freeport.py

#!/usr/bin/env python

import sys

ports = set()
for s in ("tcp","udp","tcp6","udp6"):
    with open("/proc/net/%s" % s) as f:
        for l in f.readlines()[1:]:
            ports.add(int(l.split(":")[2][:4],16))

ports = sorted(ports)

from_port = int((sys.argv[1:2]+[1,])[0])
max_port = int((sys.argv[2:3]+[2**16-1,])[0])
free_port = from_port
while free_port in ports and from_port <= max_port:
    free_port += 1

if free_port < max_port:
    print free_port