socket.error: [Errno 98] Address already in use

Nobody nobody at
Sun Sep 19 07:36:19 CEST 2010

On Sun, 19 Sep 2010 12:27:08 +1200, Lawrence D'Oliveiro wrote:

>> That's why Stevens recommends that all TCP servers use the
>> SO_REUSEADDR socket option.
> I don’t think I’ve ever used that. It seems to defeat a safety mechanism 
> which was put in for a reason.

It was put in for the benefit of clients, to prevent them from selecting
a port which they won't be able to use. At the point that the program
calls bind(), the kernel doesn't know whether the program will call
connect() or listen() next (i.e. whether it's a client or a server).

Even if you set SO_REUSEADDR, you cannot create a connection which could
be confused with an existing connection, i.e. one with the same source and
destination address and port (BSD allows this provided that the previous
connection is closed and the initial sequence number of the new connection
exceeds the final sequence number of the previous connection).

For a client, re-using the port would mean that any attempt to connect to
the same port and address as an existing TIME_WAIT connection will fail
with EADDRINUSE, so it should choose another port. This scenario is quite
likely, as a client for a particular protocol will tend to connect to a
specific remote port, and often to a small set of servers.

But a server often has to use a specific port, and its clients will
typically connect from ephemeral ports. Even if some clients insist on
trying to use a specific source port (which will fail so long as the
TIME_WAIT connections exist), the server can still serve other clients.

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