From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751873Ab1ADJIX (ORCPT ); Tue, 4 Jan 2011 04:08:23 -0500 Received: from lo.gmane.org ([80.91.229.12]:52836 "EHLO lo.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750824Ab1ADJIU (ORCPT ); Tue, 4 Jan 2011 04:08:20 -0500 X-Injected-Via-Gmane: http://gmane.org/ To: linux-kernel@vger.kernel.org From: Alexander Clouter Subject: Re: strange fragmented udp packets Date: Tue, 4 Jan 2011 08:50:54 +0000 Message-ID: References: <201012272354.44196.thomas@fjellstrom.ca> <201101031647.56077.thomas@fjellstrom.ca> X-Complaints-To: usenet@dough.gmane.org X-Gmane-NNTP-Posting-Host: chipmunk.wormnet.eu User-Agent: tin/1.9.6-20100522 ("Lochruan") (UNIX) (Linux/2.6.32-5-sparc64-smp (sparc64)) Cc: linux-net@vger.kernel.org Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thomas Fjellstrom wrote: > >> I can't explain the packets getting fragmented (or I couldn't, see below). >> And only have a guess as to why my VOIP provider isn't seeing them, I've >> heard that some firewalls explicitly block fragmented UDP as its often >> used in DoS attacks, and for little else. >> >> Here's at least what tcpdump has to say.. >> >> packet coming in: >> 23:38:26.136021 IP x.x.x.x.5060 > x.x.x.x.5060: SIP, length: 582 >> >> packet going out: >> 23:38:18.136085 IP x.x.x.x.5060 > x.x.x.x.5060: SIP, length: 544 >> 23:38:18.136169 IP x.x.x.x > x.x.x.x: udp >> >> ... >> >> After writing all that I've realized that the problem is the mtu on the nic >> attached to my cable modem, it was set to 576 for some odd reason. Changing >> the mtu to 1500 fixes my problem, but what is causing it to auto select an >> mtu of 576 anyhow? Could my modem be telling it that 576 is better for it? > > Anyone have a hint? I've checked with my sip provider and they don't have > iptables set up, so nothing should be getting in the way of these packets. For > some reason they just aren't reaching the SIP server. > pmtu? Play around with the handy utility tracepath (and tracepath6) to discover more! Cheers -- Alexander Clouter .sigmonster says: May you have many handsome and obedient sons.