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Message-ID: <20071021225329.1a971751@shodan.orpak.com> Date: Sun, 21 Oct 2007 22:53:29 +0200 From: Tal Kelrich <tal@...icgenome.com> To: Willy Tarreau <w@....eu> Cc: linux-kernel@...r.kernel.org Subject: Re: 2.4/2.6 local TCP connect oddity On Sun, 21 Oct 2007 19:29:02 +0200 Willy Tarreau <w@....eu> 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) Regards, Tal -- Tal Kelrich PGP fingerprint: 3EDF FCC5 60BB 4729 AB2F CAE6 FEC1 9AAC 12B9 AA69 Key Available at: http://www.hasturkun.com/pub.txt ---- We are each entitled to our own opinion, but no one is entitled to his own facts. -- Patrick Moynihan ---- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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