From: Phil Oester 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://asedeno.scripts.mit.edu/gitweb/?a=commitdiff_plain;h=409b545ac10d9548929557a75ad86540f59a2c83;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 Signed-off-by: Pablo Neira Ayuso --- 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));