TCP - possible for same client-side port to be used for different connections by different applications simlutaneously?

Question

Is it possible in TCP for different processes not sharing the same executable image (so no fork() for example) to use a same client-side port on Windows, Linux or OSX? This is specifically related to the socket options SO_REUSEADDR and SO_REUSEPORT set using setsockopt() I believe.

As far as I've read, I believe it is possible for the same process/image to do this, but I haven't found information as to multiple processes/images. I would imagine it is theoretically possible since each socket is defined by the 5-valued tuple [IP_PROTO, src_ip:src_port, dst_ip:dst_port]. So I would assume that, as long as multiple TCP connections sharing a client-side port are not made to the same dst_ip:dst_port, this would be theoretically possible.

score 0 · Answer 1 · answered Jan 22 '17 at 08:35

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UDP is not connection-oriented and has no real distinction between client and server, so for UDP this question doesn't make a lot of sense.

For TCP, you can use SO_REUSEADDR to bind mulitple clients to the same port, but why would you want to? Normally you leave the client unbound before making a connection and let the kernel pick an unused port for you.

answered Jan 22 '17 at 08:35

Chris Dodd

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I don't want to, but I need to know if other applications can do this as I depend on whether or not this behaviour is possible. – Æðelstan Jan 22 '17 at 08:59
So I am to understand that this is indeed possible for different applications? – Æðelstan Jan 22 '17 at 09:05
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Is it *possible*? Yes, if all sockets on the same port use `SO_REUSEADDR`/`SO_REUSEPORT`. Is it *likely*? No. – Remy Lebeau Jan 23 '17 at 04:23

TCP - possible for same client-side port to be used for different connections by different applications simlutaneously?

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