From 170080645dac61816455afad807ffeb326ce79e8 Mon Sep 17 00:00:00 2001 From: Benjamin LaHaise Date: Mon, 17 Dec 2007 22:27:36 -0800 Subject: [PATCH] [NETFILTER]: xt_TCPMSS: don't allow netfilter --setmss to increase mss 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. Signed-off-by: Benjamin LaHaise Signed-off-by: Patrick McHardy Signed-off-by: David S. Miller --- net/netfilter/xt_TCPMSS.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c index e4ee4bc81ff3..a1bc77fcd681 100644 --- a/net/netfilter/xt_TCPMSS.c +++ b/net/netfilter/xt_TCPMSS.c @@ -88,8 +88,11 @@ tcpmss_mangle_packet(struct sk_buff *skb, 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; -- 2.20.1