lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Message-ID: <20071022032145.GA14735@1wt.eu> Date: Mon, 22 Oct 2007 05:21:45 +0200 From: Willy Tarreau <w@....eu> To: Tal Kelrich <tal@...icgenome.com> Cc: linux-kernel@...r.kernel.org Subject: Re: 2.4/2.6 local TCP connect oddity On Sun, Oct 21, 2007 at 10:53:29PM +0200, Tal Kelrich wrote: > 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) 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 - 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/
Powered by blists - more mailing lists