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-rw-r--r--net/netfilter/xt_TCPMSS.c6
1 files changed, 6 insertions, 0 deletions
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));