The port 0 trick

The port 0 trick came in handy when writing eximunit, and it’s something surprisingly few developers know about, so I thought it worth recounting here:

The problem: you want to set up a web/mail/whatever server programmatically (e.g. as part of some tests). This server wants to bind to port 80/25/whatever. Your first problem is that it can’t bind to these because you’re not running your tests as root (or as an administrator on Windows).

The lazy approach at this point is to hard-code a port number over 1024, which you don’t have to be privileged to bind to. But this all falls to bits if you want to run the same test simultaneously on the same machine, or you need lots of different ports during the course of one test.

At this point, you can reach for the port 0 trick: on both Windows and Linux, if you bind a socket to port 0, the kernel will assign it a free port number somewhere above 1024. Truly well-written software (e.g. Jetty) will not only let you configure it to bind to port 0, but will make it easy to parse its logs to obtain the actual port number it got assigned. Less helpful software (Tomcat) will let you configure it to bind to port 0, but print 0 in all its logs, never the actual number. And the majority of software just won’t let you put 0 as a port number in its configuration.

At this point (subject to a slight race condition), you can grab some port numbers yourself and feed them to whatever you’re trying to configure:

def findFreePorts(howMany=1):
    """Return a list of n free port numbers on localhost"""
    results = []
    sockets = []
    for x in range(howMany):
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.bind(('localhost', 0))
        # work out what the actual port number it's bound to is
        addr, port = s.getsockname()
        results.append(port)
        sockets.append(s)

    for s in sockets:
        s.close()

    return results

The above is written in Python, but it translates trivially to any programming language which knows what a socket is.

You’ll note that the correct way to get, say, five free port numbers is to call the above method once with 5 as its argument. If you wrote a simple method which just returned one number, there would be nothing to stop it returning the same number each time you called it (the Linux kernel is rather more helpful, and will usually hand out a different number to each port 0 request, but I wouldn’t  rely on this).