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authorJesper Dangaard Brouer <brouer@redhat.com>2015-05-07 14:54:16 +0200
committerPablo Neira Ayuso <pablo@netfilter.org>2015-05-15 20:50:56 +0200
commitb3cad287d13b5f6695c6b4aab72969cd64bf0171 (patch)
tree35cb16bad9e76c3cf30e01e71b4ea5031de0a564 /include/uapi
parent595ca5880b37d4aa3c292d75531577175d36b225 (diff)
conntrack: RFC5961 challenge ACK confuse conntrack LAST-ACK transition
In compliance with RFC5961, the network stack send challenge ACK in response to spurious SYN packets, since commit 0c228e833c88 ("tcp: Restore RFC5961-compliant behavior for SYN packets"). This pose a problem for netfilter conntrack in state LAST_ACK, because this challenge ACK is (falsely) seen as ACKing last FIN, causing a false state transition (into TIME_WAIT). The challenge ACK is hard to distinguish from real last ACK. Thus, solution introduce a flag that tracks the potential for seeing a challenge ACK, in case a SYN packet is let through and current state is LAST_ACK. When conntrack transition LAST_ACK to TIME_WAIT happens, this flag is used for determining if we are expecting a challenge ACK. Scapy based reproducer script avail here: https://github.com/netoptimizer/network-testing/blob/master/scapy/tcp_hacks_3WHS_LAST_ACK.py Fixes: 0c228e833c88 ("tcp: Restore RFC5961-compliant behavior for SYN packets") Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com> Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Diffstat (limited to 'include/uapi')
-rw-r--r--include/uapi/linux/netfilter/nf_conntrack_tcp.h3
1 files changed, 3 insertions, 0 deletions
diff --git a/include/uapi/linux/netfilter/nf_conntrack_tcp.h b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
index 9993a421201c..ef9f80f0f529 100644
--- a/include/uapi/linux/netfilter/nf_conntrack_tcp.h
+++ b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
@@ -42,6 +42,9 @@ enum tcp_conntrack {
/* The field td_maxack has been set */
#define IP_CT_TCP_FLAG_MAXACK_SET 0x20
+/* Marks possibility for expected RFC5961 challenge ACK */
+#define IP_CT_EXP_CHALLENGE_ACK 0x40
+
struct nf_ct_tcp_flags {
__u8 flags;
__u8 mask;