Subject: Re: [PATCH] [IPV4]: Update MTU to all related cache entries in ip_rt_frag_needed()

From: Timo Teräs <[email-masked]>
Date: Sun, 27 Apr 2008 11:35:42 +0300

David Miller wrote:
> From: Timo_Teräs <[email-masked]>
> Date: Thu, 24 Apr 2008 13:05:27 +0300
> 
>> Add struct net_device parameter to ip_rt_frag_needed() and update MTU to
>> cache entries where ifindex is specified. This is similar to what is
>> already done in ip_rt_redirect().
>>
>> Signed-off-by: Timo Teras <[email-masked]>
> 
> I haven't forgotten about this patch, I'm just still thinking about
> possible negative side effects.  I don't think there are any, but
> I want to be sure.
> 
> This function has been a constant source of consternation.  It also
> doesn't do the right thing when the TOS is modified mid-network, and
> that's arguably wrong too but more difficult to solve efficiently than
> this ifindex keying case.

I understand your concern. It is pretty easy break routing cache with
some seemingly unrelated changes. I tried to think about possible side
effects, but concluded that this should be safe. Then again, I don't
know all the tiny details and unusual scenarios. But I have been running
a 2.6.23 kernel with (backported) patch on a production machine (router
connected to two ISPs, internal subnet and also having multiple gre
tunnels), so far I have not encountered any problems.

Cheers,
  Timo