How do you decide what port to use?

Question

This is a little subjective, as there are no rules so to speak. Every time I create a server, I think to myself, "What is the best port to use?" I guess an answer is "Any, as long as the user can change it." So, how does everyone else decide how to choose the default port? Personally, I like to use something like 8000-something if it's HTTP related, and I've noticed this is a pretty common trend. But what if 8000 is already in use? Use 8001? It seems a little ad-hoc, and I suppose it is.

Clearly I'm not the first to have asked this question; IANA maintain a port numbers list... Which leads me on to the unassigned range (48620-49150). I guess we should really be using these, but why don't more programmers do so? How do you decide which to use; if everyone started at #1, then we'd all be using 48620.

Egret

+1 for answering your own question – Trehala

Thanks - I wanted to tell everyone my thought process, so that others can call me on it if I was wrong. – Egret

Answer 1

I think you've pretty much answered your question as much as is possible; there isn't really a strict rule you can follow here beyond what you've said. But generally:

• Look at the IANA list and pick a port that's not in use.
• Pick a port number that is easy to remember.
• Don't fix the port number in your code. Some other product may have picked the same port as you and you never know when you'll have to co-exist on a server, so put the port number in a configuration file somewhere so it can be changed without a recompile if necessary. The ability to change port number can also be helpful for getting through firewalls without having to get them reconfigured. (You can always default to your chosen value if the configuration file doesn't exist.)
• There is an argument that you don't want to pick something too high as you may conflict with the range used for ephemeral ports. It's not that likely that you'll get hit by this, but it's a hard problem to debug when it happens.

(And if you want a tip for picking memorable port numbers, I once worked with someone who remembered port numbers based around the telephone extensions of his co-workers.)

Moran

"Look at the IANA list and pick a port that's not in use" or just pick a large number – Kragh

A large number under 49152 obviously, I knew that, really I did – Kragh

Answer 2

Some easy to remember and appropriately nerdy unassigned (per IANA) ports:

27182 (e)

31415 (pi)

60221 (avagadro's)

Necromancy

The top answer here says for ssh you should use a root-reserved port: unix.stackexchange.com/questions/424755/… – Ables

Answer 3

During testing... always port #666 ;)

Emilie

I take it you always run as root? – Mexican

it's reserved for the Doom game! you should respect it – Rambow

Answer 4

How about:

defaultPort = (new Random()).Next(48620, 49150);

Darwin

Answer 5

You answered your own question? Pick any unassigned port and allow the user to change it.

Trehala

Answer 6

I prefer this way: (python code following)

#!/usr/bin/env python3
import random as R
r = R.SystemRandom()
print([r.randrange(1024, 65535) for x in range(4)])

And then I pick the number which I like the most. Or course, change the range if you have some stricter limits of what are acceptable numbers.

Rambow

int.from_bytes(hashlib.sha256(b"my application name").digest()[:4], "little") & 0xFFFF – Runlet

I would not leave it up to random. Could easily pick one that is being used by ephemeral root process launchers from the system at higher ranges. – Goodwife

Answer 7

After a quick Google search to make sure it's clear, I generally just choose a number of personal significance.

Gaskins

Answer 8

I'd suggest never use a port that is a big number like 5 digits, as it might hit some other operation system processes and assigns the Ephemeral ports. You would start to get 'Already in use errors' due to its limitations.

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