Commit 409b545a authored Jun 04, 2013 by Phil Oester Committed by Pablo Neira Ayuso Jun 05, 2013

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>

parent 37bc4f8d

net/netfilter/xt_TCPMSS.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -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));