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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>
This commit is contained in:
Phil Oester 2013-06-04 05:09:27 +00:00 committed by Pablo Neira Ayuso
parent 37bc4f8dfa
commit 409b545ac1
1 changed files with 6 additions and 0 deletions
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6
net/netfilter/xt_TCPMSS.c
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@ -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));