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Date:	Sun, 21 Oct 2007 22:53:29 +0200
From:	Tal Kelrich <>
To:	Willy Tarreau <>
Subject: Re: 2.4/2.6 local TCP connect oddity

On Sun, 21 Oct 2007 19:29:02 +0200
Willy Tarreau <> wrote:

> Hi,
> On Sun, Oct 21, 2007 at 05:53:37PM +0200, Tal Kelrich wrote:
> > Hi,
> > 
> > I've run into a problem where a process trying to connect to a local
> > port within the local port range eventually ends up connected to
> > itself, with source port = dest port.
> > 
> > similar behavior can be gotten by running netcat as follows:
> > nc -p 1025 localhost 1025
> > 
> > I'm not really sure if that's a bug, but the original case was at
> > least unexpected.
> It is not a bug, it is caused by the "simultaneous connect" feature of
> TCP. Although rarely used, in TCP you can connect two clients
> together. They just have to exchange their SYN, SYN/ACK then ACK and
> bingo, they're connected. In fact, you found the easiest way to
> achieve it, by using the same port. To demonstrate the feature, I'm
> used to either temporarily block SYNs with iptables, or by unplugging
> the cable between two machines.


It still seems confusing that a connect against localhost may
randomly succeed.

Here's a better example, if somewhat violent. eventually succeeds.
(p=$((`cat /proc/sys/net/ipv4/ip_local_port_range|cut -f1`+1)); while
true ; do nc -v -v $p ; done)


Tal Kelrich
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We are each entitled to our own opinion, but no one is entitled to his
own facts.
		-- Patrick Moynihan
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