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netfilter: Mark SYN/ACK packets as invalid from original direction

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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>
This commit is contained in:
Jozsef Kadlecsik 2012-08-31 09:55:53 +00:00 committed by Pablo Neira Ayuso
parent 0626af3139
commit 64f509ce71
1 changed files with 8 additions and 11 deletions
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19
net/netfilter/nf_conntrack_proto_tcp.c
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@ -158,21 +158,18 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
* 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 },