What are the use cases of SO_REUSEADDR?

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For TCP, the primary purpose is to restart a closed/killed process on the same address.

The flag is needed because the port goes into a TIME_WAIT state to ensure all data is transferred.

If two sockets are bound to the same interface and port, and they are members of the same multicast group, data will be delivered to both sockets.

I guess an alternative use would be a security attack to try to intercept data.

(Source)

For UDP, SO_REUSEADDR is used for multicast.

More than one process may bind to the same SOCK_DGRAM UDP port if the bind() is preceded by:

int one = 1;
setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));

In this case, every incoming multicast or broadcast UDP datagram destined to
the shared port is delivered to all sockets bound to the port.

(Source)

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