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
- Add a random string (6 characters) to first DNS label of
hostname to prevent DNS caching. For example, "foo.bar.gov"
would be modified to "<random-chars>.foo.bar.gov".
- <connection>
- Define a client connection profile. Client connection
profiles are groups of OpenVPN options that describe how to connect to a
given OpenVPN server. Client connection profiles are specified within an
OpenVPN configuration file, and each profile is bracketed by
<connection> and </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, float, http-proxy,
http-proxy-option, http-proxy-retry,
http-proxy-timeout, local, lport, nobind,
port, proto, remote, rport, socks-proxy,
and socks-proxy-retry.
A defaulting mechanism exists for specifying options to apply to all
<connection> profiles. If any of the above options (with the
exception of remote ) appear outside of a <connection>
block, but in a configuration file which has one or more
<connection> blocks, the option setting will be used as a
default for <connection> blocks which follow it in the
configuration file.
For example, suppose the nobind option were placed in the sample
configuration file above, near the top of the file, before the first
<connection> block. The effect would be as if nobind
were declared in all <connection> blocks below it.
- --proto-force p
- When iterating through connection profiles, only consider
profiles using protocol p ('tcp'|'udp').
- --remote-random
- When multiple --remote address/ports are specified,
or if connection profiles are being used, initially randomize the order of
the list as a kind of basic load-balancing measure.
- --proto p
- Use protocol p for communicating with remote host.
p can be udp, tcp-client, or tcp-server.
The default protocol is udp when --proto is not specified.
For UDP operation, --proto udp should be specified on both peers.
For TCP operation, one peer must use --proto tcp-server and the other
must use --proto tcp-client. A peer started with tcp-server
will wait indefinitely for an incoming connection. A peer started with
tcp-client will attempt to connect, and if that fails, will sleep
for 5 seconds (adjustable via the --connect-retry option) and try
again infinite or up to N retries (adjustable via the
--connect-retry-max option). Both TCP client and server will
simulate a SIGUSR1 restart signal if either side resets the connection.
OpenVPN is designed to operate optimally over UDP, but TCP capability is
provided for situations where UDP cannot be used. In comparison with UDP,
TCP will usually be somewhat less efficient and less robust when used over
unreliable or congested networks.
This article outlines some of problems with tunneling IP over TCP:
http://sites.inka.de/sites/bigred/devel/tcp-tcp.html
There are certain cases, however, where using TCP may be advantageous from a
security and 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).
- --auto-proxy
- Try to sense HTTP or SOCKS proxy settings automatically. If
no settings are present, a direct connection will be attempted. If both
HTTP and SOCKS settings are present, HTTP will be preferred. If the HTTP
proxy server requires a password, it will be queried from stdin or the
management interface. If the underlying OS doesn't support an API for
returning proxy settings, a direct connection will be attempted.
Currently, only Windows clients support this option via the
InternetQueryOption API. This option exists in OpenVPN 2.1 or higher.
- --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]
- Connect to remote host through a Socks5 proxy at address
server and port port (default=1080).
- --socks-proxy-retry
- Retry indefinitely on Socks proxy errors. If a Socks proxy
error occurs, simulate a SIGUSR1 reset.
- --resolv-retry n
- If hostname resolve fails for --remote, retry
resolve for n seconds before failing.
Set n to "infinite" to retry indefinitely.
By default, --resolv-retry infinite is enabled. You can disable by
setting n=0.
- --float
- Allow remote peer to change its IP address and/or port
number, such as due to DHCP (this is the default if --remote is not
used). --float when specified with --remote allows an
OpenVPN session to initially connect to a peer at a known address, however
if packets arrive from a new address and pass all authentication tests,
the new address will take control of the session. This is useful when you
are connecting to a peer which holds a dynamic address such as a dial-in
user or DHCP client.
Essentially, --float tells OpenVPN to accept authenticated packets
from any address, not only the address which was specified in the
--remote option.
- --ipchange cmd
- Execute shell command cmd when our remote ip-address
is initially authenticated or changes.
Execute as:
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.
Note that cmd can be a shell command with multiple arguments, in
which case all OpenVPN-generated arguments will be appended to cmd
to build a command line which will be passed to the script.
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.
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
"nil".
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
- Execute shell command cmd after routes are added,
subject to --route-delay.
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
Note that cmd can be a shell command with multiple arguments.
- --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.
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.
- --redirect-gateway flags...
- (Experimental) Automatically execute routing commands to
cause all outgoing IP traffic to be redirected over the VPN.
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.
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).
Using the def1 flag is highly recommended.
- --redirect-private [flags]
- Like --redirect-gateway, but omit actually changing the
default gateway. Useful when pushing private subnets.
- --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.
- --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.
- --socket-flags flags...
- Apply the given flags to the OpenVPN transport socket.
Currently, only TCP_NODELAY is supported.
The TCP_NODELAY socket flag is useful in TCP mode, and causes the
kernel to send tunnel packets immediately over the TCP connection without
trying to group several smaller packets into a larger packet. This can
result in a considerably improvement in latency.
This option is pushable from server to client, and should be used on both
client and server for maximum effect.
- --txqueuelen n
- (Linux only) Set the TX queue length on the TUN/TAP
interface. Currently defaults to 100.
- --shaper n
- Limit bandwidth of outgoing tunnel data to n bytes
per second on the TCP/UDP port. If you want to limit the bandwidth in both
directions, use this option on both peers.
OpenVPN uses the following algorithm to implement traffic shaping: Given a
shaper rate of n bytes per second, after a datagram write of
b bytes is queued on the TCP/UDP port, wait a minimum of (b /
n) seconds before queuing the next write.
It should be noted that OpenVPN supports multiple tunnels between the same
two peers, allowing you to construct full-speed and reduced bandwidth
tunnels at the same time, routing low-priority data such as off-site
backups over the reduced bandwidth tunnel, and other data over the
full-speed tunnel.
Also note that for low bandwidth tunnels (under 1000 bytes per second), you
should probably use lower MTU values as well (see above), otherwise the
packet latency will grow so large as to 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.
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
- Shell command to run after successful TUN/TAP device open
(pre --user UID change). The up script 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 cmd can be a shell command with multiple arguments, in
which case all OpenVPN-generated arguments will be appended to cmd
to build a command line which will be passed to the shell.
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
- Shell command to run after TUN/TAP device close (post
--user UID change and/or --chroot ). Called with the same
parameters and environmental variables as the --up option above.
Note that if you reduce privileges by using --user and/or
--group, your --down script will also run at reduced
privilege.
- --down-pre
- Call --down cmd/script before, rather than after,
TUN/TAP close.
- --up-restart
- Enable the --up and --down scripts to be
called for restarts as well as initial program start. This option is
described more fully above in the --up option documentation.
- --setenv name value
- Set a custom environmental variable name=value to
pass to script.
- --setenv FORWARD_COMPATIBLE 1
- Relax config file syntax checking so that unknown
directives will trigger a warning but not a fatal error, on the assumption
that a given unknown directive might be valid in future OpenVPN versions.
This option should be used with caution, as there are good security reasons
for having OpenVPN fail if it detects problems in a config file. Having
said that, there are valid reasons for wanting new software features to
gracefully degrade when encountered by older software versions.
- --setenv-safe name value
- Set a custom environmental variable
OPENVPN_name=value to pass to script.
This directive is designed to be pushed by the server to clients, and the
prepending of "OPENVPN_" to the environmental variable is a
safety precaution to prevent a LD_PRELOAD style attack from a malicious or
compromised server.
- --script-security level [method]
- This directive offers policy-level control over OpenVPN's
usage of external programs and scripts. Lower level values are more
restrictive, higher values are more permissive. Settings for level:
0 -- Strictly no calling of external programs.
1 -- (Default) Only call built-in executables such as ifconfig, ip,
route, or netsh.
2 -- Allow calling of built-in executables and user-defined scripts.
3 -- Allow passwords to be passed to scripts via environmental
variables (potentially unsafe).
The method parameter indicates how OpenVPN should call external
commands and scripts. Settings for method:
execve -- (default) Use execve() function on Unix family OSes and
CreateProcess() on Windows.
system -- Use system() function (deprecated and less safe since the
external program command line is subject to shell expansion).
The --script-security option was introduced in OpenVPN 2.1_rc9. For
configuration file compatibility with previous OpenVPN versions, use:
--script-security 3 system
- --disable-occ
- Don't output a warning message if option inconsistencies
are detected between peers. An example of an option inconsistency would be
where one peer uses --dev tun while the other peer uses --dev
tap.
Use of this option is discouraged, but is provided as a temporary fix in
situations where a recent version of OpenVPN must connect to an old
version.
- --user user
- Change the user ID of the OpenVPN process to user
after initialization, dropping privileges in the process. This option is
useful to protect the system in the event that some hostile party was able
to gain control of an OpenVPN session. Though OpenVPN's security features
make this unlikely, it is provided as a second line of defense.
By setting user to nobody or somebody similarly unprivileged,
the hostile party would be limited in what damage they could cause. Of
course once you take away privileges, you cannot return them to an OpenVPN
session. This means, for example, that if you want to reset an OpenVPN
daemon with a SIGUSR1 signal (for example in response to a DHCP
reset), you should make use of one or more of the --persist options
to ensure that OpenVPN doesn't need to execute any privileged operations
in order to restart (such as re-reading key files or running
ifconfig on the TUN device).
- --group group
- Similar to the --user option, this option changes
the group ID of the OpenVPN process to group after
initialization.
- --cd dir
- Change directory to dir prior to reading any files
such as configuration files, key files, scripts, etc. dir should be
an absolute path, with a leading "/", and without any references
to the current directory such as "." or "..".
This option is useful when you are running OpenVPN in --daemon mode,
and you want to consolidate all of your OpenVPN control files in one
location.
- --chroot dir
- Chroot to dir after initialization. --chroot
essentially redefines dir as being the top level directory tree
(/). OpenVPN will therefore be unable to access any files outside this
tree. This can be desirable from a security standpoint.
Since the chroot operation is delayed until after initialization, most
OpenVPN options that reference files will operate in a pre-chroot context.
In many cases, the dir parameter can point to an empty directory,
however complications can result when scripts or restarts are executed
after the chroot operation.
- --setcon context
- Apply SELinux context after initialization. This
essentially provides the ability to restrict OpenVPN's rights to only
network I/O operations, thanks to SELinux. This goes further than
--user and --chroot in that those two, while being great
security features, unfortunately do not protect against privilege
escalation by exploitation of a vulnerable system call. You can of course
combine all three, but please note that since setcon requires access to
/proc you will have to provide it inside the chroot directory (e.g. with
mount --bind).
Since the setcon operation is delayed until after initialization, OpenVPN
can be restricted to just network-related system calls, whereas by
applying the context before startup (such as the OpenVPN one provided in
the SELinux Reference Policies) you will have to allow many things
required only during initialization.
Like with chroot, complications can result when scripts or restarts are
executed after the setcon operation, which is why you should really
consider using the --persist-key and --persist-tun
options.
- --daemon [progname]
- Become a daemon after all initialization functions are
completed. This option will cause all message and error output to be sent
to the syslog file (such as /var/log/messages), except for the output of
shell scripts and ifconfig commands, which will go to /dev/null unless
otherwise redirected. The syslog redirection occurs immediately at the
point that --daemon is parsed on the command line even though the
daemonization point occurs later. If one of the --log options is
present, it will supercede syslog redirection.
The optional progname parameter will cause OpenVPN to report its
program name to the system logger as progname. This can be useful
in linking OpenVPN messages in the syslog file with specific tunnels. When
unspecified, progname defaults to "openvpn".
When OpenVPN is run with the --daemon option, it will try to delay
daemonization until the majority of initialization functions which are
capable of generating fatal errors are complete. This means that
initialization scripts can test the return status of the openvpn command
for a fairly reliable indication of whether the command has correctly
initialized and entered the packet forwarding event loop.
In OpenVPN, the vast majority of errors which occur after initialization are
non-fatal.
- --syslog [progname]
- Direct log output to system logger, but do not become a
daemon. See --daemon directive above for description of
progname parameter.
- --passtos
- Set the TOS field of the tunnel packet to what the
payload's TOS is.
- --inetd [wait|nowait] [progname]
- Use this option when OpenVPN is being run from the inetd or
xinetd(8) server.
The wait/nowait option must match what is specified in the
inetd/xinetd config file. The nowait mode can only be used with
--proto tcp-server. The default is wait. The nowait
mode can be used to instantiate the OpenVPN daemon as a classic TCP
server, where client connection requests are serviced on a single port
number. For additional information on this kind of configuration, see the
OpenVPN FAQ: http://openvpn.net/faq.html#oneport
This option precludes the use of --daemon, --local, or
--remote. Note that this option causes message and error output to
be handled in the same way as the --daemon option. The optional
progname parameter is also handled exactly as in --daemon.
Also note that in wait mode, each OpenVPN tunnel requires a separate
TCP/UDP port and a separate inetd or xinetd entry. See the OpenVPN 1.x
HOWTO for an example on using OpenVPN with xinetd:
http://openvpn.net/1xhowto.html
- --log file
- Output logging messages to file, including output to
stdout/stderr which is generated by called scripts. If file already
exists it will be truncated. This option takes effect immediately when it
is parsed in the command line and will supercede syslog output if
--daemon or --inetd is also specified. This option is
persistent over the entire course of an OpenVPN instantiation and will not
be reset by SIGHUP, SIGUSR1, or --ping-restart.
Note that on Windows, when OpenVPN is started as a service, logging occurs
by default without the need to specify this option.
- --log-append file
- Append logging messages to file. If file does
not exist, it will be created. This option behaves exactly like
--log except that it appends to rather than truncating the log
file.
- --suppress-timestamps
- Avoid writing timestamps to log messages, even when they
otherwise would be prepended. In particular, this applies to log messages
sent to stdout.
- --writepid file
- Write OpenVPN's main process ID to file.
- --nice n
- Change process priority after initialization ( n
greater than 0 is lower priority, n less than zero is higher
priority).
- --fast-io
- (Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding
a call to poll/epoll/select prior to the write operation. The purpose of
such a call would normally be to block until the device or socket is ready
to accept the write. Such blocking is unnecessary on some platforms which
don't support write blocking on UDP sockets or TUN/TAP devices. In such
cases, one can optimize the event loop by avoiding the poll/epoll/select
call, improving CPU efficiency by 5% to 10%.
This option can only be used on non-Windows systems, when --proto udp
is specified, and when --shaper is NOT specified.
- --multihome
- Configure a multi-homed UDP server. This option can be used
when OpenVPN has been configured to listen on all interfaces, and will
attempt to bind client sessions to the interface on which packets are
being received, so that outgoing packets will be sent out of the same
interface. Note that this option is only relevant for UDP servers and
currently is only implemented on Linux.
Note: clients connecting to a --multihome server should always use
the --nobind option.
- --echo [parms...]
- Echo parms to log output.
Designed to be used to send messages to a controlling application which is
receiving the OpenVPN log output.
- --remap-usr1 signal
- Control whether internally or externally generated SIGUSR1
signals are remapped to SIGHUP (restart without persisting state) or
SIGTERM (exit).
signal can be set to "SIGHUP" or "SIGTERM". By
default, no remapping occurs.
- --verb n
- Set output verbosity to n (default=1). Each level
shows all info from the previous levels. Level 3 is recommended if you
want a good summary of what's happening without being swamped by output.
0 -- No output except fatal errors.
1 to 4 -- Normal usage range.
5 -- Output R and W characters to the console for each
packet read and write, uppercase is used for TCP/UDP packets and lowercase
is used for TUN/TAP packets.
6 to 11 -- Debug info range (see errlevel.h for additional
information on debug levels).
- --status file [n]
- Write operational status to file every n
seconds.
Status can also be written to the syslog by sending a SIGUSR2
signal.
- --status-version [n]
- Choose the status file format version number. Currently
n can be 1, 2, or 3 and defaults to 1.
- --mute n
- Log at most n consecutive messages in the same
category. This is useful to limit repetitive logging of similar message
types.
- --comp-lzo [mode]
- Use fast LZO compression -- may add up to 1 byte per packet
for incompressible data. mode may be "yes",
"no", or "adaptive" (default).
In a server mode setup, it is possible to selectively turn compression on or
off for individual clients.
First, make sure the client-side config file enables selective compression
by having at least one --comp-lzo directive, such as --comp-lzo
no. This will turn off compression by default, but allow a future
directive push from the server to dynamically change the on/off/adaptive
setting.
Next in a --client-config-dir file, specify the compression setting
for the client, for example:
comp-lzo yes
push "comp-lzo yes"
The first line sets the comp-lzo setting for the server side of the
link, the second sets the client side.
- --comp-noadapt
- When used in conjunction with --comp-lzo, this
option will disable OpenVPN's adaptive compression algorithm. Normally,
adaptive compression is enabled with --comp-lzo.
Adaptive compression tries to optimize the case where you have compression
enabled, but you are sending predominantly uncompressible (or
pre-compressed) packets over the tunnel, such as an FTP or rsync transfer
of a large, compressed file. With adaptive compression, OpenVPN will
periodically sample the compression process to measure its efficiency. If
the data being sent over the tunnel is already compressed, the compression
efficiency will be very low, triggering openvpn to disable compression for
a period of time until the next re-sample test.
- --management IP port [pw-file]
- Enable a TCP server on IP:port to handle daemon
management functions. pw-file, if specified, is a password file
(password on first line) or "stdin" to prompt from standard
input. The password provided will set the password which TCP clients will
need to provide in order to access management functions.
The management interface can also listen on a unix domain socket, for those
platforms that support it. To use a unix domain socket, specify the unix
socket pathname in place of IP and set port to 'unix'. While
the default behavior is to create a unix domain socket that may be
connected to by any process, the --management-client-user and
--management-client-group directives can be used to restrict
access.
The management interface provides a special mode where the TCP management
link can operate over the tunnel itself. To enable this mode, set
IP = "tunnel". Tunnel mode will cause the management
interface to listen for a TCP connection on the local VPN address of the
TUN/TAP interface.
While the management port is designed for programmatic control of OpenVPN by
other applications, it is possible to telnet to the port, using a telnet
client in "raw" mode. Once connected, type "help" for
a list of commands.
For detailed documentation on the management interface, see the
management-notes.txt file in the management folder of the OpenVPN
source distribution.
It is strongly recommended that IP be set to 127.0.0.1 (localhost) to
restrict accessibility of the management server to local clients.
- --management-client
- Management interface will connect as a TCP client to
IP:port specified by --management rather than listen as a
TCP server.
- --management-query-passwords
- Query management channel for private key password and
--auth-user-pass username/password. Only query the management
channel for inputs which ordinarily would have been queried from the
console.
- --management-forget-disconnect
- Make OpenVPN forget passwords when management session
disconnects.
This directive does not affect the --http-proxy username/password. It
is always cached.
- --management-hold
- Start OpenVPN in a hibernating state, until a client of the
management interface explicitly starts it with the hold release
command.
- --management-signal
- Send SIGUSR1 signal to OpenVPN if management session
disconnects. This is useful when you wish to disconnect an OpenVPN session
on user logoff.
- --management-log-cache n
- Cache the most recent n lines of log file history
for usage by the management channel.
- --management-client-auth
- Gives management interface client the responsibility to
authenticate clients after their client certificate has been verified. See
management-notes.txt in OpenVPN distribution for detailed notes.
- --management-client-pf
- Management interface clients must specify a packet filter
file for each connecting client. See management-notes.txt in OpenVPN
distribution for detailed notes.
- --management-client-user u
- When the management interface is listening on a unix domain
socket, only allow connections from user u.
- --management-client-group g
- When the management interface is listening on a unix domain
socket, only allow connections from group g.
- --plugin module-pathname [init-string]
- Load plug-in module from the file module-pathname,
passing init-string as an argument to the module initialization
function. Multiple plugin modules may be loaded into one OpenVPN process.
For more information and examples on how to build OpenVPN plug-in modules,
see the README file in the plugin folder of the OpenVPN source
distribution.
If you are using an RPM install of OpenVPN, see /usr/share/openvpn/plugin.
The documentation is in doc and the actual plugin modules are in
lib.
Multiple plugin modules can be cascaded, and modules can be used in tandem
with scripts. The modules will be called by OpenVPN in the order that they
are declared in the config file. If both a plugin and script are
configured for the same callback, the script will be called last. If the
return code of the module/script controls an authentication function (such
as tls-verify, auth-user-pass-verify, or client-connect), then every
module and script must return success (0) in order for the connection to
be authenticated.
Server Mode¶
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and
can be enabled with the
--mode server option. In server mode, OpenVPN
will listen on a single port for incoming client 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
- 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"
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.
- --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
- 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.
This option must be associated with a specific client instance, which means
that it must be specified either in a client instance config file using
--client-config-dir or dynamically generated using a
--client-connect script.
Remember also to include a --route directive in the main OpenVPN
config file which encloses local, so that the kernel will know to
route it to the server's TUN/TAP interface.
OpenVPN's internal client IP address selection algorithm works as follows:
1 -- Use --client-connect script generated file for static IP
(first choice).
2 -- Use --client-config-dir file for static IP (next choice).
3 -- Use --ifconfig-pool allocation for dynamic IP (last
choice).
- --iroute network [netmask]
- Generate an internal route to a specific client. The
netmask parameter, if omitted, defaults to 255.255.255.255.
This directive can be used to route a fixed subnet from the server to a
particular client, regardless of where the client is connecting from.
Remember that you must also add the route to the system routing table as
well (such as by using the --route directive). The reason why two
routes are needed is that the --route directive routes the packet
from the kernel to OpenVPN. Once in OpenVPN, the --iroute directive
routes to the specific client.
This option must be specified either in a client instance config file using
--client-config-dir or dynamically 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 script
- Run script on client connection. The script is
passed the common name and IP address of the just-authenticated client as
environmental variables (see environmental variable section below). The
script is also passed the pathname of a freshly created temporary file as
$1 (i.e. the first command line argument), to be used by the script 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
$1.
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
- 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 script 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.
- --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.
- --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 script or shell command cmd to validate client
virtual addresses or routes.
cmd will be executed with 3 parameters:
[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 script method
- Require the client to provide a username/password (possibly
in addition to a client certificate) for authentication.
OpenVPN will execute script as a shell command to validate the
username/password provided by the client.
If method is set to "via-env", OpenVPN will call
script with the environmental variables username and
password set to the username/password strings provided by the
client. Be aware that this method is insecure on some platforms which make
the environment of a process publicly visible to other unprivileged
processes.
If method is set to "via-file", OpenVPN will write the
username and password to the first two lines of a temporary file. The
filename will be passed as an argument to script, and the file will
be automatically deleted by OpenVPN after the script returns. The location
of the temporary file is controlled by the --tmp-dir option, and
will default to the current directory if 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.
- --no-name-remapping
- Allow Common Name, X509 Subject, and username strings to
include any printable character including space, but excluding control
characters such as tab, newline, and carriage-return.
By default, OpenVPN will remap any character other than alphanumeric,
underbar ('_'), dash ('-'), dot ('.'), and slash ('/') to underbar ('_').
The X509 Subject string as returned by the tls_id environmental
variable, can additionally contain colon (':') or equal ('=').
While name remapping is performed for security reasons to reduce the
possibility of introducing string expansion security vulnerabilities in
user-defined authentication scripts, this option is provided for those
cases where it is desirable to disable the remapping feature. Don't use
this option unless you know what you are doing!
- --port-share host port
- 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.
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.
- --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.
- --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 specified, 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.
- --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
or
openvpn --test-crypto --secret key --verb 9
This option is very useful to test OpenVPN after it has been ported to a new
platform, or to isolate problems in the compiler, OpenSSL crypto library,
or OpenVPN's crypto code. Since it is a self-test mode, problems with
encryption and authentication can be debugged independently of network and
tunnel issues.
TLS Mode Options:¶
TLS mode is the most powerful crypto mode of OpenVPN in both security and
flexibility. TLS mode works by establishing 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.
- --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 ).
- --key file
- Local peer's private key in .pem format. Use the private
key which was generated when you built your peer's certificate (see
-cert file above).
- --pkcs12 file
- Specify a PKCS #12 file containing local private key, local
certificate, and root CA certificate. This option can be used instead of
--ca, --cert, and --key.
- --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:
0 (default) -- Try to determind automatically.
1 -- Use sign.
2 -- Use sign recover.
4 -- Use decrypt.
8 -- Use unwrap.
- --cryptoapicert select-string
- Load the certificate and private key from the Windows
Certificate System Store (Windows 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
- Execute shell 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.
Note that cmd is a command line and as such may (if enclosed in
quotes) contain whitespace separated arguments. The first word of
cmd is the shell command to execute and the remaining words are its
arguments. When cmd is executed two arguments are appended, as
follows:
cmd certificate_depth X509_NAME_oneline
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 fieldname
- Field in x509 certificate subject to be used as username
(default=CN). Fieldname will be uppercased before matching. When
this option is used, the --tls-remote option will match against the chosen
fieldname instead of the CN.
- --tls-remote name
- Accept connections only from a host with X509 name or
common name equal to name. The remote host must also pass all other
tests of verification.
NOTE: Because tls-remote may test against a common name prefix, only
use this option when you are using OpenVPN with a custom CA certificate
that is under your control. Never use this option when your client
certificates are signed by a third party, such as a commercial web CA.
Name can also be a common name prefix, for example if you want a client to
only accept 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.
- --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, --tls-remote, or --tls-verify.
- --remote-cert-ku v...
- Require that peer certificate was signed with an explicit
key usage.
This is a useful security option for clients, to ensure that the host they
connect to is a 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 ).
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
--remote-cert-tls, --tls-remote, or --tls-verify.
- --crl-verify crl
- 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.
- --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|'env'
- 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, the
pathname will be set to "C:\WINDOWS"
The special string 'env' indicates that the pathname should be read
from the SystemRoot environmental variable.
- --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
NeonSurge@abyss.com)
DISABLE-NBT -- Disable Netbios-over-TCP/IP.
Note that if --dhcp-option is pushed via --push to a
non-windows client, the option will be saved in the client's environment
before the up script is called, under the name
"foreign_option_{n}".
- --tap-sleep n
- Cause OpenVPN to sleep for n seconds immediately
after the TAP-Win32 adapter state is set to "connected".
This option is intended to be used to troubleshoot problems with the
--ifconfig and --ip-win32 options, and is used to give the
TAP-Win32 adapter time to come up before Windows IP Helper API operations
are applied to it.
- --show-net-up
- Output OpenVPN's view of the system routing table and
network adapter list to the syslog or log file after the TUN/TAP adapter
has been brought up and any routes have been added.
- --dhcp-renew
- Ask Windows to renew the TAP adapter lease on startup. This
option is normally unnecessary, as Windows automatically triggers a DHCP
renegotiation on the TAP adapter when it comes up, however if you set the
TAP-Win32 adapter Media Status property to "Always Connected",
you may need this flag.
- --dhcp-release
- Ask Windows to release the TAP adapter lease on shutdown.
This option has the same caveats as --dhcp-renew above.
- --register-dns
- Run net stop dnscache, net start dnscache, ipconfig
/flushdns and ipconfig /registerdns on 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.
The following options exist to support IPv6 tunneling in peer-to-peer and
client-server mode. As of now, this is just very basic documentation of the
IPv6-related options. More documentation can be found on
http://www.greenie.net/ipv6/openvpn.html.
- --ifconfig-ipv6 ipv6addr/bits ipv6remote
- configure IPv6 address ipv6addr/bits on the ``tun''
device. The second parameter is used as route target for
--route-ipv6 if no gateway is specified.
- --route-ipv6 ipv6addr/bits [gateway] [metric]
- setup IPv6 routing in the system to send the specified IPv6
network into OpenVPN's ``tun'' device
- --server-ipv6 ipv6addr/bits
- convenience-function to enable a number of IPv6 related
options at once, namely --ifconfig-ipv6, --ifconfig-ipv6-pool,
--tun-ipv6 and --push tun-ipv6 Is only accepted if ``--mode
server'' or ``--server'' is set.
- --ifconfig-ipv6-pool ipv6addr/bits
- Specify an IPv6 address pool for dynamic assignment to
clients. The pool starts at ipv6addr and increments by +1 for every
new client (linear mode). The /bits setting controls the size of
the pool.
- --ifconfig-ipv6-push ipv6addr/bits ipv6remote
- for ccd/ per-client static IPv6 interface configuration,
see --client-config-dir and --ifconfig-push for more
details.
- --iroute-ipv6 ipv6addr/bits
- for ccd/ per-client static IPv6 route configuration, see
--iroute for more details how to setup and use this, and how
--iroute and --route interact.
SCRIPTING AND ENVIRONMENTAL VARIABLES¶
OpenVPN exports a series of environmental variables for use by user-defined
scripts.
Script Order of Execution¶
- --up
- Executed after TCP/UDP socket bind and TUN/TAP open.
- --tls-verify
- Executed when we have a still untrusted remote peer.
- --ipchange
- Executed after connection authentication, or remote IP
address change.
- --client-connect
- Executed in --mode server mode immediately after
client authentication.
- --route-up
- Executed after connection authentication, either
immediately after, or some number of seconds after as defined by the
--route-delay option.
- --client-disconnect
- Executed in --mode server mode on client instance
shutdown.
- --down
- Executed after TCP/UDP and TUN/TAP close.
- --learn-address
- Executed in --mode server mode whenever an IPv4
address/route or MAC address is added to OpenVPN's internal routing
table.
- --auth-user-pass-verify
- Executed in --mode server mode on new client
connections, when the client is still untrusted.
String Types and Remapping¶
In certain cases, OpenVPN will perform remapping of characters in strings.
Essentially, any characters outside the set of permitted characters for each
string type will be converted to underbar ('_').
Q: Why is string remapping necessary?
A: It's an important security feature to prevent the malicious coding of
strings from untrusted sources to be passed as parameters to scripts, saved in
the environment, used as a common name, translated to a filename, etc.
Q: Can string remapping be disabled?
A: Yes, by using the
--no-name-remapping option, however this
should be considered an advanced option.
Here is a brief rundown of OpenVPN's current string types and the permitted
character class for each string:
X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at
('@'), colon (':'), slash ('/'), and equal ('='). Alphanumeric is defined as a
character which will cause the C library isalnum() function to return true.
Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at
('@').
--auth-user-pass username: Same as Common Name, with one exception:
starting with OpenVPN 2.0.1, the username is passed to the
OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw form, without string
remapping.
--auth-user-pass password: Any "printable" character except CR
or LF. Printable is defined to be a character which will cause the C library
isprint() function to return true.
--client-config-dir filename as derived from common name or username:
Alphanumeric, underbar ('_'), dash ('-'), and dot ('.') except for
"." or ".." as standalone strings. As of 2.0.1-rc6, the at
('@') 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_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".
n is the OpenVPN route number, starting from 1.
If the network or gateway are resolvable DNS names, their IP address
translations will be recorded rather than their names as denoted on the
command line or configuration file.
- peer_cert
- Temporary file name containing the client certificate upon
connection. Useful in conjunction with --tls-verify
- script_context
- Set to "init" or "restart" prior to
up/down script execution. For more information, see documentation for
--up.
- script_type
- Prior to execution of any script, this variable is set to
the type of script being run. It can be one of the following: up, down,
ipchange, route-up, tls-verify, auth-user-pass-verify,
client-connect, client-disconnect, or learn-address.
- 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_id_{n}
- A series of certificate fields from the remote peer, where
n is the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script.
- tls_serial_{n}
- The serial number of the certificate from the remote peer,
where n is the verification level. Only set for TLS connections.
Set prior to execution of --tls-verify script. This is in the form
of a hex string like "37AB46E0", which is suitable for doing
serial-based OCSP queries (with OpenSSL, you have to prepend
"0x" to the string). If something goes wrong while reading the
value from the certificate it will be an empty string, so your code should
check that. See the contrib/OCSP_check/OCSP_check.sh script for an
example.
- tun_mtu
- The MTU of the TUN/TAP device. Set prior to --up or
--down script execution.
- trusted_ip (or trusted_ip6)
- Actual IP address of connecting client or peer which has
been authenticated. Set prior to 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=me@myhost.mydomain
X509_0_CN=Test-Client
X509_0_O=OpenVPN-TEST
X509_0_ST=NA
X509_0_C=KG
X509_1_emailAddress=me@myhost.mydomain
X509_1_O=OpenVPN-TEST
X509_1_L=BISHKEK
X509_1_ST=NA
X509_1_C=KG
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 shell
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 <info@openvpn.net>.
SEE ALSO¶
dhcpcd(8),
ifconfig(8),
openssl(1),
route(8),
scp(1) ssh(1)
NOTES¶
This product includes software developed by the OpenSSL Project (
http://www.openssl.org/ )
For more information on the TLS protocol, see
http://www.ietf.org/rfc/rfc2246.txt
For more information on the LZO real-time compression library see
http://www.oberhumer.com/opensource/lzo/
COPYRIGHT¶
Copyright (C) 2002-2010 OpenVPN Technologies, Inc. This program is free
software; you can redistribute it and/or modify it under the terms of the GNU
General Public License version 2 as published by the Free Software Foundation.
AUTHORS¶
James Yonan <jim@yonan.net>
