From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?ISO-8859-1?Q?=22Thomas_B=2E_R=FCcker=22?= Subject: UDP checksum broken since 2.6.18? Date: Tue, 22 May 2007 21:47:22 +0000 Message-ID: <465364EA.3020100@openpma.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: =?ISO-8859-1?Q?Philipp_G=FChring?= To: netdev@vger.kernel.org Return-path: Received: from vpn-da.werderstrasse.de ([217.195.43.101]:57483 "EHLO dk0td.afthd.tu-darmstadt.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1756427AbXEVV6A (ORCPT ); Tue, 22 May 2007 17:58:00 -0400 Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org hi, a friend of mine recently contacted me about what he at first thought were IPv6 issues with some java software. As it turns out it probably is a general IP issue with the Linux kernel: He wrote this piece of c which sends an UDP packet to 127.28.50.50 - http://www2.futureware.at/~philipp/udp-problem.c Packets generated by this code were captured by tcpdump and wireshark. When feeding the dump into wireshark it says: "Checksum: 0x62fd [incorrect, should be 0xe4f3]" for the udp packet. We've tested this on several kernel versions. Wireshark reports checksum broken: Linux version 2.6.18-4-vserver-686 (Debian 2.6.18.dfsg.1-12) (waldi@debian.org) (gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)) #1 SMP Mon Mar 26 19:55:22 UTC 2007 Linux version 2.6.19-dm8tbr-1 (root@oberon) (gcc version 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)) #3 SMP PREEMPT Sun Dec 3 18:31:00 CET 2006 - (that's vanilla) Linux version 2.6.21.1-dm8tbr-1 (root@oberon) (gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)) #3 SMP Fri May 18 09:04:55 CEST 2007 - (that's vanilla + dscape patch) Wireshark reports checksum ok: Linux version 2.6.16.13-4-default (geeko@buildhost) (gcc version 4.1.0 (SUSE Linux)) #1 Wed May 3 04:53:23 UTC 2006 Linux version 2.6.17-11-386 (root@terranova) (gcc version 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)) #2 Tue Mar 13 23:30:30 UTC 2007 (Ubuntu 2.6.17-11.37-386) So my guess is something between 2.6.17 and 2.6.18 broke. Second option is: The way you are supposed to send UDP packets changed in 2.6.18 and sun javavm and that piece of c are broken for the same reason. Third option: everything is perfectly ok, the UDP checksum is computed in a different way since 2.6.18 - due to some reason I don't know - and Wireshark is broken. We'd be grateful for some enlightment. Cheers Thomas PS: please keep me CCed - I'm not subscribed to the netdev ml.