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 0c228e83 ("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: 0c228e83 ("tcp: Restore RFC5961-compliant behavior for SYN packets") Signed-off-by: NJesper Dangaard Brouer <brouer@redhat.com> Acked-by: NJozsef Kadlecsik <kadlec@blackhole.kfki.hu> Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
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