How to reuse TCP listening socket immediately after it was connected at least once?
descentspb at gmail.com
Sun May 24 16:44:49 CEST 2009
Roy Smith wrote:
> In article <gvb8fn$7gm$1 at lust.ihug.co.nz>,
> Lawrence D'Oliveiro <ldo at geek-central.gen.new_zealand> wrote:
>> In message <mailman.651.1243154739.8015.python-list at python.org>, Igor Katson
>>> I have written a socket server and some arbitrary clients. When I
>>> shutdown the server, and do socket.close(), I cannot immediately start
>>> it again cause it has some open sockets in TIME_WAIT state. It throws
>>> address already in use exception at me.
>> There's a reason for that. It's to ensure that there are no leftover packets
>> floating around the Internet somewhere, that you might mistakenly receive
>> and think they were part of a new connection, when they were in fact part of
>> an old one.
> In theory, that is indeed the reason for the TIME_WAIT state. In practice,
> however, using SO_REUSEADDR is pretty safe, and common practice.
> You've got several things working in your favor. First, late-delivery of
> packets is pretty rare. Second, if some late packet were to arrive, the
> chances of them having the same local and remote port numbers as an
> existing connection is slim. And, finally, the TCP sequence number won't
> line up.
> One thing to be aware of is that SO_REUSEADDR isn't 100% portable. There
> are some systems (ISTR HP-UX) which use SO_REUSEPORT instead of
> SO_REUSEADDR. The original specifications weren't very clear, and some
> implementers read them in strange ways. Some of that old code continues in
> use today. I only mention this because if you try SO_REUSEADDR and it's
> not doing what you expect, it's worth trying SO_REUSEPORT (or both) to see
> what happens on your particular system.
>> The right thing to do is try to ensure that all your connections are
>> properly closed at shutdown. That may not be enough (if your server crashes
>> due to bugs), so the other thing you need to do is retry the socket open,
>> say, at 30-second intervals, until it succeeds.
> That may be a reasonable thing to do for production code, but when you're
> building and debugging a server, it's a real pain to not be able to restart
> it quickly whenever you want (or need) to.
Thanks for a great answer, Roy!
More information about the Python-list