Discovering free ports

Question

I wrote an server application in erlang and a client in C#. They communicate through 3 TCP ports. Port numbers are hardcoded. Now I'd like to do this dynamically. This is my first time doing network programming, so please pardon my inability to use proper terminology :-D

What I would like to do is make a supervisor which would accept a TCP connection from a client on a previously known port (say, 10000, or whatever), then find 3 free ports, start a server application on those 3 ports and tell the client those port numbers so client can connect to the server.

My particular problem is: how do I find 3 ports which are not in use? (clarification: which module:fun() to use to find a free port?)

My general problem is: I'm sure this kind of stuff (one server allocating ports and redirecting clients) is quite common network programming problem and there should be a bunch of (erlang-specific or general) resources about this, but I just don't have the terminology to google it out.

Answer 1

According to the Erlang documentation here, if the Port argument to the gen_tcp:listen/2 function is 0, then the OS will assign any available port to the socket. The latter can then be retreived using inet:port/1 .

You can therefore do something like this :

{ok, Listen} = gen_tcp:listen(0, [Options]),
Port = inet:port(Listen).

Answer 2

just in case you didn't know that - you dont have to allocate new ports for each client, it's perfectly fine to have all clients to connect to same ports

UPDATE:

if there is a reason to allocate new ports for incoming clients then it's far beyond your first "introduction to network programming" program.

separate ports could mean you want to completely isolate environments of different groups of clients. it's comparable to providing completely different IP addresses to connect to. if you want to write a simple ping-pong program - you don't need it. and i honestly believe you will never need to use such solution in your whole life - that's how incredibly rarely it is.

regarding cpu/ports overhead - allocating ports and starting a server that listens to that port is already far bigger overhead than accepting clients on same port.