What exactly does SO_REUSEADDR do

This socket option tells the kernel that even if this port is busy (in   the TIME_WAIT state), go ahead and reuse it anyway.  If it is busy,   but with another state, you will still get an address already in use   error.  It is useful if your server has been shut down, and then   restarted right away while sockets are still active on its port.  You   should be aware that if any unexpected data comes in, it may confuse   your server, but while this is possible, it is not likely.    It has been pointed out that "A socket is a 5 tuple (proto, local   addr, local port, remote addr, remote port).  SO_REUSEADDR just says   that you can reuse local addresses.  The 5 tuple still must be   unique!" by Michael Hunter (mphunter@qnx.com).  This is true, and this   is why it is very unlikely that unexpected data will ever be seen by   your server.  The danger is that such a 5 tuple is still floating   around on the net, and while it is bouncing around, a new connection   from the same client, on the same system, happens to get the same   remote port.  This is explained by Richard Stevens in ``2.7 Please   explain the TIME_WAIT state.''.