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

Nobody nobody at
Tue Sep 21 14:02:01 CEST 2010

On Mon, 20 Sep 2010 12:00:41 +1200, Lawrence D'Oliveiro wrote:

>> However, some clients choose their own source ports. E.g. rlogin/rsh use
>> privileged (low-numbered) ports, and you can't get the kernel to choose a
>> random privileged port for you.
> But nobody uses rlogin/rsh any more,

They did when the bind() and SO_REUSEADDR semantics were developed. If
they were doing it now, chances are that SO_REUSEADDR would be enabled by

> and who would attach any trustworthy 
> meaning to a connection coming from a remote low-numbered source port?

If you receive a connection with a low-numbered source port and it
*isn't* legitimate, then someone has got root (in which case any other
authentication mechanism isn't safe either) or someone has hijacked the
IP address (you do know that low-numbered ports are only meaningful for
systems under your control, right?).

Using a firewall rule which only allows connections from a low port on
specific IP addresses certainly isn't any worse than a rule which only
allows connections from any port on specific IP addresses. That's true
regardless of whether the protocol includes other authentication

>> If you're writing a server which listens on a known port, you *should* be
>> using SO_REUSEADDR to avoid unnecessary delays in start-up. The kernel
>> will automatically reject packets relating to stale connections, and your
>> server should be accepting any new connections ASAP.
> That makes it sound like SO_REUSEADDR should really be a superfluous option. 
> But it’s not.

Well it is mostly superfluous. It should always be enabled for a server
listening on a known port, and doesn't matter for a client which uses
ephemeral ports.

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