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Re: Determining whether a TCP connection is up

From: Lars Magne Ingebrigtsen
Subject: Re: Determining whether a TCP connection is up
Date: Mon, 05 Aug 2013 19:55:55 +0200
User-agent: Gnus/5.130008 (Ma Gnus v0.8) Emacs/24.3.50 (gnu/linux)

Julien Danjou <address@hidden> writes:

> It would help a lot:
> - connections would be stuck in the *background*, avoiding getting your
>   Emacs frozen;

Not really.  Network chatter is issued when users want something to
happen, like entering a group and the like. 

> - connections will eventually time out (the kernel does have a TCP
>   timeout), and you will be able to reschedule the operation since you
>   used a queue.

Heh heh.  After waiting half an hour?

> Also, sqending regular NOOP command should triggers early detection of
> IP changes. If I'm not mistaken, if the kernel thinks you have a TCP
> session established, but your NATed public IP address changed, the
> remote server is going to send you a RST or something like that as soon
> as it received something, and the kernel will consider the connection as
> no more valid and will close it.

Nope.  This no longer happens in modern network setups.  Like my
original article said, it's common for people to run firewalls that drop
packages that don't belong to TCP sessions.  So you get no RST.

(domestic pets only, the antidote for overdose, milk.)
  No Gnus T-Shirt for sale: http://ingebrigtsen.no/no.php
  and http://lars.ingebrigtsen.no/2013/08/twenty-years-of-september.html

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