pptp uses very few linux-specific features, and should be easily
portable to most Unix systems.

Two things to watch:
  1)
  The code in pty.[ch] to find a free pty/tty pair.  This was
  conceptually borrowed from the xterm sources, which need to
  do more or less the same thing.  *But* the xterm sources had
  a heck of a lot more system-specific #defines to cover all
  the eccentric unix variants out there.  If you are porting
  this code to a non-unix system, I would recommend downloading
  the xterm sources to find out how pty.[ch] should look for
  your system.  Xterm is in the standard X distribution, or
  you can download SRRMs from ftp.redhat.com.

  Configure the xterm sources for your machine, and then preprocess
  main.c with these configuration options.  E.g. I did:

    [cananian@cananian xterm-sb_right-ansi-3d]# make main.o
    rm -f main.o
    gcc -c -O2 -fno-strength-reduce -m486     -I/usr/X11R6/include
    -Dlinux -D__i386__ -D_POSIX_SOURCE -D_BSD_SOURCE -D_SVID_SOURCE
    -DX_LOCALE  -DFUNCPROTO=15 -DNARROWPROTO    -DUTMP -DUSE_TTY_GROUP
    -DOSMAJORVERSION=2             -DOSMINORVERSION=0 main.c

  So the appropriate preprocessing command would be:

    [cananian@cananian xterm-sb_right-ansi-3d]# gcc -E -O2
    -fno-strength-reduce -m486 -I/usr/X11R6/include -Dlinux -D__i386__
    -D_POSIX_SOURCE -D_BSD_SOURCE -D_SVID_SOURCE -DX_LOCALE
    -DFUNCPROTO=15 -DNARROWPROTO -DUTMP -DUSE_TTY_GROUP
    -DOSMAJORVERSION=2 -DOSMINORVERSION=0 main.c > main.CPP 

  Grok through the sources to figure out how the ptys are allocated on
  your machine.  I suspect many people will want to look carefully at
  the function pty_search(), but there are architectures that have
  other, built-in, functions for doing the same thing.

  Add the code to pty.[ch] with the proper #ifdefs, mail a patch back
  to me at <cananian@alumni.princeton.edu> and you're good to go!

  2) The pptp_gre_copy function opens an IP protocol socket with:

     s = socket(AF_INET, SOCK_RAW, PPTP_PROTO);

  where PPTP_PROTO is #define'd in the code to be 47.  I *think* that
  this should work across Unix variants, but if your system has a
  different method for opening a non-TCP, raw-IP-protocol-47 socket,
  then you'll have to make some changes here, and perhaps in
  decaps_gre and encaps_gre as well.

OK. Those are the only two potential non-portabilities I can think
of.  I should really be using automake/autoconf, of course, as well.
  --Scott 15Dec1997
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C. Scott Ananian <cananian@alumni.princeton.edu>