Generate deterministic *nix ports for services

Allocate ports.md

Example usage:

$ python gimme_port.py
usage: gimme_port.py [-r|--resolve] NAME

$ python gimme_port.py muhc.ca.http
29595

$ python gimme_port.py staging.muhc.ca.http
port 31079 is in use by 'nc' (pid: 28569), use --resolve

$ python gimme_port.py --resolve staging.muhc.ca.http
42820

gimme_port.py

#!/usr/bin/env python
# exit codes: 0 success, 1 bad usage, 2 port in use

import sys
import subprocess
import hashlib
import os

# upper and lower bounds on generated port numbers
PORT_LOWER = 2000
PORT_UPPER = 65535

def err_usage():
    sys.stderr.write('usage: %s [-r|--resolve] NAME\n' % sys.argv[0])
    sys.exit(1)


def err_port_in_use(port, pid, prog):
    msg = "port %d is in use by '%s' (pid: %s), use --resolve\n" % (port, prog, pid)
    sys.stderr.write(msg)
    sys.exit(2)


# takes a string and converts it to a valid *nix port.
def port_gen(name):
    md5_int = int(hashlib.md5(name).hexdigest()[:8], 16)
    return (md5_int % (PORT_UPPER - PORT_LOWER)) + PORT_LOWER


# gives a dict (of ints to strings) containing the ports corresponding to
# all UDP and listening TCP sockets currently in use. Values are pid/programs as
# reported by netstat.
def occupied_ports():
    # netstat options:
    # --ip: address family (must proceed other options). includes raw, udp and tcp sockets.
    # -l, --listening: show only listening sockets.  (These are omitted by default.)
    # -n, --numeric: do not try to resolve socket addresses (address and port number of local end).
    # -p, --program: include a column including PID/PROGRAM for each socket.
    lines = subprocess.check_output(['netstat', '--ip', '-lnp']).split('\n')[2:]
    records = [line.split() for line in lines]
    ports = {int(record[3].split(':')[-1]): record[-1] for record in records if len(record)}
    return ports


def main(name, resolve=False):
    in_use = occupied_ports()
    port = port_gen(name)
    if port not in in_use:
        return port
    else:
        if resolve:
            max_port = max(in_use) if in_use else PORT_LOWER
            return max_port + 10
        else:
            [pid, prog] = in_use[port].split('/')
            err_port_in_use(port, pid, prog)

if __name__ == '__main__':
    if os.getuid() != 0:
        sys.stderr.write('user is not root; sudo please!\n')
        sys.exit(1)
    args = sys.argv[1:]
    if len(args) < 1:
        err_usage()
    resolve = False
    if args[0] == '-r' or args[0] == '--resolve':
        args.pop(0)
        resolve = True
    if len(args) != 1:
        err_usage()
    print(main(args[0], resolve=resolve))