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IPv6 in OpenVPN

This page describes IPv6 support in OpenVPN.

Overview

Starting officially in the 2.3.0 release, OpenVPN supports IPv6 inside the tunnel, and can optionally be configured with IPv6 as a transport protocol for the tunneled data. There were some unofficial developer patches for the 2.2.x series that added partial IPv6 support (Debian in particular chose to integrate these patches into some of their builds.)

Providing IPv6 outside the tunnel

To connect to your server over ipv6 (ipv6 transport) use this on both sides:

proto udp6

Providing IPv6 inside the tunnel

This section walks through providing IPv6 connectivity inside the tunnel; this will discuss a routed setup; a bridged (dev tap) setup is not recommended in general, and users doing so are presumably advanced enough to know what they're doing.

Requirements

A few things must be met in order to use IPv6:

  • An existing and functional OpenVPN configuration (use the official howto if you don't yet have this.)
  • Both client and server must support IPv6; most modern systems these-days include this support already

Additionally:

  • Recommended A routed IPv6 network block that will reach the host configured as the OpenVPN server
  • alternatively, check section "Splitting a single routable IPv6 netblock" below

Details: IPv6 routed block

In a routed setup, you must use a unique routed network range, just like when routing with IPv4. Most ISPs should have a facility to obtain a routed block on request. It is recommended to use a /64 for your OpenVPN subnet.

Assuming the OpenVPN server has:

  • IPv6 IP of 2001:db8:0:abc::100/64 on its LAN interface
  • Routed block: 2001:db8:0:123::/64

Additional OpenVPN config

Config stanza using the helper

Add the following to a functioning OpenVPN config:

server-ipv6 2001:db8:0:123::/64

Config stanza with expanded directives

Add the following to a functioning OpenVPN config:

tun-ipv6
push tun-ipv6
ifconfig-ipv6 2001:db8:0:123::1 2001:db8:0:123::2

Pushing IPv6 routes

Pushing routes over the tunnel works much like in IPv4:

push "route-ipv6 2001:db8:0:abc::/64"
push "route-ipv6 2000::/3"

Splitting a single routable IPv6 netblock

Typically /64 IPv6 netblocks are assigned, leaving a large address space.

Split netblock configuration

Assume the original IPv6 netblock on your OpenVPN server is 2001:db8:0:123::/64.

  1. Check your NIC uses no addresses in the upper /65 block.
  2. Re-assign the new restricted netblock – lower part.
  3. Assign the higher part of the restricted netblock to OpenVPN.
  4. Restart the VPN.

Client issues

Android 4.4.x

Android 4.4.x has a known bug related to the tun0 interface which affects IPv6 connectivity.

iOS 9

iOS 9 broke redirect-gateway if used with IPv6 tunnels and no IPv4 traffic goes inside the tunnel. To workaround, use:

redirect-gateway ipv6

This option works only on Android and iOS OpenVPN Connect clients (OpenVPN 3) and OpenVPN 2.4 (development version) and has no effect for OpenVPN 2.3.