From Linux man page:
SO_REUSEADDR Specifies that the rules used in validating addresses supplied to bind() should allow reuse of local addresses, if this is supported by the protocol. This option takes an int value. This is a Boolean option
When should I use it? Why does "reuse of local addresses" give?
Thanks, Ray
SO_REUSEADDR allows your server to bind to an address which is in a
TIME_WAIT state.
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.
From unixguide.net
TCP's primary design goal is to allow reliable data communication in the face of packet loss, packet reordering, and -- this is important -- packet duplication.
It's fairly obvious how a TCP/IP network stack deals with all this while the connection is up, but there's an edge case that happens just after the connection closes. What happens if a packet sent right at the end of the conversation is duplicated and delayed, such that the 4-way close (FINs and ACKs, both ways) packets get to the receiver first? The stack dutifully closes down its connection. Then later, the delayed duplicate packet shows up. What should the stack do?
More importantly, what should it do if the program that owned that connection immediately dies, then another starts up wanting the same IP address and TCP port number?
One option is to not allow that for at least 2 times the maximum round-trip-time for a packet. This is the default behavior of all common TCP/IP stacks. 2*RTT is between 30 and 120 seconds, typically. (This is the TIME_WAIT period.) After that time, the stack assumes that any rogue packets have been dropped en route due to their TTL expiring, so it leaves the TIME_WAIT state, allowing that IP/port combo to be reused.
The other option is to allow the new program to re-bind to that IP/port combo. In stacks with BSD sockets interfaces, you have to ask for this behavior by setting the SO_REUSEADDR option before you call bind().
SO_REUSEADDR is most commonly set in server programs.
A common pattern is that you change a server configuration file, and so need to kill and then immediately restart that server to make it reload its configuration. Without SO_REUSEADDR, the bind() call will fail if there were connections to it open when you killed it, so those connections will hold the port in the TIME_WAIT state for 30-120 seconds. The safe thing to do is wait out the TIME_WAIT period, but in practice this isn't a big enough risk that it's worth doing that. It's better to get the server back up immediately so as to not miss any more incoming connections than necessary.
When you create a socket, you don't really own it. The OS (TCP stack) creates it for you and gives you a handle (file descriptor) to access it. When your socket is closed, it might take time for the OS to "notice" it.
If you try to create multiple sockets with the same ip:port pair really quick, you get the "address already in use" error. Using SO_REUSEADDR will get rid of this error.