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Date: Fri, 29 Jun 2007 14:03:07 +0200
From: Patrick McHardy <kaber@trash.net>
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To: Andreas Steinmetz <ast@domdv.de>
CC: Linux Kernel Mailinglist <linux-kernel@vger.kernel.org>,
       netfilter-devel@lists.netfilter.org
Subject: Re: mss to pmtu clamping partially broken?
References: <4684E853.20900@domdv.de> <4684EF6A.7070109@trash.net> <4684F3D8.1080800@domdv.de>
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Andreas Steinmetz wrote:
> Patrick McHardy wrote:
> 
>>Andreas Steinmetz wrote:
>>
>>>[...]
>>>The tcpdump on the client shows that the mss of the incoming syn reply
>>>packet is *NOT* clamped to the ppp interface mtu.
>>
>>
>>You forgot to mention *how* you're clamping the MSS. Using
>>TCPMSS? Do you have a rule for incoming packets?
>>
> 
> 
> The relevant iptables commands I do use for masquerading and clamping are:
> 
> iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
> iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS \
> 	--clamp-mss-to-pmtu


Two things here:

- tcpdumps on ppp0 will show unclamped packets since they haven't
been forwarded yet

- assuming you have ethernet internally, the PMTU from your router
to the internal hosts is 1500, so it won't do any clamping.

Does that explain it?

A useful thing for TCPMSS for routers would be to clamp to the
minimum of the PMTU of both directions. But thats not supported
so far.