Pietro Battiston

I often connect to Internet from a connection which blocks, by default, outgoing connections except for some given ports.

For instance, port 80 (http) is open, as are 443 (https), 995 (pop3s) and a few others… but port 22 (ssh) is closed. So if I want to be able to connect via ssh to a server of mine, I must configure the server to listen (also) on a different port… non blocked. Configuring the server to listen on multiple ports is trivial, this page is devoted to the problem of understanding which port can be used.

My rudimentary solution is composed of 3 parts:

1. a Python script for opening ports, which must be ran on the server
2. a Python script for probing ports, which must be ran from the client
3. (optional) a PHP and a Python scripts for driving the server script in a way which is accessible from the blocked connection: via http

The script just runs a dummy service on each port for N (provided as argument) to N+257 (I don't check more than 257 ports at the same time, to not incur in the restrictions imposed on the maximum number of opened files).

  #! /usr/bin/python
  
  import socket, time, sys
  
  try:
      start = int( sys.argv[1] )
  except (IndexError, ValueError):
      print "usage: %s first_port" % sys.argv[0]
      sys.exit(1)
  
  sockets = []
  
  log = open('log.txt', 'w')
  
  for port in range(start, start + 257):
      s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
      try:
      # You should probably edit the following line!
          s.bind( (socket.gethostname(), port) )
          s.listen(1)
      except Exception as exc:
          print port, exc
      sockets.append( s )
  
  time.sleep(3600)
  
  log.close()