csocketstcpudp

What are the use cases of SO_REUSEADDR?

I have used SO_REUSEADDR to have my server which got terminated to restart without complaining that the socket is already in use. I was wondering: are there other uses of SO_REUSEADDR? Has anyone used the socket option for other than said purpose?

Solution

  • 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)