linux-kernel - Re: mss to pmtu clamping partially broken?

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Date:	Fri, 29 Jun 2007 14:06:43 +0200
From:	Andreas Steinmetz <ast@...dv.de>
To:	Patrick McHardy <kaber@...sh.net>
CC:	Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
	netfilter-devel@...ts.netfilter.org
Subject: Re: mss to pmtu clamping partially broken?

Patrick McHardy wrote:
> 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
> 

That is true, I know this.

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

Yep, internal PMTU is 1500, still the incoming packets are clamped to
1452 on the one line and not clamped on the other.

> 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.
> 

I wonder, as somteimes it gets clamped. If it would never have been
clamped I wouldn't have asked.

-- 
Andreas Steinmetz                       SPAMmers use robotrap@...dv.de
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