Subject: Re: iptables CLAMP MSS to PMTU not working?

From: Timo Teras <[email-masked]>
Date: Thu, 12 Jul 2012 13:24:19 +0300

To reply my self for some additional notes.

On Thu, 12 Jul 2012 12:00:21 +0300 Timo Teras <[email-masked]> wrote:

> We recently noticed that CLAMPMSS to path MTU does not seem to be
> working properly. Most recently tested version is linux-3.3.6 which
> does not work. linux-2.6.35 works for sure, but I suspect it to have
> broken somewhere around 3.0'ish with the inetpeer changes.
> 
> In my case, the destination is on gre tunnel (that gets routed to
> Internet over IPsec transport mode).
> 
> 'ip route' command verifies that in both boxes the path-MTU is
> detected properly. That, is on both cases the static route MTU is
> higher. And after large packets sent, ICMP frag-needed is received
> and the cache route is updated properly.
> 
> On the new kernel, I get info like:
> # ip route get 10.x.x.x
> 10.x.x.x via 172.16.y.y dev gre1  src 172.16.z.z 
>     cache  expires 68sec ipid 0x3153 mtu 1422

CLAMP MSS sets MSS to 1432. Which implies MTU 1472. This matches the
gre1 interface MTU:

14: gre1: <UP,LOWER_UP> mtu 1472 qdisc noqueue state UNKNOWN 

So apparently CLAMPMSS is honoring the static route for gre1, instead
of the cached pmtu route.

> And the older kernel:
> # ip route get 10.x.x.x
> 10.x.x.x via 172.16.y.y dev gre1  src 172.16.z.z 
>     cache  expires 595sec ipid 0xd241 mtu 1422 advmss 1432 hoplimit 64
> 
> For some reason, iptables CLAMPMSS seems to set incorrect MSS for this
> route (or maybe it's using the static route instead?).

And in this case MSS is set to 1382. That is, it's properly calculated
from the path MTU (1422-40=1382). I would expect the advmss of the
cached route to get updated on the TCP connects on the older kernels
(the above paste is after pinging with large packets and no TCP
connection done for the cached entry).

- Timo