[PATCH 2/2] tcp: Acknowledge keep-alive segments, ignore them for the rest

[Stefano Brivio <sbrivio@redhat.com>]

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.

ACK these keep-alive segments, reset the activity timeout, and ignore
them for the rest.

At some point, we could think of implementing an approximation of
keep-alive segments on outbound sockets, for example by setting
TCP_KEEPIDLE to 1, and a large TCP_KEEPINTVL, so that we send a single
keep-alive segment at approximately the same time, and never reset the
connection. That's beyond the scope of this fix, though.

Reported-by: Tim Besard <tim.besard@gmail.com>
Link: https://github.com/containers/podman/discussions/24572
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tcp.c b/tcp.c
index f357920..1eb85bb 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1763,6 +1763,20 @@ 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_send_flag(c, conn, ACK);
+			tcp_timer_ctl(c, conn);
+
+			if (p->count == 1)
+				return 1;
+
+			continue;
+		}
+
 		ack_seq = ntohl(th->ack_seq);
 
 		if (th->ack) {
-- 
@@ -1763,6 +1763,20 @@ 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_send_flag(c, conn, ACK);
+			tcp_timer_ctl(c, conn);
+
+			if (p->count == 1)
+				return 1;
+
+			continue;
+		}
+
 		ack_seq = ntohl(th->ack_seq);
 
 		if (th->ack) {
-- 
2.43.0