From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin LaHaise Subject: [PATCH] don't allow netfilter --setmss to increase mss Date: Tue, 4 Dec 2007 17:45:35 -0500 Message-ID: <20071204224535.GY27007@kvack.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@vger.kernel.org To: David Miller Return-path: Received: from kanga.kvack.org ([66.96.29.28]:37165 "EHLO kanga.kvack.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750898AbXLDWqG (ORCPT ); Tue, 4 Dec 2007 17:46:06 -0500 Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-ID: Hi folks, When terminating DSL connections for an assortment of random customers, I've found it necessary to use iptables to clamp the MSS used for connections to work around the various ICMP blackholes in the greater net. Unfortunately, the current behaviour in Linux is imperfect and actually make things worse, so I'm proposing the following: increasing the MSS in a packet can never be a good thing, so make --set-mss only lower the MSS in a packet. Yes, I am aware of --clamp-mss-to-pmtu, but it doesn't work for outgoing connections from clients (ie web traffic), as it only looks at the PMTU on the destination route, not the source of the packet (the DSL interfaces in question have a 1442 byte MTU while the destination ethernet interface is 1500 -- there are problematic hosts which use a 1300 byte MTU). Reworking that is probably a good idea at some point, but it's more work than this is. Thoughts? Would it be better to add a new flag? -ben Signed-off-by: Benjamin LaHaise diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c index d40f7e4..411c482 100644 --- a/net/netfilter/xt_TCPMSS.c +++ b/net/netfilter/xt_TCPMSS.c @@ -88,8 +88,11 @@ tcpmss_mangle_packet(struct sk_buff **pskb, oldmss = (opt[i+2] << 8) | opt[i+3]; - if (info->mss == XT_TCPMSS_CLAMP_PMTU && - oldmss <= newmss) + /* Never increase MSS, even when setting it, as + * doing so results in problems for hosts that rely + * on MSS being set correctly. + */ + if (oldmss <= newmss) return 0; opt[i+2] = (newmss & 0xff00) >> 8;