OpenVPN
Copyright (C) 2002-2021 OpenVPN Inc <sales@openvpn.net>

  OpenVPN has been written to try to avoid features
  that are not standardized well across different
  OSes, so porting OpenVPN itself will probably be
  straightforward if a tun or tap driver already exists.

  Where special OS features are used, they are usually
  bracketed with #ifdef HAVE_SOME_FUNCTION.

PLATFORM STATUS:

  * Linux 2.2+ (supported)
  * Solaris (supported)
  * OpenBSD 3.0 (supported but pthreads are broken)
  * Max OS X Darwin
  * FreeBSD
  * NetBSD
  * Windows
  * 64 bit platforms -- I have heard reports that
    OpenVPN runs on Alpha Linux and FreeBSD.
  * ARM -- I have heard of at least one case
    where OpenVPN was successfully built and
    run on the ARM architecture.

PORTING NOTES:

  * Make sure that OpenSSL will build on your
    platform.
  * Make sure that a tun or tap virtual device
    driver exists for your platform.  See
    http://vtun.sourceforge.net/tun/ for examples
    of tun and tap drivers that have been written
    for Linux, Solaris, and FreeBSD.
  * Make sure you have autoconf 2.50+ and
    automake 1.6+.
  * Edit configure.ac, adding platform specific
    config code, and a TARGET_YOUROS define.
  * Add platform-specific includes to syshead.h.
  * Add an #ifdef TARGET_YOUROS to the do_ifconfig()
    function in tun.c to generate a correct "ifconfig"
    command for your platform.  Note that OpenVPN
    determines the ifconfig path at ./configure time.
  * Add an ifconfig_order() variant for your OS so
    openvpn knows whether to call ifconfig before
    or after tun/tap dev open.
  * Add an #ifdef TARGET_YOUROS block in tun.c and define
    the open_tun, close_tun, read_tun, and write_tun
    functions.  If your tun/tap virtual device is
    sufficiently generic, you may be able to use the
    default case.
  * Add appropriate code to route.c to handle
    the route command on your platform.  This
    is necessary for the --route option to
    work correctly.
  * After you successfully build OpenVPN, run
    the loopback tests as described in INSTALL.
  * For the next test, confirm that the UDP socket
    functionality is working independently of the
    tun device, by doing something like:
      ./openvpn --remote localhost --verb 9 --ping 1 --dev null
  * Now try with --remote [a real host]
  * Now try with a real tun/tap device, you will
    need to figure out the appropriate ifconfig
    command to use once openvpn has opened the tun/tap
    device.
  * Once you have simple tests working on the tun device,
    try more complex tests such as using TLS mode.
  * Stress test the link by doing ping -f across it.
  * Make sure that packet fragmenting is happening
    correctly by doing a ping -s 2000 or higher.
  * Ensure that OpenVPN on your platform will talk
    to OpenVPN on other platforms such as Linux.
    Some tun/tap driver implementations will prepend
    unnecessary stuff onto the datagram that must be
    disabled with an explicit ioctl call if cross-platform
    compatibility is to be preserved.  You can see some
    examples of this in tun.c.
  * If your system supports pthreads, try building
    with ./configure --enable-pthread and do a stress
    test in TLS mode.
  * Try the ultimate stress test which is --gremlin
    --reneg-sec 10 in TLS mode (preferably with pthreads
    enabled), then do a flood ping across the tunnel
    (ping -f remote-endpoint) in both directions and let
    it run overnight.  --gremlin will induce massive
    corruption and packet loss, but you win if you
    wake up the next morning and both peers are still
    running and occasionally even succeeding in their
    attempted once-per-10-seconds TLS handshake. 
  * When it's working, submit your patch to
    <openvpn-devel@lists.sourceforge.net>
    and rejoice :)