From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754411AbXJVDV7 (ORCPT ); Sun, 21 Oct 2007 23:21:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751675AbXJVDVw (ORCPT ); Sun, 21 Oct 2007 23:21:52 -0400 Received: from 1wt.eu ([62.212.114.60]:3477 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751308AbXJVDVv (ORCPT ); Sun, 21 Oct 2007 23:21:51 -0400 Date: Mon, 22 Oct 2007 05:21:45 +0200 From: Willy Tarreau To: Tal Kelrich Cc: linux-kernel@vger.kernel.org Subject: Re: 2.4/2.6 local TCP connect oddity Message-ID: <20071022032145.GA14735@1wt.eu> References: <20071021175337.605e843e@shodan.orpak.com> <20071021172901.GV10199@1wt.eu> <20071021225329.1a971751@shodan.orpak.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20071021225329.1a971751@shodan.orpak.com> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 21, 2007 at 10:53:29PM +0200, Tal Kelrich wrote: > 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. > > > > Hi, > > 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 127.0.0.1 $p ; done) Of course, for the same reason. If you reduce the ip_local_port_range, it will even succeed more often. This is because the source port is choosen before the first packet is sent, so when it is sent, it reaches a pending connection (itself). I can understand that it is confusing when you see it as a single connection, but try to imagine (or reproduce) with 2 machines, then translate that to the localhost with a single and same connection. You may even draw the exchanges on paper, an you will notice that "each end" of the connection gets its SYN-SYN/ACK-ACK sequence. You may also tcpdump on loopback if that helps. Regards, Willy