From: Phil Oester <kernel@linuxace.com>
Date: Tue, 4 Jun 2013 05:09:27 +0000 (+0000)
Subject: netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option
X-Git-Tag: v3.10-rc6~18^2~15^2~2
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=409b545a;p=linux.git

netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option

The clamp-mss-to-pmtu option of the xt_TCPMSS target can cause issues
connecting to websites if there was no MSS option present in the
original SYN packet from the client. In these cases, it may add a
MSS higher than the default specified in RFC879. Fix this by never
setting a value > 536 if no MSS option was specified by the client.

This closes netfilter's bugzilla #662.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index a75240f0d42b..afaebc766933 100644
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -125,6 +125,12 @@ tcpmss_mangle_packet(struct sk_buff *skb,
 
 	skb_put(skb, TCPOLEN_MSS);
 
+	/* RFC 879 states that the default MSS is 536 without specific
+	 * knowledge that the destination host is prepared to accept larger.
+	 * Since no MSS was provided, we MUST NOT set a value > 536.
+	 */
+	newmss = min(newmss, (u16)536);
+
 	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 	memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));