linux-kernel - Re: Revert 462fb2af9788a82a534f8184abfde31574e1cfa0 (bridge : Sanitize skb before it enters the IP stack)

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Message-ID: <14456.1400561749@turing-police.cc.vt.edu>
Date:	Tue, 20 May 2014 00:55:49 -0400
From:	Valdis.Kletnieks@...edu
To:	David Newall <davidn@...idnewall.com>
Cc:	Florian Westphal <fw@...len.de>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	bridge@...ts.linux-foundation.org
Subject: Re: Revert 462fb2af9788a82a534f8184abfde31574e1cfa0 (bridge : Sanitize skb before it enters the IP stack)

On Mon, 19 May 2014 23:49:22 +0930, David Newall said:

> How does a packet get fragmented in this case?  Does it only happen when
> bridging to a device with smaller MTU?  That scenario sounds quite
> un-bridge-like.  It also sounds like something that can be handled by
> real routing.

Which doesn't change the fact that you *will* get clowns who take a box that
has a 10G card on a jumbogram-enabled subnet that's running with an MTU of
9000, and a 1G at MTU 1500 on the other, and try to bridge rather than route.
(Did you know that you can actually mount an NFS filesystem across that? And
that ls and cat and friends will work *just fine*? Until you hit a file that's
more than 1.5 in size, that is. And when you do a traceroute to the wedged
client, it tells you it's on the 10G network, so you have no idea why you're
seeing an MTU issue.  Don't ask how I know this - let's just say that
supporting HPC users is never boring. :)

So yes, we *do* need to do something sensible there - either frag the packet
on the way out, or something.  It *would* be nice if we could drop the
packet and send an ICMP Frag Needed back - except it's unclear what IP
you use as the source address for the ICMP....

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