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