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jamieb
Asked: 2011-10-09 19:30:17 +0800 CST

(Linux) Can multiple processes bind to the same TCP port on a per-use basis?

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As part of a pilot project, I am attempting to set-up a thin client environment for a team of developers using NoMachine. Each developer will login to the same Linux box and do development via an X session. Currently, each developer runs their own HTTP daemon on their local workstation that listens on 127.0.0.1:5000. However, if I move everyone onto the same machine this obviously creates a problem with port conflicts.

Ideally, I'd like to keep their workflow the same. If I have to assign everyone a unique port, it's just going to create a lot of grief and confusion. Is there a way to do this? Can different processes bind to the same port on a per-user basis? I discovered a way to use iptables to do port redirection on a per-user basis, but this only solves part the problem:

iptables -t nat -I OUTPUT --src 0/0 --dst 127.0.0.1 -p tcp --dport 5000 -m owner --uid-owner userA -j REDIRECT --to-ports 5001

This solution still doesn't allow different processes to bind to the same port. And I'm not even sure that I'm on the right track here by looking for an iptables solution. Any suggestions? Is there maybe a hack that be applied in userland? Thanks!

linux iptables tcp

4 Answers

  1. Best Answer

    They can't bind to the same port.

    Bind each process to its own port, and then dispatch INPUT port 5000 to 5001, 5002, 5003 depending on your conditions.

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  2. TCP server addresses

    The address of a listening TCP server consists of (IP, port) (for IP in IPv4 or IPv6), where IP can be the wild-card IP "*", that is the address 0.

    A TCP client will connect to a specific (server_IP, server_port) pair (no wild-card here). There must be exactly one listening socket with either:

    • this exact (server_IP, server_port) address
    • or the wild-card IP, and the same port; ie the listening socket will have the address (0, server_port)

    The TCP stack does not want to have to make arbitrary choices between sockets, so it will not allow the creation of two different listening TCP sockets at the same time which could accept the same connexion attempt from a TCP client.

    Usable address space

    The good new is that the space for IPv4 local-host addresses is quite large: exactly 224 - 2 = **16777214 different IPv4 addresses are reserved for this job, from 127.0.0.1 to 127.255.255.254.

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  3. How many people developers do you have? If it is not that many, how about setting up KVM (or Xen/VMware) on the server and have them use their own virtual machine (each with it's own IP, of course).

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  4. You can use a different IP for each user. Most of the programs allow to bind on different IPs on the same port. You will need to add those IPs to a network interface.

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