EXTERNAL: OSError: [Errno 48] Address already in use

Marko Rauhamaa marko at pacujo.net
Tue Jul 3 10:01:12 EDT 2018

Chris Angelico <rosuav at gmail.com>:

> On Tue, Jul 3, 2018 at 10:26 PM, Marko Rauhamaa <marko at pacujo.net> wrote:
>> It's crucial that the killed party is the server for the situation to
>> arise.
>> That's why polite clients close their end of the connection before
>> the server. Whoever closes first will suffer the TIME-WAIT state.
> The same thing can happen with clients AND servers, but it's very
> common with clients to allow an arbitrary port number to be chosen for
> you. To trigger this problem from the client end, you would need to
> use the same port. An attacker can do this easily.

No, I mean that the TIME-WAIT state is triggered asymmetrically. See RFC
793 page 23. Whoever sends FIN first will end up in TIME-WAIT for 2*MSL.

A client that doesn't send FIN first will cause an accumulation of
connection objects (TCBs) at the server end. It can also prevent the
server from being restarted quickly.

The SO_REUSEADDR discussion has no direct relationship with computer or
network security. In fact, I don't see any reason why it's not always
on for TCP. Maybe it's done so UDP and TCP (and other protocols) would
work similarly.


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