Can two applications listen to the same port?

Question

Can two applications on the same machine bind to the same port and IP address? Taking it a step further, can one app listen to requests coming from a certain IP and the other to another remote IP? I know I can have one application that starts off two threads (or forks) to have similar behavior, but can two applications that have nothing in common do the same?

Answer 1

In principle, no.

It's not written in stone; but it's the way all APIs are written: the app opens a port, gets a handle to it, and the OS notifies it (via that handle) when a client connection (or a packet in UDP case) arrives.

If the OS allowed two apps to open the same port, how would it know which one to notify?

But... there are ways around it:

1. As Jed noted, you could write a 'master' process, which would be the only one that really listens on the port and notifies others, using any logic it wants to separate client requests.
   • On Linux and BSD (at least) you can set up 'remapping' rules that redirect packets from the 'visible' port to different ones (where the apps are listening), according to any network related criteria (maybe network of origin, or some simple forms of load balancing).

Answer 2

Yes Definitely. As far as i remember From kernel version 3.9 (Not sure on the version) onwards support for the SO_REUSEPORT was introduced. SO_RESUEPORT allows binding to the exact same port and address, As long as the first server sets this option before binding its socket.

It works for both TCP and UDP. Refer to the link for more details: SO_REUSEPORT

Note: Accepted answer no longer holds true as per my opinion.

Answer 3

You can have one application listening on one port for one network interface. Therefore you could have:

1. httpd listening on remotely accessible interface, e.g. 192.168.1.1:80
2. another daemon listening on 127.0.0.1:80

Sample use case could be to use httpd as a load balancer or a proxy.

Answer 4

Yes.

1. Multiple listening TCP sockets, all bound to the same port, can co-exist, provided they are all bound to different local IP addresses. Clients can connect to whichever one they need to. This excludes 0.0.0.0 (INADDR_ANY).

2. Multiple accepted sockets can co-exist, all accepted from the same listening socket, all showing the same local port number as the listening socket.

3. Multiple UDP sockets all bound to the same port can all co-exist provided either the same condition as at (1) or they have all had the SO_REUSEADDR option set before binding.

4. TCP ports and UDP ports occupy different namespaces, so the use of a port for TCP does not preclude its use for UDP, and vice versa.

Reference: Stevens & Wright, TCP/IP Illustrated, Volume II.