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openvpn

Section: Maintenance Commands (8)
Updated: 17 November 2008
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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 bridging,
TCP or UDP tunnel transport through proxies or NAT,
support for dynamic IP addresses and DHCP,
scalability to hundreds or thousands of users,
and portability to most major OS platforms.

OpenVPN is tightly bound to the OpenSSL library, and derives much
of its crypto capabilities from it.

OpenVPN supports
conventional encryption
using a pre-shared secret key
(Static Key mode)

or
public key security
(SSL/TLS mode)

using client & server certificates.
OpenVPN also
supports non-encrypted TCP/UDP tunnels.

OpenVPN is designed to work with the
TUN/TAP

virtual networking interface that exists on most platforms.

Overall, OpenVPN aims to offer many of the key features of IPSec but
with a relatively lightweight footprint.

OPTIONS

OpenVPN allows any option to be placed either on the command line
or in a configuration file. Though all command line 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 comments.

Note that OpenVPN 2.0 and higher performs backslash-based shell
escaping for characters not in single quotations,
so the following mappings should be observed:

\\       Maps to a single backslash character (\).
\"       Pass a literal doublequote character ("), don‘t
         interpret it as enclosing a parameter.
\[SPACE] Pass a literal space or tab character, don‘t
         interpret it as a parameter delimiter.

For example on Windows, use double backslashes to represent pathnames:

secret "c:\\OpenVPN\\secret.key"

For examples of configuration files, see http://openvpn.net/examples.html

Here is an example configuration file:

#
# Sample OpenVPN configuration file for
# using a pre-shared static key.
#
# ‘#‘ or ‘;‘ may be used to delimit comments.

# Use a dynamic tun device.
dev tun

# Our remote peer
remote mypeer.mydomain

# 10.1.0.1 is our local VPN endpoint
# 10.1.0.2 is our remote VPN endpoint
ifconfig 10.1.0.1 10.1.0.2

# Our pre-shared static key
secret static.key

Tunnel Options:

--mode m

Set OpenVPN major mode. By default, OpenVPN runs in point-to-point mode ("p2p"). OpenVPN 2.0 introduces a new mode ("server") which implements a multi-client server capability.

--local host

Local host name or IP address for bind. If specified, OpenVPN will bind to this address only. If unspecified, 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 <connection> documentation below.

The OpenVPN client will try to connect to a server at host:port in the order specified by the list of --remote 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, providing a sort of basic load-balancing and failover capability.

--remote-random-hostname

Prepend a random string (6 bytes, 12 hex characters) to hostname to prevent DNS caching. For example, "foo.bar.gov" would be modified to "<random-chars>.foo.bar.gov".

<connection>

Define a client connection profile. Client connection profiles are groups of OpenVPN options that describe how to connect to a given OpenVPN server. Client connection profiles are specified within an OpenVPN configuration file, and each profile is bracketed by <connection> and </connection>.

An OpenVPN client will try each connection profile sequentially until it achieves a successful connection.

--remote-random can be used to initially "scramble" the connection list.

Here is an example of connection profile usage:

client
dev tun

<connection>
remote 198.19.34.56 1194 udp
</connection>

<connection>
remote 198.19.34.56 443 tcp
</connection>

<connection>
remote 198.19.34.56 443 tcp
http-proxy 192.168.0.8 8080
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, explicit-exit-notify, float, fragment, http-proxy, http-proxy-option, http-proxy-retry, http-proxy-timeout, link-mtu, local, lport, mssfix, mtu-disc, nobind, port, proto, remote, rport, socks-proxy, socks-proxy-retry, tun-mtu and tun-mtu-extra.

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 robustness perspective, such as tunneling non-IP or application-level UDP protocols, or tunneling protocols which don‘t possess a built-in reliability layer.

--connect-retry n

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 authentication 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] [authfile]

Connect to remote host through a Socks5 proxy at address server and port port (default=1080). authfile (optional) is a file containing a username and password on 2 lines, or "stdin" to prompt from console.

--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 follows:

cmd ip_address port_number

Don‘t use --ipchange in --mode server mode. Use a --client-connect script instead.

See the "Environmental Variables" section below for additional parameters passed as environmental variables.

If you are running in a dynamic IP address environment where the IP addresses of either peer could change without 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 official IANA port number assignment for OpenVPN and has been used since version 2.0-beta17. Previous versions used port 5000 as the default.

--lport port

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 automatically push your chosen topology setting to clients as well. This directive can also be manually pushed to clients. Like the --dev directive, this directive must always be compatible between client and server.

mode can be one of:

net30 -- Use a point-to-point topology, by allocating one /30 subnet per client. This is designed to allow point-to-point semantics when some or all of the connecting clients might be Windows systems. This is the default on OpenVPN 2.0.

p2p -- Use a point-to-point topology where the remote endpoint of the client‘s tun interface always points to the 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 systems. 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 Windows as well. Only available when server and clients are OpenVPN 2.1 or higher, or OpenVPN 2.0.x which has been manually patched with the --topology directive code. When used on Windows, requires version 8.2 or higher of the TAP-Win32 driver. When used on *nix, requires that the tun driver supports an ifconfig(8) command which sets a subnet instead of a remote endpoint IP address.

This option exists in OpenVPN 2.1 or higher.

--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 OpenVPN cannot figure out whether node is a TUN or TAP device based on the name, you should also specify --dev-type tun or --dev-type tap.

Under Mac OS X this option can be used to specify the default tun implementation. Using --dev-node utun forces usage of the native Darwin tun kernel support. Use --dev-node utunN to select a specific utun instance. To force using the tun.kext (/dev/tunX) use --dev-node tun. When not specifying a --dev-node option openvpn will first try to open utun, and fall back to tun.kext.

On Windows systems, select the TAP-Win32 adapter which is named node in the Network Connections Control Panel or the raw GUID of the adapter enclosed by braces. The --show-adapters option under Windows can also be used to enumerate all available TAP-Win32 adapters and will show both the network connections control panel name and the GUID for each TAP-Win32 adapter.

--lladdr address

Specify the link layer address, more commonly known as the MAC address. Only applied to TAP devices.

--iproute cmd

Set alternate command to execute instead of default iproute2 command. May be used in order to execute OpenVPN in 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 negotiation with the OpenVPN server-side LAN.

--route-metric m

Specify a default metric m for use with --route.

--route-delay [n] [w]

Delay n seconds (default=0) after connection establishment, before adding routes. If n is 0, routes will be added 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 environmental 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 --ifconfig, --route, and --route-gateway.

--client-nat snat|dnat network netmask alias

This pushable client option sets up a stateless one-to-one NAT rule on packet addresses (not ports), and is useful in cases where routes or ifconfig settings pushed to the client would create an IP numbering conflict.

network/netmask (for example 192.168.0.0/255.255.0.0) defines the local view of a resource from the client 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-gateway 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 gateway.

bypass-dhcp -- Add a direct route to the DHCP server (if it is non-local) which bypasses the tunnel (Available on Windows clients, may not be available on non-Windows clients).

bypass-dns -- Add a direct route to the DNS server(s) (if they are non-local) which bypasses the tunnel (Available on Windows clients, may not be available on non-Windows clients).

block-local -- Block access to local LAN when the tunnel is active, except for the LAN gateway itself. This is 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 configuration.
OpenVPN will send ping packets of various sizes
to the remote peer and measure the largest packets
which were successfully received. The
--mtu-test

process normally takes about 3 minutes to complete.

--fragment max

Enable internal datagram fragmentation so
that no UDP datagrams are sent which
are larger than
max

bytes.

The
max

parameter is interpreted in the same way as the
--link-mtu

parameter, i.e. the UDP packet size after 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.
Note that this will only work if mode is set to p2p.
If you want to limit the bandwidth
in both directions, use this option on both peers.

OpenVPN uses the following algorithm to implement
traffic shaping: Given a shaper rate of
n

bytes per second, after a datagram write of
b

bytes is queued on the TCP/UDP port, wait a minimum of
(b / n)

seconds before queuing the next write.

It should be noted that OpenVPN supports multiple
tunnels between the same two peers, allowing you
to construct full-speed and reduced bandwidth tunnels
at the same time,
routing low-priority data such as off-site backups
over the reduced bandwidth tunnel, and other data
over the full-speed tunnel.

Also note that for low bandwidth tunnels
(under 1000 bytes per second), you should probably
use lower MTU values as well (see above), otherwise
the packet latency will grow so large as to trigger
timeouts in the TLS layer and TCP connections running
over the tunnel.

OpenVPN allows
n

to be between 100 bytes/sec and 100 Mbytes/sec.

--inactive n [bytes]

Causes OpenVPN to exit after
n

seconds of inactivity on the TUN/TAP device. The time length of
inactivity is measured since the last incoming or outgoing tunnel
packet. The default value is 0 seconds, which disables this feature.

If the optional
bytes

parameter is included,
exit if less than
bytes

of combined in/out traffic are produced on the tun/tap device
in
n

seconds.

In any case, OpenVPN‘s internal ping packets (which are just
keepalives) and TLS control packets are not considered
"activity", nor are they counted as traffic, as they are used
internally by OpenVPN and are not an indication of 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 cryptographically secure.

This option has two intended uses:

(1) Compatibility
with stateful firewalls. The periodic ping will ensure that
a stateful firewall rule which allows OpenVPN UDP packets to
pass will not time out.

(2) To provide a basis for the remote to test the existence
of its peer using the
--ping-exit

option.

--ping-exit n

Causes OpenVPN to exit after
n

seconds pass without reception of a ping
or other packet from remote.
This option can be combined with
--inactive, --ping,

and
--ping-exit

to create a two-tiered inactivity disconnect.

For example,

openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60

when used on both peers will cause OpenVPN to exit within 60
seconds if its peer disconnects, but will exit after one
hour if no actual tunnel data is exchanged.

--ping-restart n

Similar to
--ping-exit,

but trigger a
SIGUSR1

restart after
n

seconds pass without reception of a ping
or other packet from remote.

This option is useful in cases
where the remote peer has a dynamic IP address and
a low-TTL DNS name is used to track the IP address using
a service such as
http://dyndns.org/

+ a dynamic DNS client such
as
ddclient.

If the peer cannot be reached, a restart will be triggered, causing
the hostname used with
--remote

to be re-resolved (if
--resolv-retry

is also specified).

In server mode,
--ping-restart, --inactive,

or any other type of internally generated signal will always be
applied to
individual client instance objects, never to whole server itself.
Note also in server mode that any internally generated signal
which would normally cause a restart, will cause the deletion
of the client instance object instead.

In client mode, the
--ping-restart

parameter is set to 120 seconds by default. This default will
hold until the client pulls a replacement value from the server, based on
the
--keepalive

setting in the server configuration.
To disable the 120 second default, set
--ping-restart 0

on the client.

See the signals section below for more information
on
SIGUSR1.

Note that the behavior of
SIGUSR1

can be modified by the
--persist-tun, --persist-key, --persist-local-ip,

and
--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 detected 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 virtual memory paging operations which occur under most modern operating systems. It ensures that even if an attacker was able to crack the box running OpenVPN, he would not be able to scan the system swap file to recover previously used ephemeral keys, which are used for a period of time governed by the --reneg options (see below), then are discarded.

The downside of using --mlock is that it will reduce the amount of physical memory available to other applications.

--up cmd

Run command cmd after successful TUN/TAP device open (pre --user UID change).

The up command is useful for specifying route commands which route IP traffic destined for private 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 initialization 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‘ --persist-tun --up-restart

Note that OpenVPN also provides the --ifconfig option to automatically ifconfig the TUN device, eliminating the need to define an --up script, unless you also want to configure routes in the --up script.

If --ifconfig is also specified, OpenVPN will pass the ifconfig local and remote endpoints on the command line to the --up script so that they can be used to configure routes such as:

route add -net 10.0.0.0 netmask 255.255.255.0 gw $5

--up-delay

Delay TUN/TAP open and possible --up script execution until after TCP/UDP connection establishment with peer.

In --proto udp mode, this option normally requires the use of --ping to allow connection initiation to be sensed in the absence of tunnel data, since UDP is a "connectionless" protocol.

On Windows, this option will delay the TAP-Win32 media state transitioning to "connected" until connection 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 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.

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.

It is also possible to tag a single directive so as not to trigger a fatal error if the directive isn‘t recognized. To do this, prepend the following before the directive: setenv opt

Versions prior to OpenVPN 2.3.3 will always ignore options set with the setenv opt directive.

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 connections. All client connections will be routed through a single tun or tap interface. This mode is designed for scalability and should be able to support hundreds or even thousands of clients on sufficiently fast hardware. SSL/TLS authentication must be used in this mode.

--server network netmask [‘nopool‘]

A helper directive designed to simplify the configuration of OpenVPN‘s server mode. This directive will set up an OpenVPN server which will allocate addresses to clients out of the given network/netmask. The server itself will take the ".1" address of the given network for use as the server-side endpoint of the local TUN/TAP interface.

For example, --server 10.8.0.0 255.255.255.0 expands as follows:

 mode server
 tls-server
 push "topology [topology]"

 if dev tun AND (topology == net30 OR topology == p2p):
   ifconfig 10.8.0.1 10.8.0.2
   if !nopool:
     ifconfig-pool 10.8.0.4 10.8.0.251
   route 10.8.0.0 255.255.255.0
   if client-to-client:
     push "route 10.8.0.0 255.255.255.0"
   else if topology == net30:
     push "route 10.8.0.1"

 if dev tap OR (dev tun AND topology == subnet):
   ifconfig 10.8.0.1 255.255.255.0
   if !nopool:
     ifconfig-pool 10.8.0.2 10.8.0.254 255.255.255.0
   push "route-gateway 10.8.0.1"
   if route-gateway unset:
     route-gateway 10.8.0.2

Don‘t use --server if you are ethernet bridging. Use --server-bridge instead.

--server-bridge gateway netmask pool-start-IP pool-end-IP

--server-bridge [‘nogw‘]

A helper directive similar to --server which is designed to simplify the configuration of OpenVPN‘s server mode in ethernet bridging configurations.

If --server-bridge is used without any parameters, it will enable a DHCP-proxy mode, where connecting OpenVPN clients will receive an IP address for their TAP adapter from the DHCP server running on the OpenVPN server-side LAN. Note that only clients that support the binding of a DHCP client with the TAP adapter (such as Windows) can support this mode. The optional nogw flag (advanced) indicates that gateway information should not be pushed to the client.

To configure ethernet bridging, you must first use your OS‘s bridging capability to bridge the TAP interface with the ethernet NIC interface. For example, on Linux this is done with the brctl tool, and with Windows XP it is done in the Network Connections Panel by selecting the ethernet and TAP adapters and right-clicking on "Bridge Connections".

Next you you must manually set the IP/netmask on the bridge interface. The gateway and netmask parameters to --server-bridge can be set to either the IP/netmask of the bridge interface, or the IP/netmask of the default 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 cannot 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 specified either in a client instance config file using --client-config-dir or dynamically generated using a --client-connect script.

--ifconfig-pool start-IP end-IP [netmask]

Set aside a pool of subnets to be dynamically allocated to connecting clients, similar to a DHCP server. For tun-style tunnels, each client will be given a /30 subnet (for interoperability with Windows clients). For tap-style tunnels, individual addresses will be allocated, and the optional netmask parameter will also be pushed to clients.

--ifconfig-pool-persist file [seconds]

Persist/unpersist ifconfig-pool data to file, at seconds intervals (default=600), as well as on program startup and shutdown.

The goal of this option is to provide a long-term association between clients (denoted by their common name) and the virtual IP address assigned to them from the ifconfig-pool. Maintaining a long-term association is good for clients because it allows them to effectively use the --persist-tun option.

file is a comma-delimited ASCII file, formatted as <Common-Name>,<IP-address>.

If seconds = 0, file will be treated as read-only. This is useful if you would like to treat file as a configuration file.

Note that the entries in this file are treated by OpenVPN as suggestions only, based on past associations between a common name and IP address. They do not guarantee that the given common name will always receive the given IP address. If you want guaranteed assignment, use --ifconfig-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).

-- Use
--client-config-dir

file for static IP (next choice).

-- 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 interface.

When this option is used, each client will "see" the other
clients which are currently connected. Otherwise, each
client will only see the server. Don‘t use this option
if you want to firewall tunnel traffic using
custom, per-client rules.

--duplicate-cn

Allow multiple clients with the same common name to concurrently connect.
In the absence of this option, OpenVPN will disconnect a client instance
upon connection of a new client having the same common name.

--client-connect cmd

Run
command cmd

on client connection.

cmd

consists of a path to script (or executable program), optionally
followed by arguments. The path and arguments may be single- or double-quoted
and/or escaped using a backslash, and should be separated by one or more spaces.

The command is passed the common name
and IP address of the just-authenticated client
as environmental variables (see environmental variable section
below). The command is also passed
the pathname of a freshly created temporary file as the last argument
(after any arguments specified in
cmd

), to be used by the command
to pass dynamically generated config file directives back to OpenVPN.

If the script wants to generate a dynamic config file
to be applied on the server when the client connects,
it should write it to the file named by the last argument.

See the
--client-config-dir

option below for options which
can be legally used in a dynamically generated config file.

Note that the return value of
script

is significant. If
script

returns a non-zero error status, it will cause the client
to be disconnected.

--client-disconnect cmd

Like
--client-connect

but called on client instance shutdown. Will not be called
unless the
--client-connect

script and plugins (if defined)
were previously called on this instance with
successful (0) status returns.

The exception to this rule is if the
--client-disconnect

command or plugins are cascaded, and at least one client-connect
function succeeded, then ALL of the client-disconnect functions for
scripts and plugins will be called on client instance object deletion,
even in cases where some of the related client-connect functions returned
an 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_control_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 ultimately fail to authenticate.

This is an imperfect solution however, because in a real DoS scenario, legitimate connections might also be refused.

For the best protection against DoS attacks in server mode, use --proto udp and --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.

cmd

If
method

is set to "via-env", OpenVPN will call
script

with the environmental variables
username

and
password

set to the username/password strings provided by the client.
Be aware that this method is insecure on some platforms which
make the environment of a process publicly visible to other
unprivileged processes.

If
method

is set to "via-file", OpenVPN will write the username and
password to the first two lines of a temporary file. The filename
will be passed as an argument to
script,

and the file will be automatically deleted by OpenVPN after
the script returns. The location of the temporary file is
controlled by the
--tmp-dir

option, and will default to the current directory if unspecified.
For security, consider setting
--tmp-dir

to a volatile storage medium such as
/dev/shm

(if available) to prevent the username/password file from touching the hard drive.

The script should examine the username
and password,
returning a success exit code (0) if the
client‘s authentication request is to be accepted, or a failure
code (1) to reject the client.

This directive is designed to enable a plugin-style interface
for extending OpenVPN‘s authentication capabilities.

To protect against a client passing a maliciously formed
username or password string, the username string must
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 underbar (‘_‘).

Care must be taken by any user-defined scripts to avoid
creating a security vulnerability in the way that these
strings are handled. Never use these strings in such a way
that they might be escaped or evaluated by a shell interpreter.

For a sample script that performs PAM authentication, see
sample-scripts/auth-pam.pl

in the OpenVPN source distribution.

--opt-verify

Clients that connect with options that are incompatible
with those of the server will be disconnected.

Options that will be compared for compatibility include
dev-type, link-mtu, tun-mtu, proto, 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 responsibility to the user-defined authentication
module/script to accept or deny the client based on other factors
(such as the setting of X509 certificate fields). When this option is used,
and a connecting client does not submit a username/password, the user-defined
authentication module/script will see the username and password as being set
to empty strings (""). The authentication module/script MUST have logic
to detect this condition and respond accordingly.

--client-cert-not-required

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] (DEPRECATED)

Until OpenVPN v2.3 the format of the X.509 Subject fields was formatted
like this:

/C=US/L=Somewhere/CN=John Doe/emailAddress=[email protected]

In addition the old behaviour was to remap any character other than
alphanumeric, underscore (‘_‘), dash (‘-‘), dot (‘.‘), and slash (‘/‘) to
underscore (‘_‘). The X.509 Subject string as returned by the
tls_id

environmental 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=[email protected]

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 Representation of Distinguished
Names.

The
no-remapping

mode flag can be used with the
--compat-names

option to be compatible with the now deprecated --no-name-remapping option.
It is only available at the server. When this mode flag is used, the Common Name,
Subject, and username strings are allowed to include any printable character
including space, but excluding control characters such as tab, newline, and
carriage-return. no-remapping is only available on the server side.

Please note:

This option is immediately deprecated. It is only implemented
to make the transition to the new formatting less intrusive. It will be
removed either in OpenVPN v2.4 or v2.5. So please make sure you use the
--verify-x509-name

option instead of
--tls-remote

as soon as possible and update your scripts where necessary.

--no-name-remapping (DEPRECATED)

The
--no-name-remapping

option is an alias for
--compat-names no-remapping.

It ensures compatibility with server configurations using the
--no-name-remapping

option.

Please note:

This option is now deprecated. It will be removed either in OpenVPN v2.4
or v2.5. So please make sure you support the new X.509 name formatting
described with the
--compat-names

option as soon as possible.

--port-share host port [dir]

When run in TCP server mode, share the OpenVPN port with
another application, such as an HTTPS server. If OpenVPN
senses a connection to its port which is using a non-OpenVPN
protocol, it will proxy the connection to the server at
host:port.

Currently only designed to work with HTTP/HTTPS,
though it would be theoretically possible to extend to
other protocols such as ssh.

dir

specifies an optional directory where a temporary file with name N
containing content C will be dynamically 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 directive is equivalent to:

 pull
 tls-client

--pull

This option must be used on a client which is connecting to a multi-client server. It indicates to OpenVPN that it should accept options pushed by the server, provided they are part of the legal set of pushable options (note that the --pull option is implied by --client ).

In particular, --pull allows the server to push routes to the client, so you should not use --pull or --client in situations where you don‘t trust the server to have control over the client‘s routing table.

--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/settings.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 username/password provided by the client.

--auth-retry type

Controls how OpenVPN responds to username/password verification errors such as the client-side response to an AUTH_FAILED message from the server or verification failure of the private key password.

Normally used to prevent auth errors from being fatal on the client side, and to permit username/password requeries in case of error.

An AUTH_FAILED message is generated by the server if the client fails --auth-user-pass authentication, or if the server-side --client-connect script returns an error status when the client tries to connect.

type can be one of:

none -- Client will exit with a fatal error (this is the default).

nointeract --

Client will retry the connection without requerying for an
--auth-user-pass

username/password. Use this option for unattended clients.

interact --

Client will requery for an
--auth-user-pass

username/password and/or private key password before 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 challenge/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

parameter, will also support 2048 bit key file generation
using the
--genkey

option.

Static key encryption mode has certain advantages,
the primary being ease of configuration.

There are no certificates
or certificate authorities or complicated negotiation handshakes and protocols.
The only requirement is that you have a pre-existing secure channel with
your peer (such as
ssh

) to initially copy the key. This requirement, along with the
fact that your key never changes unless you manually generate a new one,
makes it somewhat less secure than TLS mode (see below). If an attacker
manages to steal your key, everything that was ever encrypted with
it is compromised. Contrast that to the perfect forward secrecy features of
TLS mode (using Diffie Hellman key exchange), where even if an attacker
was able to steal your private key, he would gain no information to help
him decrypt past sessions.

Another advantageous aspect of Static Key encryption mode is that
it is a handshake-free protocol
without any 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 OpenVPN‘s pseudo-random number needs.

--engine [engine-name]

Enable OpenSSL hardware-based crypto engine functionality.

If
engine-name

is specified,
use a specific crypto engine. Use the
--show-engines

standalone option to list the crypto engines which are
supported by OpenSSL.

--no-replay

(Advanced) Disable OpenVPN‘s protection against replay attacks.
Don‘t use this option unless you are prepared to make
a tradeoff of greater efficiency in exchange for less
security.

OpenVPN provides datagram replay protection by default.

Replay protection is accomplished
by tagging each outgoing datagram with an identifier
that is guaranteed to be unique for the key being used.
The peer that receives the datagram will check for
the uniqueness of the identifier. If the identifier
was already received in a previous datagram, OpenVPN
will drop the packet. Replay protection is important
to defeat attacks such as a SYN flood attack, where
the attacker listens in the wire, intercepts a TCP
SYN packet (identifying it by the context in which
it occurs in relation to other packets), then floods
the receiving peer with copies of this packet.

OpenVPN‘s replay protection is implemented in slightly
different ways, depending on the key management mode
you have selected.

In Static Key mode
or when using an CFB or OFB mode cipher, OpenVPN uses a
64 bit unique identifier that combines a time stamp with
an incrementing sequence number.

When using TLS mode for key exchange and a CBC cipher
mode, OpenVPN uses only a 32 bit sequence number without
a time stamp, since OpenVPN can guarantee the uniqueness
of this value for each key. As in IPSec, if the sequence number is
close to wrapping back to zero, OpenVPN will trigger
a new key exchange.

To check for replays, OpenVPN uses
the
sliding window

algorithm used
by IPSec.

--replay-window n [t]

Use a replay protection sliding-window of size
n

and a time window of
t

seconds.

By default
n

is 64 (the IPSec default) and
t

is 15 seconds.

This option is only relevant in UDP mode, i.e.
when either
--proto udp

is 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 protection altogether).

(b)

If a packet arrives out of order, it will only be accepted if the difference
between its sequence number and the highest sequence number received
so far is less than
n.

(c)

If a packet arrives out of order, it will only be accepted if it arrives no later
than
t

seconds after any packet containing a higher sequence number.

If you are using a network link with a large pipeline (meaning that
the product of bandwidth and latency is high), you may want to use
a larger value for
n.

Satellite links in particular often require this.

If you run OpenVPN at
--verb 4,

you will see the message "Replay-window backtrack occurred [x]"
every time the 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 number window.

OpenVPN adds to the IPSec model by limiting the window size in time as well as
sequence space.

OpenVPN also adds TCP transport as an option (not offered by IPSec) in which
case OpenVPN can adopt a very strict 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 session
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 function,
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 isolate problems in the compiler, OpenSSL
crypto library, or OpenVPN‘s crypto code. Since it is a self-test mode,
problems with encryption and authentication can be debugged independently
of network and tunnel issues.

TLS Mode Options:

TLS mode is the most powerful crypto mode of OpenVPN in both security and flexibility.
TLS mode works by establishing 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 forward secrecy.

To use TLS mode, each peer that runs OpenVPN should have its own local
certificate/key pair (
--cert

and
--key

), signed by the root certificate which is specified
in
--ca.

When two OpenVPN peers connect, each presents its local certificate to the
other. Each peer will then check that its 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 certificate
ca.crt.

For testing purposes only, the OpenVPN distribution includes a sample
CA certificate (ca.crt).
Of course you should never use
the test certificates and test keys distributed with OpenVPN in a
production environment, since by virtue of the fact that
they are distributed with OpenVPN, they are totally insecure.

--capath dir

Directory containing trusted certificates (CAs and CRLs).
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 commercial service such as thawte.com (in which case you will be
helping to finance the world‘s second space tourist :).
To generate a certificate,
you can use a command such as:

openssl req -nodes -new -keyout mycert.key -out mycert.csr

If your certificate authority private key lives on another machine, copy
the certificate signing request (mycert.csr) to this other machine (this can
be done over an insecure channel such as email). Now sign the certificate
with a command such as:

openssl ca -out mycert.crt -in mycert.csr

Now copy the certificate (mycert.crt)
back to the peer which initially generated the .csr file (this
can be over a public medium).
Note that the
openssl ca

command reads the location of the certificate authority key from its
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
certificate chain without trusting them to verify the peer-submitted
certificate, as would be the case if the certs were placed in the
ca

file.

--key file

Local peer‘s private key in .pem format. Use the private key which was generated
when you built your peer‘s certificate (see
-cert file

above).

--tls-version-min version [‘or-highest‘]

Enable TLS version negotiation, and set the minimum
TLS version we will accept from the peer (default is "1.0").
Examples for version
include "1.0", "1.1", or "1.2". If ‘or-highest‘ is specified
and version is not recognized, we will only accept the highest TLS
version supported by the local SSL implementation.

If this options is not set, the code in OpenVPN 2.3.4 will default
to using TLS 1.0 only, without any version negotiation. This reverts
the beaviour to what OpenVPN versions up to 2.3.2 did, as it turned
out that TLS version negotiation can lead to handshake problems due
to new signature algorithms in TLS 1.2.

--tls-version-max version

Set the maximum TLS version we will use (default is the highest version
supported). Examples for version include "1.0", "1.1", or "1.2".

--pkcs12 file

Specify a PKCS #12 file containing local private key,
local certificate, and root CA certificate.
This option can be used instead of
--ca, --cert,

and
--key.

Not available with 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 following:

(default) -- Try to determine automatically.

-- Use sign.

-- Use sign recover.

-- Use decrypt.

-- Use unwrap.

--cryptoapicert select-string

Load the certificate and private key from the
Windows Certificate System Store (Windows/OpenSSL Only).

Use this option instead of
--cert

and
--key.

This makes
it possible to use any smart card, supported by Windows, but also any
kind of certificate, residing in the Cert Store, where you have access to
the private key. This option has been tested with a couple of different
smart cards (GemSAFE, Cryptoflex, and Swedish Post Office eID) on the
client side, and also an imported PKCS12 software certificate on the
server side.

To select a certificate, based on a substring search in the
certificate‘s subject:

cryptoapicert

"SUBJ:Peter Runestig"

To select a certificate, based on certificate‘s thumbprint:

cryptoapicert

"THUMB:f6 49 24 41 01 b4 ..."

The thumbprint hex string can easily be copy-and-pasted from the Windows
Certificate Store GUI.

--key-method m

Use data channel key negotiation method
m.

The key method must match on both sides of the connection.

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

seconds (default=2). When OpenVPN sends a control
packet to its peer, it will expect to receive an
acknowledgement within
n

seconds or it will retransmit the packet, subject
to a TCP-like exponential backoff algorithm. This parameter
only applies to control channel packets. Data channel
packets (which carry encrypted tunnel data) are never
acknowledged, sequenced, or 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 connection
is still using the default value of 3600 seconds, meaning that the renegotiation will
still occur once per 3600 seconds. The solution is to increase --reneg-sec on both the
client and server, or set it to 0 on one side of the connection (to disable), and to
your chosen value on the other side.

--hand-window n

Handshake Window -- the TLS-based key exchange must finalize within
n

seconds
of handshake initiation by any peer (default = 60 seconds).
If the handshake fails
we will attempt to reset our connection with our peer and try again.
Even in the event of handshake failure we will still use
our expiring key for up to
--tran-window

seconds to maintain continuity of transmission of tunnel
data.

--tran-window n

Transition window -- our old key can live this many seconds
after a new a key renegotiation begins (default = 3600 seconds).
This feature allows for a graceful transition from old to new
key, and removes the key renegotiation 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 packets 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 signature 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,
including packets which are sent before the TLS level has had a chance
to authenticate the peer.
The result is that packets without
the correct signature can be dropped immediately upon reception,
before they have a chance to consume additional system resources
such as by initiating a TLS handshake.
--tls-auth

can be strengthened by adding the
--replay-persist

option which will keep OpenVPN‘s replay protection state
in a file so that it is not lost across restarts.

It should be emphasized that this feature is optional and that the
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 private keys.

If
file

is specified, read the password from the first line of
file.

Keep in mind that storing your password in a file
to a certain extent invalidates the extra security provided by
using an encrypted key (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.

cmd

When
cmd

is executed two arguments are appended after any arguments specified in
cmd

, as follows:

cmd certificate_depth subject

These arguments are, respectively, the current certificate depth and
the X509 common name (cn) of the peer.

This feature is useful if the peer you want to trust has a certificate
which was signed by a certificate authority who also signed many
other certificates, where you don‘t necessarily want to trust all of them,
but rather be 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 [ext:]fieldname

Field in the X.509 certificate subject to be used as the username (default=CN).
Typically, this option is specified with
fieldname

as either of the following:

--x509-username-field

emailAddress

--x509-username-field ext:subjectAltName

The first example uses the value of the "emailAddress" attribute in the
certificate‘s Subject field as the username. The second example uses
the
ext:

prefix to signify that the X.509 extension
fieldname

"subjectAltName" be searched for an rfc822Name (email) field to be used
as the username. In cases where there are multiple email addresses
in
ext:fieldname,

the last occurrence is chosen.

When this option is used, the
--verify-x509-name

option will match against the chosen
fieldname

instead of the Common Name.

Please note:

This option has a feature which will convert an all-lowercase
fieldname

to uppercase characters, e.g., ou -> OU. A mixed-case
fieldname

or one having the
ext:

prefix will be left as-is. This automatic upcasing feature
is deprecated and will be removed in a future release.

--tls-remote name (DEPRECATED)

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 connections 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 associated
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.

Please also note:

This option is now deprecated. It will be removed either in OpenVPN v2.4
or v2.5. So please make sure you support the new X.509 name formatting
described with the
--compat-names

option as soon as possible by updating your configurations to use
--verify-x509-name

instead.

--verify-x509-name name type

Accept connections only if a host‘s X.509 name is equal to
name.

The remote host must also pass all other tests of verification.

Which X.509 name is compared to
name

depends on the setting of type.
type

can be "subject" to match the complete subject DN (default),
"name" to match a subject RDN or "name-prefix" to match a subject RDN prefix.
Which RDN is verified as name depends on the
--x509-username-field

option. But it defaults to the common name (CN), e.g. a certificate with a
subject DN "C=KG, ST=NA, L=Bishkek, CN=Server-1" would be matched by:

--verify-x509-name ‘C=KG, ST=NA, L=Bishkek, CN=Server-1‘

and
--verify-x509-name Server-1 name

or you could use
--verify-x509-name Server- name-prefix

if you want a client to only accept connections to "Server-1", "Server-2", etc.

--verify-x509-name

is a useful replacement for the
--tls-verify

option to verify the remote host, because
--verify-x509-name

works in a
--chroot

environment without any dependencies.

Using a name prefix is a useful alternative to managing
a CRL (Certificate Revocation List) on the client, since it allows the client
to refuse all certificates except for those associated
with designated servers.

NOTE:

Test against a name prefix only when you are using OpenVPN with
a custom CA certificate that is under your control.
Never use this option with type "name-prefix" when your client certificates
are signed by a third party, such as a commercial web CA.

--x509-track attribute

Save peer X509
attribute

value in environment for use by plugins and management interface.
Prepend a ‘+‘ to
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 designated server.

See the easy-rsa/build-key-server script for an example
of how to generate a certificate with the
nsCertType

field set to "server".

If the server certificate‘s nsCertType field is set
to "server", then the clients can verify this with
--ns-cert-type server.

This is an important security precaution to protect against
a man-in-the-middle attack where an authorized client
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, --verify-x509-name,

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 designated 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 designated server.

The extended key usage should be encoded in oid notation, or
OpenSSL symbolic representation.

--remote-cert-tls client|server

Require that peer certificate was signed with an explicit
key usage

and
extended key usage

based on RFC3280 TLS rules.

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

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 ).

or
--tls-verify.

--crl-verify crl [‘dir‘]

Check peer certificate against the file
crl

in PEM format.

A CRL (certificate revocation list) is used when a particular key is
compromised but when the overall PKI is still intact.

Suppose you had a PKI consisting of a CA, root certificate, and a number of
client certificates. Suppose a laptop computer containing a client key and
certificate was stolen. By adding the stolen certificate to the CRL file,
you could reject any connection which attempts to use it, while preserving the
overall integrity of the PKI.

The only time when it would be necessary to rebuild the entire PKI from scratch would be
if the root certificate key itself was compromised.

If the optional
dir

flag is specified, enable a different mode where
crl

is a directory containing files named as 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 setting the TCP/IP properties since it uses the well-known
DHCP protocol. There are, however, two prerequisites for using
this mode: (1) The TCP/IP properties for the TAP-Win32
adapter must be set to "Obtain an IP address automatically," and
(2) OpenVPN needs to claim an IP address in the subnet for use
as the virtual DHCP server address. By default in
--dev tap

mode, OpenVPN will
take the normally unused first address in the subnet. For example,
if your subnet is 192.168.4.0 netmask 255.255.255.0, then
OpenVPN will take the IP address 192.168.4.0 to use as the
virtual DHCP server address. In
--dev tun

mode, OpenVPN will cause the DHCP server to masquerade as if it were
coming from the remote endpoint. The optional offset parameter is
an integer which is > -256 and < 256 and which defaults to 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 masquerade 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 prevents routes involving the TAP-Win32 adapter from
being lost when the system goes to sleep. The default
lease time is one year.

netsh --

Automatically set the IP address and netmask using
the Windows command-line "netsh"
command. This method appears to work correctly on
Windows XP but not Windows 2000.

ipapi --

Automatically set the IP address and netmask using the
Windows IP Helper API. This approach
does not have ideal semantics, though testing has indicated
that it works okay in practice. If you use this option,
it is best to leave the TCP/IP properties for the TAP-Win32
adapter in their default state, i.e. "Obtain an IP address
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 [email protected])

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 connection 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 service wrapper
when a given OpenVPN configuration is being run as a service.

exit-event

is the name of a Windows global event object, and OpenVPN will continuously
monitor the state of this event object and exit when it becomes signaled.

The second parameter indicates the initial state of
exit-event

and normally defaults to 0.

Multiple OpenVPN processes can be simultaneously executed with the same
exit-event

parameter. In any case, the 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 necessary 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. All options are modeled after their IPv4
counterparts, so more detailed explanations given there apply here
as well (except for
--topology

, which has no effect on IPv6).

--ifconfig-ipv6 ipv6addr/bits ipv6remote

configure IPv6 address
ipv6addr/bits

on the ``tun‘‘ device. The second parameter is used as route target for
--route-ipv6

if no gateway is specified.

--route-ipv6 ipv6addr/bits [gateway] [metric]

setup IPv6 routing in the system to send the specified IPv6 network
into OpenVPN‘s ``tun‘‘ 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. Due to implementation details,
the pool size must be between /64 and /112.

--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 OpenVPN‘s
internal routing table.

--auth-user-pass-verify

Executed in
--mode server

mode on new client connections, when the client is
still untrusted.

String Types and Remapping

In certain cases, OpenVPN will perform remapping of characters
in strings. Essentially, any characters outside the set of
permitted characters for each string type will be converted
to underbar (‘_‘).

Why is string remapping necessary?

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.

Can string remapping be disabled?

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 (‘@‘) character has
been added as well for compatibility with the common name character class.

Environmental variable names:

Alphanumeric or underbar (‘_‘).

Environmental variable values:

Any printable character.

For all cases, characters in a string which are not members of the legal
character class for that string type will be remapped to underbar (‘_‘).

Environmental Variables

Once set, a variable is persisted
indefinitely until it is reset by a new value or a restart,

As of OpenVPN 2.0-beta12, in server mode, environmental
variables set by OpenVPN
are scoped according to the client objects
they are
associated with, so there should not be any issues with
scripts having access to stale, previously set variables
which refer to different client instances.

bytes_received

Total number of bytes received from client during VPN session.
Set prior to execution of the
--client-disconnect

script.

bytes_sent

Total number of bytes sent to client during VPN session.
Set prior to execution of the
--client-disconnect

script.

common_name

The X509 common name of an authenticated client.
Set prior to execution of
--client-connect, --client-disconnect,

and
--auth-user-pass-verify

scripts.

config

Name of first
--config

file.
Set on program initiation and reset on SIGHUP.

daemon

Set to "1" if the
--daemon

directive is specified, or "0" otherwise.
Set on program initiation and reset on SIGHUP.

daemon_log_redirect

Set to "1" if the
--log

or
--log-append

directives are specified, or "0" otherwise.
Set on program initiation and reset on SIGHUP.

dev

The actual name of the TUN/TAP device, including
a unit number if it exists.
Set prior to
--up

or
--down

script execution.

foreign_option_{n}

An option pushed via
--push

to a client which does not natively support it,
such as
--dhcp-option

on a non-Windows system, will be recorded to this
environmental variable sequence prior to
--up

script execution.

ifconfig_broadcast

The broadcast address for the virtual
ethernet segment which is derived from the
--ifconfig

option when
--dev tap

is used.
Set prior to OpenVPN calling the
ifconfig

or
netsh

(windows version of ifconfig) commands which
normally 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
normally occurs prior to
--up

script execution.

ifconfig_remote

The remote VPN endpoint IP address specified in the
--ifconfig

option (second parameter) when
--dev tun

is used.
Set prior to OpenVPN calling the
ifconfig

or
netsh

(windows version of ifconfig) commands which
normally occurs prior to
--up

script execution.

ifconfig_netmask

The subnet mask of the virtual ethernet segment
that is specified as the second parameter to
--ifconfig

when
--dev tap

is being used.
Set prior to OpenVPN calling the
ifconfig

or
netsh

(windows version of ifconfig) commands which
normally occurs prior to
--up

script execution.

ifconfig_pool_local_ip

The local
virtual IP address for the TUN/TAP tunnel taken from an
--ifconfig-push

directive if specified, or 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 specified, or otherwise from
the ifconfig pool (controlled by the
--ifconfig-pool

config file directive).
Only set for
--dev tap

tunnels.
This option is set on the server prior to execution
of the
--client-connect

and
--client-disconnect

scripts.

ifconfig_pool_remote_ip

The remote
virtual IP address for the TUN/TAP tunnel taken from an
--ifconfig-push

directive if specified, or 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 execution only when the
via-env

modifier is specified, and deleted from the environment
after the script returns.

proto

The
--proto

parameter.
Set on program initiation and reset on SIGHUP.

remote_{n}

The
--remote

parameter.
Set on program initiation and reset on SIGHUP.

remote_port_{n}

The remote port number, specified by
--port

or
--rport.

Set on program initiation and reset on SIGHUP.

route_net_gateway

The pre-existing default IP gateway in the system routing
table.
Set prior to
--up

script execution.

route_vpn_gateway

The default gateway used by
--route

options, as specified in either the
--route-gateway

option or the second parameter to
--ifconfig

when
--dev tun

is specified.
Set prior to
--up

script execution.

route_{parm}_{n}

A set of variables which define each route to be added, and
are set prior to
--up

script execution.

parm

will be one of "network", "netmask", "gateway", or "metric".

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).

is the OpenVPN route number, starting from 1.

If the network or gateway are resolvable DNS names,
their IP address translations will be recorded rather
than their names as denoted on the command line
or configuration file.

peer_cert

Temporary file name containing the client certificate upon
connection. Useful in conjunction with --tls-verify

script_context

Set to "init" or "restart" prior to up/down script execution.
For more information, see
documentation for
--up.

script_type

Prior to execution of any script, this variable is set to the type of
script being run. It can be one of the 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

(controlled by
--inactive

option),
ping-exit

(controlled by
--ping-exit

option),
ping-restart

(controlled 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 execution of the
--client-disconnect

script.

time_unix

Client connection timestamp, formatted as a unix integer
date/time value.
Set prior to execution of the
--client-connect

script.

tls_digest_{n}

Contains the certificate SHA1 fingerprint/digest hash value,
where
n

is the verification level. Only set for TLS connections. Set prior
to execution of
--tls-verify

script.

tls_id_{n}

A series of certificate fields from the remote peer,
where
n

is the verification level. Only set for TLS 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 decimal string like "933971680", which is
suitable for doing serial-based OCSP queries (with OpenSSL, do not
prepend "0x" to the string) If something goes wrong while reading
the value from the certificate it will be an empty string, so your
code should check that.
See the contrib/OCSP_check/OCSP_check.sh script for an example.

tls_serial_hex_{n}

Like
tls_serial_{n},

but in hex form (e.g. "12:34:56:78:9A").

tun_mtu

The MTU of the TUN/TAP device.
Set prior to
--up

or
--down

script execution.

trusted_ip (or trusted_ip6)

Actual IP address of connecting client or peer which has been authenticated.
Set prior to execution of
--ipchange, --client-connect,

and
--client-disconnect

scripts.
If using ipv6 endpoints (udp6, tcp6),
trusted_ip6

will be set instead.

trusted_port

Actual port number of connecting client or peer which has been authenticated.
Set prior to execution of
--ipchange, --client-connect,

and
--client-disconnect

scripts.

untrusted_ip (or untrusted_ip6)

Actual IP address of connecting client or peer which has not been authenticated
yet. Sometimes used to
nmap

the 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 endpoints (udp6, tcp6),
untrusted_ip6

will be set instead.

untrusted_port

Actual port number of connecting client or peer which has not been authenticated
yet.
Set prior to execution of
--tls-verify

and
--auth-user-pass-verify

scripts.

username

The username provided by a connecting client.
Set prior to
--auth-user-pass-verify

script execution only when the
via-env

modifier is specified.

X509_{n}_{subject_field}

An X509 subject field from the remote peer certificate,
where
n

is the verification level. Only set for TLS connections. Set prior
to execution of
--tls-verify

script. This variable is similar to
tls_id_{n}

except the component X509 subject fields are broken out, and
no string remapping occurs on these field values (except for remapping
of control characters to "_").
For example, the following variables would be set on the
OpenVPN server using the sample client certificate
in sample-keys (client.crt).
Note that the verification level is 0 for the client certificate
and 1 for the CA certificate.

X509_0_emailAddress=[email protected]
X509_0_CN=Test-Client
X509_0_O=OpenVPN-TEST
X509_0_ST=NA
X509_0_C=KG
X509_1_emailAddress=[email protected]
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 authenticated 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 stdout otherwise).

SIGINT, SIGTERM

Causes OpenVPN to exit gracefully.

TUN/TAP DRIVER SETUP

If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP driver already installed. If so, there are still a few things you need to do:

Make device: mknod /dev/net/tun c 10 200

Load driver: modprobe tun

EXAMPLES

Prior to running these examples, you should have OpenVPN installed on two machines with network connectivity between them. If you have not yet installed OpenVPN, consult the INSTALL file included in the OpenVPN distribution.

TUN/TAP Setup:

If you are using Linux 2.4 or higher, make the tun device node and load the tun module:

: mknod /dev/net/tun c 10 200

: modprobe tun

If you installed from RPM, the mknod step may be omitted, because the RPM install does that for you.

Only Linux 2.4 and newer are supported.

For other platforms, consult the INSTALL file at http://openvpn.net/install.html for more information.

Firewall Setup:

If firewalls exist between the two machines, they should be set to forward UDP port 1194 in both directions. If you do not have control over the firewalls between the two machines, you may still be able to use OpenVPN by adding --ping 15 to each of the openvpn commands used below in the examples (this will cause each peer to send out a UDP ping to its remote peer once every 15 seconds which will cause many stateful firewalls to forward packets in both directions without an explicit firewall rule).

If you are using a Linux iptables-based firewall, you may need to enter the following command to allow incoming packets on the TUN device:

: iptables -A INPUT -i tun+ -j ACCEPT

See the firewalls section below for more information on configuring firewalls for use with OpenVPN.

VPN Address Setup:

For purposes of our example, our two machines will be called may.kg and june.kg. If you are constructing 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 meaning in the context of the VPN. Each machine will use the tunnel endpoint of the other machine to access it over the VPN. In our example, the tunnel endpoint for 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 endpoint 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 subnet 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 certificates 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 negotiation.

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 seconds. If n is less than the stateful firewall connection timeout, you can maintain an OpenVPN connection indefinitely without explicit firewall rules.

You should also add firewall rules to allow incoming IP traffic on TUN or TAP devices such as:

: iptables -A INPUT -i tun+ -j ACCEPT

to allow input packets from tun devices,

: iptables -A FORWARD -i tun+ -j ACCEPT

to allow input packets from tun devices to be forwarded to other hosts on the local network,

: iptables -A INPUT -i tap+ -j ACCEPT

to allow input packets from tap devices, and

: iptables -A FORWARD -i tap+ -j ACCEPT

to allow input packets from tap devices to be forwarded to other hosts on the local network.

These rules are secure if you use packet authentication, since no incoming packets will arrive on a TUN or TAP virtual 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 <[email protected]>.

NOTES

This product includes software developed by the OpenSSL Project ( http://www.openssl.org/ )

For more information on the TLS protocol, see http://www.ietf.org/rfc/rfc2246.txt

For more information on the LZO real-time compression library see http://www.oberhumer.com/opensource/lzo/

COPYRIGHT

Copyright (C) 2002-2010 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.