How to Choose a Port Number? - network-programming

Question

I'm writing a program which uses ZeroMQ to communicate with other running programs on the same machine. I want to choose a port number at run time to avoid the possibility of collisions. Here is an example of a piece of code I wrote to accomplish this.
#!/usr/bin/perl -Tw
use strict;
use warnings;
my %in_use;
{
local $ENV{PATH} = '/bin:/usr/bin';
%in_use = map { $_ => 1 } split /\n/, qx(
netstat -aunt |\
awk '{print \$4}' |\
grep : |\
awk -F: '{print \$NF}'
);
}
my ($port) = grep { not $in_use{$_} } 50_000 .. 59_999;
print "$port is available\n";
The procedure is:
invoke netstat -aunt
parse the result
choose the first port on a fixed range which doesn't appear on netstat list.
Is there a system utility better suited to accomplishing this?

Answer 1

context = zmq.Context()
socket = context.socket(zmq.ROUTER)
port_selected = socket.bind_to_random_port('tcp://*', min_port=6001, max_port=6004, max_tries=100)

Answer 2

First of all, from your code it looks like you are trying to choose a port between 70000 and 79999. You do know that port numbers only go up to 65535, right? :-)
You can certainly do it this way, even though there are a couple of problems with the approach. The first problem is that netstat output differs between different operating systems so it's hard to do it portably. The second problem is that you still need to wrap the code in a loop which tries again to find a new port number in case it was not possible to bind to the chosen port number, because there's a race condition between ascertaining that the port is free and actually binding to it.
If the library you are using allows you to specify the port number as 0 and allows you to call getsockname() on the socket after it is bound, then you should just do that. Using 0 makes the system choose any free port number, and with getsockname() you can find out which port it chose.
Failing that, it would probably actually be more efficient to not bother calling netstat and just try to find to different port numbers in a loop. If you succeed, break from the loop. If you fail, increment the port number by 1, go back, and try again.