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

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Message-ID: <537C5F71.6000204@davidnewall.com>
Date:	Wed, 21 May 2014 17:40:25 +0930
From:	David Newall <davidn@...idnewall.com>
To:	Valdis.Kletnieks@...edu
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 20/05/14 14:25, Valdis.Kletnieks@...edu wrote:
> So yes, we*do*  need to do something sensible there - either frag the packet
> on the way out, or something.

I think the problem is that a bridge cannot be used across incompatible 
media.  That's the job of a router.

A bridge should act like a bridge, not a router.  Fragmenting the packet 
is wrong; that's IP's job.  Dropping the packet is also arguably wrong; 
that's the real device-driver's job.  What seems right to me is to act 
like a bridge and forward packets by looking inside of them *no more 
than is necessary*.  Looking beyond MAC address is perhaps too much.

We can finish the job of processing IP options, or at least in this 
scenario, but that seems wrong-headed and invites more work as more 
problems are discovered; or we could remove the half-hearted attempt it 
currently does and leave the bridge as a simple bridge.

This problem wouldn't occur if all devices in a bridge were required to 
be compatible media; particularly identical MTU.
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