What is the meaning of SO_REUSEADDR (setsockopt op

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• Socket options SO_REUSEADDR and SO_REUSEPORT, how do they differ? Do they mean the same across all major operating systems? 1 answer

From the 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?

Answer 1

TCP's primary design goal is to allow reliable data communication in the face of packet loss, packet reordering, and — key, here — 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 shutdown packets get to the receiver before the delayed packet? 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?

There are a couple of choices:

1. Disallow reuse of that IP/port combo for at least 2 times the maximum time a packet could be in flight. In TCP, this is usually called the 2×MSL delay. You sometimes also see 2×RTT, which is roughly equivalent.

   This is the default behavior of all common TCP/IP stacks. 2×MSL is typically between 30 and 120 seconds. (This is the TIME_WAIT period.) After that time, the stack assumes that any rogue packets have been dropped en route due to expired TTLs, so it leaves the TIME_WAIT state, allowing that IP/port combo to be reused.

2. Allow the new program to re-bind to that IP/port combo. In stacks with BSD sockets interfaces — essentially all Unixes and Unix-like systems, plus Windows via Winsock — you have to ask for this behavior by setting the SO_REUSEADDR option via setsockopt() before you call bind().

SO_REUSEADDR is most commonly set in server programs.

The reason is, a common pattern is that you change a server configuration file and need to restart that server to make it reload its configuration. Without SO_REUSEADDR, the bind() call in the restarted program's new instance will fail if there were connections open to the previous instance when you killed it. Those connections will hold the TCP port in the TIME_WAIT state for 30-120 seconds, so you fall into case 1 above.

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.

Answer 2

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 take time for the OS to "fully close it" while it goes through several states. As EJP mentioned in the comments, the longest delay is usually from the TIME_WAIT state. This extra delay is required to handle edge cases at the very end of the termination sequence and make sure the last termination acknowledgement either got through or had the other side reset itself because of a timeout. Here you can find some extra considerations about this state. The main considerations are pointed out as follow :

Remember that TCP guarantees all data transmitted will be delivered, if at all possible. When you close a socket, the server goes into a TIME_WAIT state, just to be really really sure that all the data has gone through. When a socket is closed, both sides agree by sending messages to each other that they will send no more data. This, it seemed to me was good enough, and after the handshaking is done, the socket should be closed. The problem is two-fold. First, there is no way to be sure that the last ack was communicated successfully. Second, there may be "wandering duplicates" left on the net that must be dealt with if they are delivered.

If you try to create multiple sockets with the same ip:port pair really quick, you get the "address already in use" error because the earlier socket will not have been fully released. Using SO_REUSEADDR will get rid of this error as it will override checks for any previous instance.

Answer 3

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