netfilter: Mark SYN/ACK packets as invalid from original direction
Clients should not send such packets. By accepting them, we open
up a hole by wich ephemeral ports can be discovered in an off-path
attack.
See: "Reflection scan: an Off-Path Attack on TCP" by Jan Wrobel,
http://arxiv.org/abs/1201.2074
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index a5ac11e..aba98f9 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -158,21 +158,18 @@
* sCL -> sSS
*/
/* sNO, sSS, sSR, sES, sFW, sCW, sLA, sTW, sCL, sS2 */
-/*synack*/ { sIV, sIV, sIG, sIG, sIG, sIG, sIG, sIG, sIG, sSR },
+/*synack*/ { sIV, sIV, sSR, sIV, sIV, sIV, sIV, sIV, sIV, sSR },
/*
* sNO -> sIV Too late and no reason to do anything
* sSS -> sIV Client can't send SYN and then SYN/ACK
* sS2 -> sSR SYN/ACK sent to SYN2 in simultaneous open
- * sSR -> sIG
- * sES -> sIG Error: SYNs in window outside the SYN_SENT state
- * are errors. Receiver will reply with RST
- * and close the connection.
- * Or we are not in sync and hold a dead connection.
- * sFW -> sIG
- * sCW -> sIG
- * sLA -> sIG
- * sTW -> sIG
- * sCL -> sIG
+ * sSR -> sSR Late retransmitted SYN/ACK in simultaneous open
+ * sES -> sIV Invalid SYN/ACK packets sent by the client
+ * sFW -> sIV
+ * sCW -> sIV
+ * sLA -> sIV
+ * sTW -> sIV
+ * sCL -> sIV
*/
/* sNO, sSS, sSR, sES, sFW, sCW, sLA, sTW, sCL, sS2 */
/*fin*/ { sIV, sIV, sFW, sFW, sLA, sLA, sLA, sTW, sCL, sIV },