tcp: Properly ignore keep-alive segments

RFC 9293, 3.8.4 says:

   Implementers MAY include "keep-alives" in their TCP implementations
   (MAY-5), although this practice is not universally accepted.  Some
   TCP implementations, however, have included a keep-alive mechanism.
   To confirm that an idle connection is still active, these
   implementations send a probe segment designed to elicit a response
   from the TCP peer.  Such a segment generally contains SEG.SEQ =
   SND.NXT-1 and may or may not contain one garbage octet of data.  If
   keep-alives are included, the application MUST be able to turn them
   on or off for each TCP connection (MUST-24), and they MUST default to
   off (MUST-25).

but currently, tcp_data_from_tap() is not aware of this and will
schedule a fast re-transmit on the second keep-alive (because it's
also a duplicate ACK), ignoring the fact that the sequence number was
rewinded to SND.NXT-1.

All we have to do with those segments is to reset the activity
timeout, and ignore them for the rest. We can't affect the outbound
keep-alive behaviour, other than enabling or disabling keep-alives
with SO_KEEPALIVE, because it's controlled by sysctls.

Link: https://github.com/containers/podman/discussions/24572
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/tcp.c b/tcp.c
index f357920..4a89c6c 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1763,6 +1763,19 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 			continue;
 
 		seq = ntohl(th->seq);
+		if (SEQ_LT(seq, conn->seq_from_tap) && len <= 1)
+			flow_trace(conn,
+				   "keep-alive sequence: %u, previous: %u",
+				   seq, conn->seq_from_tap);
+
+			tcp_timer_ctl(c, conn);
+
+			if (p->count == 1)
+				return 1;
+
+			continue;
+		}
+
 		ack_seq = ntohl(th->ack_seq);
 
 		if (th->ack) {