From 238c69f9af458e41dea5ad8c988dbf65b05b5172 Mon Sep 17 00:00:00 2001
From: Stefano Brivio <sbrivio@redhat.com>
Date: Tue, 19 Nov 2024 20:53:44 +0100
Subject: [PATCH] tcp: Acknowledge keep-alive segments, ignore them for the
rest
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>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
---
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) {