linux-kernel - Re: strange fragmented udp packets

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Message-ID: <ek3bv7-3m6.ln1@chipmunk.wormnet.eu>
Date:	Tue, 4 Jan 2011 08:50:54 +0000
From:	Alexander Clouter <alex@...riz.org.uk>
To:	linux-kernel@...r.kernel.org
Cc:	linux-net@...r.kernel.org
Subject: Re: strange fragmented udp packets

Thomas Fjellstrom <thomas@...llstrom.ca> 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 
<phun>discover</phun> more!

Cheers

-- 
Alexander Clouter
.sigmonster says: May you have many handsome and obedient sons.

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