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OpenVPN lwipovpn tun/tap emulator

Overview

This is a small helper tool based on lwIP - A Lightweight TCP/IP stack to emulate a tun/tap device in userspace without having an effect on the system that the lwip is running on.

This allows:

  • testing a VPN connection without root
  • pinging the connected client
  • running tests against the client and the lwip stack
  • using lwip example applications
    • lwip http server
    • lwip iperf server
    • ...
  • better automated testing

Features

  • IPv4 Support
  • IPv6 Support
  • automatic configuration of IPv4 and IPv6 addresses
  • tap or tun emulation

Enabled apps

lwip comes a number of demo/default apps. lwipovpn has enabled most of them to be helpful in testing. Further apps can be enabled/implemented to allow even more testing.

Limitations

  • Data through is limited. Iperf performance is at 20 MBit/s. This is not a problem for the indented purpose of this tool which is for testing.
  • Windows port is currently missing. It should be possible to use pipes instead of a socketpair with AF_UNIX/SOCK_DGRAM to support Windows. This might also require adding a length header to ensure that always full packets are read/written.
  • LWIP does not have a netmask for IPv6 addresses. Addresses are always assumed to be /64
  • Routes are ignored and lwipovpn assumes everything to be reachable via OpenVPN

Git checkout

Be sure that lwipovpn has been checked with submodule recursion enabled like this:

git clone --recursive https://github.com/openvpn/lwipovpn

or by enabling them in already checkout clone by using the git submodule commands:

cd lwipovpn
git submodule init
git submodule update

Building

This project uses the CMake build system.

cmake -B lwipovpnbuild -S lwipovpn
cmake --build lwipovpnbuild

This should result in a lwipovpnbuild/lwipovpn binary that can be used with OpenVPN master and OpenVPN 2.7.x like this:

openvpn --config client.ovpn --dev-node unix:lwipovpnbuild/lwipovpn

Implementation

The OpenVPN process will call (socketpair(AF_UNIX, SOCK_DGRAM, 0, fds) for connection between OpenVPN and lwipovpn and execute ovpnlwip and pass the second fd to the process. The rest of the configuration is passed as environment variables.

  • TUNTAP_SOCKET_FD: the fd number of the AF_UNIX socket
  • TUN_UNIXAF_PATH: alternative to TUNTAP_SOCKET_FD containing a unix domain socket file path.
  • TUNTAP_DEV_TYPE: the type of device to emulate: tap or tun
  • TUNTAP_MTU: MTU of the emulated devices
  • ifconfig_gateway: Gateway address. Derived from route-gateway in OpenVPN
  • ifconfig_local, ifconfig_netmask, ifconfig_netmask, ifconfig_ipv6_local, ifconfig_ipv6_netbits as described in the openvpn manual page.

Both the OpenVPN --dev-node unix: and the lwipovpn can be used for other purposes but especially lwipovpn is currently very OpenVPN specific as it uses the OpenVPN environment variables names.

License

This tool is under the same (3-Clause BSD License)[COPYING] as lwIP itself.