> documentation below.

The OpenVPN client will try to connect to a server at host:port in the order specified by the
list of --remote options.

proto indicates the protocol to use when connecting with the remote, and may be "tcp" or "udp".

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, one will be randomly chosen, pro‐
viding a sort of basic load-balancing and failover capability.

--remote-random-hostname
Add a random string (6 characters) to first DNS label of 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 </connec‐
tion>.

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
http-proxy-retry
</connection>

<connection>
remote 198.19.36.99 443 tcp
http-proxy 192.168.0.8 8080
http-proxy-retry
</connection>

persist-key
persist-tun
pkcs12 client.p12
ns-cert-type 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, float, http-proxy, http-proxy-option,
http-proxy-retry, http-proxy-timeout, local, lport, nobind, port, proto, remote, rport, socks-
proxy, and socks-proxy-retry.

A defaulting mechanism exists for specifying options to apply to all <connection> profiles. If
any of the above options (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 infinite 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 robust‐
ness perspective, such as tunneling non-IP or application-level UDP protocols, or tunneling pro‐
tocols which don't possess a built-in reliability layer.

--connect-retry n
For --proto tcp-client, take n as the number of seconds to wait between connection retries
(default=5).

--connect-timeout n
For --proto tcp-client, set connection timeout to n seconds (default=10).

--connect-retry-max n
For --proto tcp-client, take n as the number of retries of connection attempt (default=infinite).

--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 required, authfile is a file containing a username and password on 2 lines, or
"stdin" to prompt from console.

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 authenti‐
cation method, but to reject weak authentication protocols such as HTTP Basic Authentication.

--http-proxy-retry
Retry indefinitely on HTTP proxy errors. If an HTTP proxy error occurs, simulate a SIGUSR1
reset.

--http-proxy-timeout n
Set proxy timeout to n seconds, default=5.

--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.

--socks-proxy server [port]
Connect to remote host through a Socks5 proxy at address server and port port (default=1080).

--socks-proxy-retry
Retry indefinitely on Socks proxy errors. If a Socks proxy error occurs, simulate a SIGUSR1
reset.

--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 fol‐
lows:

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 notice, 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 for both local and remote. The current default of 1194 represents the offi‐
cial IANA port number assignment for OpenVPN and has been used since version 2.0-beta17. Previ‐
ous versions used port 5000 as the default.

--lport port
TCP/UDP port number for bind.

--rport port
TCP/UDP port number for remote.

--bind Bind to local address and port. This is the default unless any of --proto tcp-client , --http-
proxy or --socks-proxy are used.

--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 auto‐
matically push your chosen topology setting to clients as well. This directive can also be manu‐
ally 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 local 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 sys‐
tems. This mode is functionally equivalent to the --ifconfig-pool-linear directive which is
available in OpenVPN 2.0 and is now deprecated.

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 Win‐
dows 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.

--tun-ipv6
Build a tun link capable of forwarding IPv6 traffic. Should be used in conjunction with --dev
tun or --dev tunX. A warning will be displayed if no specific IPv6 TUN support for your OS has
been compiled into OpenVPN.

See below for further IPv6-related configuration options.

--dev-node node
Explicitly set the device node rather than using /dev/net/tun, /dev/tun, /dev/tap, etc. If Open‐
VPN 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.

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 unprivileged environment.

--ifconfig l rn
Set TUN/TAP adapter parameters. l is the IP address of the local VPN endpoint. For TUN devices,
rn is the IP address of the remote VPN endpoint. For TAP devices, rn is the subnet mask of the
virtual ethernet segment which is being created or connected to.

For TUN devices, which facilitate virtual point-to-point IP connections, 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, --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 configuration 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 --ifconfig-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 automatically 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 portable 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 --ifconfig 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.

--max-routes n
Allow a maximum number of n --route options to be specified, either in the local configuration
file, or pulled from an OpenVPN server. By default, n=100.

--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 negotia‐
tion 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 immediately 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.

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.

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 envi‐
ronmental variables.

--route-nopull
When used with --client or --pull, accept options pushed by server EXCEPT for routes and dhcp
options 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 --ifcon‐
fig, --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 cre‐
ate 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 perspective, 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-gate‐
way or the second parameter 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 servers 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 gate‐
way.

bypass-dhcp -- Add a direct route to the DHCP server (if it is non-local) which bypasses the tun‐
nel (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 accomplished by routing the local LAN (except for the LAN gateway address) into
the tunnel.

--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 parameter 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 particular 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 configura‐
tion. 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 encapsulation 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 OpenVPN 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 encapsulation overhead has been added in, but not including the UDP header itself.

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 fragmentation 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 active 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. Currently defaults to 65536 bytes.

--rcvbuf size
Set the TCP/UDP socket receive buffer size. Currently defaults to 65536 bytes.

--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 considerably 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. 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 trig‐
ger 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 actual 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 crypto‐
graphically secure.

This option has two intended uses:

(1) Compatibility with stateful firewalls. The periodic ping will ensure that a stateful fire‐
wall 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 --persist-remote-ip options.

Also note that --ping-exit and --ping-restart are mutually exclusive and cannot be used together.

--keepalive n m
A helper directive designed to simplify the expression of --ping and --ping-restart in server
mode configurations.

The server timeout is set twice the value of the second argument. This ensures that a timeout is
dectected on client side before the server side drops the connection.

For example, --keepalive 10 60 expands as follows:

if mode server:
ping 10
ping-restart 120
push "ping 10"
push "ping-restart 60"
else
ping 10
ping-restart 60

--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 OpenVPN 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 vir‐
tual 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).

The up command is useful for specifying route commands which route IP traffic destined for pri‐
vate subnets which exist 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 parameter 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.

The following standalone example shows how the --up script can be called in both an initializa‐
tion and restart context. (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' --per‐
sist-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 establish‐
ment with peer.

In --proto udp mode, this option normally requires the use of --ping to allow connection initia‐
tion 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 establishment, 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 con‐
sists 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 sepa‐
rated 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 privilege.

--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 described 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 assumption 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.

--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 "OPEN‐
VPN_" to the environmental variable is a safety precaution to prevent a LD_PRELOAD style attack
from a malicious or compromised server.

--script-security level [method]
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).

The method parameter indicates how OpenVPN should call external commands and scripts. Settings
for method:

execve -- (default) Use execve() function on Unix family OSes and CreateProcess() on Windows.
system -- Use system() function (deprecated and less safe since the external program command line
is subject to shell expansion).

The --script-security option was introduced in OpenVPN 2.1_rc9. For configuration file compati‐
bility with previous OpenVPN versions, use: --script-security 3 system

--disable-occ
Don't output a warning message if option inconsistencies are detected between peers. An example
of an option inconsistency 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 option is useful to protect the system in the event that some hostile party
was able to gain control of an OpenVPN session. 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 privileged 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 standpoint.

Since the chroot operation is delayed until after initialization, most OpenVPN options that ref‐
erence files will operate 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.

--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 vulnerable system call. You can of course com‐
bine all three, but please note that since setcon requires access to /proc you will have to pro‐
vide 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 Reference 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 output to be sent to the syslog file (such as /var/log/messages), except for
the output of scripts and ifconfig commands, which will go to /dev/null unless otherwise redi‐
rected. 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 supercede syslog redirection.

The optional progname parameter will cause OpenVPN to report its program name to the system log‐
ger as progname. This can be useful in linking OpenVPN messages in the syslog file with specific
tunnels. When unspecified, progname defaults to "openvpn".

When OpenVPN is run with the --daemon option, it will try to delay daemonization until the major‐
ity 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 initialized and entered the packet for‐
warding event loop.

In OpenVPN, the vast majority of errors which occur after initialization are non-fatal.

--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 information 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 output 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 entry. 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 already exists it will be truncated. This option takes effect immediately when
it is parsed in the command line and will supercede 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 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 priority).

--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 specified.

--multihome
Configure a multi-homed UDP server. This option can be used when OpenVPN has been configured to
listen on all interfaces, and will attempt to bind client sessions to the interface on which
packets are being received, so that outgoing packets will be sent out of the same interface.
Note that this option is only relevant for UDP servers and currently is only implemented on
Linux.

Note: clients connecting to a --multihome server should always use the --nobind option.

--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.

--status-version [n]
Choose the status file format version number. Currently n can be 1, 2, or 3 and defaults to 1.

--mute n
Log at most n consecutive messages in the same category. This is useful to limit repetitive log‐
ging of similar message types.

--comp-lzo [mode]
Use fast LZO compression -- may add up to 1 byte per packet for incompressible data. mode may be
"yes", "no", or "adaptive" (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 directive, 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. Normally, adaptive compression is enabled with --comp-lzo.

Adaptive compression tries to optimize the case where you have compression enabled, but you are
sending predominantly uncompressible (or pre-compressed) packets over the tunnel, such as an FTP
or rsync transfer of a large, compressed file. With adaptive compression, OpenVPN will periodi‐
cally 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 IP port [pw-file]
Enable a TCP server on IP:port to handle daemon management functions. pw-file, if specified, is
a password file (password on first line) or "stdin" to prompt from standard input. The password
provided will set the password which TCP clients will need to provide in order to access manage‐
ment functions.

The management interface can also listen on a unix domain socket, for those platforms that sup‐
port it. To use a unix domain socket, specify the unix socket pathname in place of IP and set
port to 'unix'. While the default behavior is to create a unix domain socket that may be con‐
nected to by any process, the --management-client-user and --management-client-group directives
can be used 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 = "tunnel". Tunnel mode will cause the manage‐
ment interface to listen for a TCP connection on the local VPN address of the TUN/TAP interface.

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.

It is strongly recommended that IP be set to 127.0.0.1 (localhost) to restrict accessibility of
the management server to local clients.

--management-client
Management interface will connect as a TCP/unix domain client to IP:port specified by --manage‐
ment 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 con‐
sole.

--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-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 generate 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 mod‐
ule initialization function. Multiple plugin modules may be loaded into one OpenVPN process.

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 actual 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 authentication function (such as tls-verify,
auth-user-pass-verify, or client-connect), then every module and script must return success (0)
in order for the connection to be authenticated.

Server Mode
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and can be enabled with the
--mode server option. In server mode, OpenVPN will listen on a single port for incoming client connec‐
tions. 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 suffi‐
ciently fast hardware. SSL/TLS authentication must be used in this mode.

--server network netmask
A helper directive designed to simplify the configuration of OpenVPN's server mode. This direc‐
tive will set up an OpenVPN server which will allocate addresses to clients out of the given net‐
work/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 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.1"

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 Open‐
VPN's server mode in ethernet bridging configurations.

If --server-bridge is used without any parameters, it will enable a DHCP-proxy mode, where con‐
necting 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 gateway/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 allocate 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 can‐
not be pushed because the client needs to know them before the connection to the server can be
initiated.

This is a partial list of options which can currently be pushed: --route, --route-gateway,
--route-delay, --redirect-gateway, --ip-win32, --dhcp-option, --inactive, --ping, --ping-exit,
--ping-restart, --setenv, --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-peer-info
Push additional information about the client to server. The additional information consists of
the following data:

IV_VER=<version> -- the client OpenVPN version

IV_HWADDR=<mac address> -- the MAC address of clients default gateway

IV_LZO_STUB=1 -- if client was built with LZO stub capability

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 spec‐
ified 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 --per‐
sist-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 address. If you want guaranteed assignment, use --ifcon‐
fig-push

--ifconfig-pool-linear
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.

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 generated 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 already 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 inter‐
face.

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.

The command is passed the common name and IP address of the just-authenticated client as environ‐
mental 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 error 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 exists, 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, --iroute,
--ifconfig-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 communicate 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_con‐
trol_file when using deferred 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 output 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_NODELAY 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 ulti‐
mately 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 --tls-auth.

--learn-address cmd
Run command cmd to validate client virtual addresses or routes.

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 authenticated 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.

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 unspeci‐
fied. 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 authentica‐
tion capabilities.

To protect against a client passing a maliciously formed username or password string, the user‐
name string must consist 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 password string will be converted to under‐
bar ('_').

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 interpreter.

For a sample script that performs PAM authentication, see sample-scripts/auth-pam.pl in the Open‐
VPN source distribution.

--opt-verify
Clients that connect with options that are incompatible with those of the server will be discon‐
nected.

Options that will be compared for compatibility include dev-type, link-mtu, tun-mtu, proto, tun-
ipv6, ifconfig, comp-lzo, fragment, 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 respon‐
sibility 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
connecting client does not submit a username/password, the user-defined authentication mod‐
ule/script will see the username and password as being set to empty strings (""). The authenti‐
cation module/script MUST have logic to detect this condition and respond accordingly.

--client-cert-not-required
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.

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.

If you don't use this directive, but you also specify an --auth-user-pass-verify 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]
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 behavivour 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 variable, 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 implemented for compatibility reasons when using older plug-ins or scripts which
does not handle the new formatting or UTF-8 characters.

In OpenVPN v2.3 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 v2.3 also does not do the character remapping which happened
earlier. This new format 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 Represen‐
tation of Distinguished Names.

As a backwards compatibility for the removed --no-name-remapping feature in older OpenVPN ver‐
sions, the no-remapping mode flag can be used with the --compat-names option. When this mode
flag is used, the Common Name, Subject, and username strings are allowed to include any printable
character including space, but excluding control characters such as tab, newline, and carriage-
return. It ensures compatibility with the --no-name-remapping option of OpenVPN versions before
v2.3.

Please note: This option will not be around for a long time. It is only implemented to make the
transition to the new formatting less intrusive. It will be removed either in OpenVPN v2.4 or
v2.5. So please make sure you start the process to support the new formatting as soon as possi‐
ble.

--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 generated 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 determine 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 configuration.

--client
A helper directive designed to simplify the configuration of OpenVPN's client mode. This direc‐
tive 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.

--auth-user-pass [up]
Authenticate with server using username/password. up is a file containing username/password on 2
lines (Note: OpenVPN will only read passwords from a file if it has been built with the --enable-
password-save configure option, or on Windows by defining ENABLE_PASSWORD_SAVE in win/set‐
tings.in).

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 user‐
name/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 user‐
name/password requeries in case of error.

An AUTH_FAILED message is generated by the server if the client fails --auth-user-pass authenti‐
cation, 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 user‐
name/password. Use this option for unattended clients.
interact -- Client will requery for an --auth-user-pass username/password and/or private key
password before attempting 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 chal‐
lenge/response protocol.

--server-poll-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.

--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 attempts that the client will try to
resend the exit notification message. OpenVPN will not send any exit notifications unless this
option is enabled.

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, cipher-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 encryption/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 parame‐
ter, 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 distinguishing signature or feature (such as a header or protocol handshake sequence)
that would mark the ciphertext packets 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 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.

OpenVPN's usage of HMAC is to first encrypt a packet, then HMAC the resulting ciphertext.

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 authentication.

For more information on HMAC see http://www.cs.ucsd.edu/users/mihir/papers/hmac.html

--cipher alg
Encrypt packets with cipher algorithm alg. The default is BF-CBC, an abbreviation for Blowfish
in Cipher Block Chaining mode. Blowfish has the advantages of being fast, very secure, and
allowing key sizes of up to 448 bits. Blowfish is designed to be used in situations where keys
are changed infrequently.

For more information on blowfish, see http://www.counterpane.com/blowfish.html

To see other ciphers that are available with OpenVPN, use the --show-ciphers option.

OpenVPN supports the CBC, CFB, and OFB cipher modes, however CBC is recommended and CFB and OFB
should be considered advanced modes.

Set alg=none to disable encryption.

--keysize n
Size of cipher key in bits (optional). If unspecified, defaults to cipher-specific default. The
--show-ciphers option (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 (default=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 Open‐
VPN'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
(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 data‐
gram, 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 (identify‐
ing 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 man‐
agement mode you have selected.

In Static Key mode or when using an CFB or OFB mode cipher, OpenVPN uses a 64 bit unique identi‐
fier 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 specifed, or no
--proto option is specified.

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 protec‐
tion 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 sec‐
onds 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 maximum 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 num‐
ber 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 attitude 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 operational 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 ses‐
sion is stopped and restarted, it will reject 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
(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 PolarSSL's RNG.

Enabling prediction resistance causes the RNG to reseed in each call for random. Reseeding this
often can quickly deplete the kernel entropy pool.

If you need this option, please consider running a daemon that adds entropy to the kernel pool.

Note that this option only works with PolarSSL versions greater than 1.1.

--test-crypto
Do a self-test of OpenVPN's crypto options by encrypting and decrypting test packets using the
data channel encryption options specified above. This option does not require a peer to func‐
tion, and therefore can be specified without --dev or --remote.

The typical usage of --test-crypto would be something like this:

openvpn --test-crypto --secret key

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 iso‐
late 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 net‐
work 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 control 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 for‐
ward 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 partner 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 session keys, and the tunnel will begin passing data.

The OpenVPN distribution contains a set of scripts for managing RSA certificates & keys, located in the
easy-rsa subdirectory.

The easy-rsa package is also rendered in web form here: http://openvpn.net/easyrsa.html

--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 multiple 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 cer‐
tificate 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). Available with OpenSSL version >=
0.9.7 dev. Not available with PolarSSL.

--dh file
File containing Diffie Hellman parameters in .pem format (required for --tls-server only). Use

openssl dhparam -out dh1024.pem 1024

to generate your own, or use the existing dh1024.pem file included with the OpenVPN distribution.
Diffie Hellman parameters may be considered public.

--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 com‐
mercial service such as thawte.com (in which case you will be helping to finance the world's sec‐
ond 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 configuration 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 certifi‐
cate 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 certificate (see -cert file above).

--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 PolarSSL.

--verify-hash hash
Specify SHA1 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

--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.

--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 follow‐
ing:

0 (default) -- Try to determind 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 certifi‐
cate, 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
Use data channel key negotiation method m. The key method must match on both sides of the con‐
nection.

After OpenVPN negotiates a TLS session, a new set of keys for protecting the tunnel data channel
is generated and exchanged 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.

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 parallel 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 cer‐
tificates and verifying the certificate (or other authentication info provided) of the other
side. The --key-method parameter has no effect on this process.

(2) After the TLS connection is established, the tunnel session keys are separately negotiated
over the existing secure TLS channel. Here, --key-method determines the derivation of the tunnel
session keys.

--tls-cipher l
A list l of allowable TLS ciphers delimited by a colon (":"). If you require a high level of
security, you may want to set this parameter manually, to prevent a version rollback attack where
a man-in-the-middle attacker tries to force two peers to negotiate to the lowest level of secu‐
rity they both support. Use --show-tls to see a list of supported TLS ciphers.

--tls-timeout n
Packet retransmit timeout on TLS control channel if no acknowledgment from remote within n sec‐
onds (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 exponen‐
tial backoff algorithm. This parameter only applies to control channel packets. Data channel
packets (which carry encrypted tunnel data) are never acknowledged, sequenced, or retransmitted
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). 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.

--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 reauthorize 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 connec‐
tion 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 initi‐
ation 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 sequence 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 protect
against DoS attacks.

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 key file which can be in one of two formats:

(1) An OpenVPN static key file generated by --genkey (required if direction parameter is used).

(2) A freeform passphrase file. In this case the HMAC key will be derived by taking a secure
hash of this file, similar to the md5sum(1) or sha1sum(1) commands.

OpenVPN will first try format (1), and if the file fails to parse as a static key file, format
(2) will be used.

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 pack‐
ets from any IP address, 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 sending. 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 always 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 sig‐
nature and a unique ID for replay protection. 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, includ‐
ing 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 passphrase/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.

--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 allows 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 pri‐
vate 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 (Note: OpenVPN will only read passwords from a file if it has been built with the
--enable-password-save configure option, or on Windows by defining ENABLE_PASSWORD_SAVE in
win/settings.in).

--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.

This directive does not affect the --http-proxy username/password. It is always cached.

--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 (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.

When cmd is executed two arguments are appended after any arguments specified in cmd , as fol‐
lows:

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 cer‐
tificate authority who also signed many other certificates, where you don't necessarily want to
trust all of them, but rather be selective 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 fieldname
Field in x509 certificate subject to be used as username (default=CN). Fieldname will be upper‐
cased before matching. When this option is used, the --tls-remote option will match against the
chosen fieldname instead of the CN.

--tls-remote name
Accept connections only from a host with X509 name or common name equal to name. The remote host
must also pass all other tests of verification.

NOTE: Because tls-remote may test against a common name prefix, only use this option when you are
using OpenVPN with a custom CA certificate that is under your control. Never use this option
when your client certificates are signed by a third party, such as a commercial web CA.

Name can also be a common name prefix, for example if you want a client to only accept connec‐
tions to "Server-1", "Server-2", etc., you can simply use --tls-remote Server

Using a common 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 asso‐
ciated with designated servers.

--tls-remote is a useful replacement for the --tls-verify option to verify the remote host,
because --tls-remote works in a --chroot environment too.

--x509-track attribute
Save peer X509 attribute value in environment for use by plugins and management interface.
Prepend a '+' to attribute 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. Not available with PolarSSL.

--ns-cert-type client|server
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 des‐
ignated 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 attempts 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, --tls-remote, or --tls-verify.

--remote-cert-ku v...
Require that peer certificate was signed with an explicit key usage.

This is a useful security option for clients, to ensure that the host they connect to is a desig‐
nated server.

The key usage should be encoded in hex, more than one key usage can be specified.

--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 desig‐
nated 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 desig‐
nated server.

The --remote-cert-tls client option is equivalent to --remote-cert-ku 80 08 88 --remote-cert-eku
"TLS Web Client Authentication"

The key usage is digitalSignature and/or keyAgreement.

The --remote-cert-tls server option is equivalent to --remote-cert-ku a0 88 --remote-cert-eku
"TLS Web Server Authentication"

The key usage is digitalSignature and ( keyEncipherment or keyAgreement ).

--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 contain‐
ing files named as revoked 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 rejected.

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 (TLS used only as a control channel). The TLS ciphers will be
sorted from highest preference (most secure) to lowest.

--show-engines
(Standalone) Show currently available hardware-based crypto acceleration engines supported by the
OpenSSL library.

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 exist 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 environment 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 option 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 generated by the kernel. This mode is probably the "cleanest" solution for set‐
ting 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 0. 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 mas‐
querade as the IP address at broadcast address + offset. The Windows ipconfig /all command can
be used to show what Windows thinks the DHCP server address is. OpenVPN will "claim" this
address, so make sure to use a free address. Having 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 pre‐
vents 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" com‐
mand. 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 prac‐
tice. 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 automatically."

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
"unstick" 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 address. Repeat this option to set secondary DNS
server addresses.

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 single 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 communicate. 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 courtesy 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 environment 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.

--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 the same caveats as
--dhcp-renew above.

--register-dns
Run net stop dnscache, net start dnscache, ipconfig /flushdns and ipconfig /registerdns on con‐
nection initiation. This is known to kick Windows into recognizing 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 automatically 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 ser‐
vice 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 controlling 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 after 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 neces‐
sary for the TAP-Win32 driver to impose certain 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.

--verb option can be used BEFORE this option to produce debugging information.

IPv6 Related Options
The following options exist to support IPv6 tunneling in peer-to-peer and client-server mode. As of
now, this is just very basic documentation of the IPv6-related options. More documentation can be found
on http://www.greenie.net/ipv6/openvpn.html.

--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'' device

--server-ipv6 ipv6addr/bits
convenience-function to enable a number of IPv6 related options at once, namely --ifconfig-ipv6,
--ifconfig-ipv6-pool, --tun-ipv6 and --push tun-ipv6 Is only accepted if ``--mode server'' or
``--server'' is set.

--ifconfig-ipv6-pool ipv6addr/bits
Specify an IPv6 address pool for dynamic assignment to clients. The pool starts at ipv6addr and
increments by +1 for every new client (linear mode). The /bits setting controls the size of the
pool.

--ifconfig-ipv6-push ipv6addr/bits ipv6remote
for ccd/ per-client static IPv6 interface configuration, see --client-config-dir and --ifconfig-
push for more details.

--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 Open‐
VPN'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 parameters 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 ('='). Alphanumeric 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 2.0.1-rc6, the at ('@') char‐
acter 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 pro‐
gram 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 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 ver‐
sion of ifconfig) commands which normally occurs 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 execution.

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 OpenVPN 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 nor‐
mally 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 ifcon‐
fig) 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 otherwise from the ifconfig pool (controlled by the --ifconfig-pool config file
directive). Only set for --dev tun tunnels. 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 speci‐
fied, or otherwise from the ifconfig pool (controlled by the --ifconfig-pool config file direc‐
tive). 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 otherwise 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, 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 execu‐
tion 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 execu‐
tion.

route_vpn_gateway
The default gateway used by --route options, as specified in either the --route-gateway option or
the second parameter to --ifconfig when --dev tun is specified. Set prior to --up script execu‐
tion.

route_{parm}_{n}
A set of variables which define each route to be added, and are set prior to --up script execu‐
tion.

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 documen‐
tation 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 following: up, down, ipchange, route-up, tls-verify, auth-user-pass-verify, client-
connect, client-disconnect, or learn-address. Set prior to execution of any script.

signal The reason for exit or restart. Can be one of sigusr1, sighup, sigterm, sigint, inactive (con‐
trolled by --inactive option), ping-exit (controlled by --ping-exit option), ping-restart (con‐
trolled by --ping-restart option), connection-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-connect script.

time_duration
The duration (in seconds) of the client session which is now disconnecting. Set prior to execu‐
tion 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_id_{n}
A series of certificate fields from the remote peer, where n is the verification level. Only set
for TLS connections. 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 connections. Set prior to execution of --tls-verify script. This is in the form
of a hex string like "37AB46E0", which is suitable for doing serial-based OCSP queries (with
OpenSSL, you have to 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 con‐
trib/OCSP_check/OCSP_check.sh script for an example.

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 execu‐
tion of --ipchange, --client-connect, and --client-disconnect scripts. If using ipv6 endpoints
(udp6, tcp6), trusted_ip6 will be set instead.

trusted_port
Actual port number of connecting client or peer which has been authenticated. Set prior to exe‐
cution 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 connecting 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 end‐
points (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 execu‐
tion 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 connections. Set prior to execution of --tls-verify script. This variable is simi‐
lar to tls_id_{n} except the component X509 subject fields are broken out, and no string remap‐
ping occurs on these field values (except for remapping of control characters to "_"). For exam‐
ple, the following variables would be set on the OpenVPN server using the sample client certifi‐
cate in sample-keys (client.crt). Note that the verification level is 0 for the client certifi‐
cate 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, --secret and --tls-auth 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 reopen 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 authenti‐
cated remote IP address/port based on --persist-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 std‐
out 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 connec‐
tivity 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 direc‐
tions. 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 may.kg and june.kg. If you are construct‐
ing a VPN over the internet, then replace may.kg and june.kg 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 mean‐
ing 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 may.kg will be 10.4.0.1 and for june.kg,
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) independently of the VPN, by choosing whether to use (a) the VPN end‐
point address or (b) the public internet address, to access the remote host. For example if you are on
may.kg and you wish to connect to june.kg via ssh without using the VPN (since ssh has its own built-in
security) you would use the command ssh june.kg. However in the same scenario, you could also use the
command telnet 10.4.0.2 to create a telnet session with june.kg 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 sub‐
net for either of the tunnel endpoints, you will get a weird feedback loop.

Example 1: A simple tunnel without security
On may:

openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 9

On june:

openvpn --remote may.kg --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 may:

ping 10.4.0.2

On june:

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 OpenVPN run quietly.

Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)
First build a static key on may.

openvpn --genkey --secret key

This command will build a random key file called key (in ascii format). Now copy key to june over a
secure medium such as by using the scp(1) program.

On may:

openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 5 --secret key

On june:

openvpn --remote may.kg --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 may:

ping 10.4.0.2

On june:

ping 10.4.0.1

Example 3: A tunnel with full TLS-based security
For this test, we will designate may as the TLS client and june 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 may and june (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 may:

openvpn --remote june.kg --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 june:

openvpn --remote may.kg --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 may:

ping 10.4.0.2

On june:

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 negotia‐
tion.

For production operations, a key renegotiation interval of 60 seconds is probably too frequent. Omit
the --reneg-sec 60 option 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 may and june 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 may's
private subnet is 10.0.0.0/24 and june'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 may:

route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2

On june:

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 sec‐
onds. If n is less than the stateful firewall connection timeout, you can maintain an OpenVPN connec‐
tion 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 device 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://openvpn.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/open‐
source/lzo/

COPYRIGHT
Copyright (C) 2002-2010 OpenVPN Technologies, 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>

17 November 2008 openvpn(8)

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