From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765270AbXF2MGw (ORCPT ); Fri, 29 Jun 2007 08:06:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761334AbXF2MGp (ORCPT ); Fri, 29 Jun 2007 08:06:45 -0400 Received: from hermes.domdv.de ([193.102.202.1]:3526 "EHLO hermes.domdv.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761296AbXF2MGo (ORCPT ); Fri, 29 Jun 2007 08:06:44 -0400 Message-ID: <4684F5D3.8020506@domdv.de> Date: Fri, 29 Jun 2007 14:06:43 +0200 From: Andreas Steinmetz User-Agent: Mail/News 1.5.0.7 (X11/20060915) MIME-Version: 1.0 To: Patrick McHardy CC: Linux Kernel Mailinglist , 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> <4684F4FB.9070807@trash.net> In-Reply-To: <4684F4FB.9070807@trash.net> X-Enigmail-Version: 0.94.1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org 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@domdv.de