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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?
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It is possible. You just have to bind on the right IP address/interface each service using the same port. Ports (be them UDP or TCP) have their own pool per IP address. You can listen on the same port if you change: IP address or protocol (UDP or TCP).
Only one app can listen on a socket in a sane way. The OS allows multiple apps to listen on the same port if you specify special options but that's insane. Accepted connections are then dispatched to different applications in an unspecified (i.e. random) way.
Answers. Yes, you can. If you use the same SSL certificate for both Web Applications, you simply bind it appropriately.
A connected socket is assigned to a new (dedicated) port, so it means that the number of concurrent connections is limited by the number of ports, unless multiple sockets can share the same port.
You can only have one application listening on the same port at one time. Now if you had 2 network cards, you could have one application listen on the first IP and the second one on the second IP using the same port number. For UDP (Multicasts), multiple applications can subscribe to the same port.
Now if you had 2 network cards, you could have one application listen on the first IP and the second one on the second IP using the same port number. For UDP (Multicasts), multiple applications can subscribe to the same port.
If you have a listening socket in an application process and you fork that process, the socket will be inherited, so technically there will be now two processes listening the same port. Show activity on this post. And even though I have not made a connection to the socket, I cannot listen twice on the same port, in spite of the reuseaddr option.
You could accomplish this by writing a "master" process, which accepts and processes all connections, then hands them off to your two applications who need to listen on the same port. This is the approach that Web servers and such take, since many processes need to listen to 80.
The answer differs depending on what OS is being considered. In general though:
For TCP, no. You can only have one application listening on the same port at one time. Now if you had 2 network cards, you could have one application listen on the first IP and the second one on the second IP using the same port number.
For UDP (Multicasts), multiple applications can subscribe to the same port.
Edit: Since Linux Kernel 3.9 and later, support for multiple applications listening to the same port was added using the SO_REUSEPORT option. More information is available at this lwn.net article.
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