[PATCH] tcp: Don't reset outbound connection on SYN retries

[Stefano Brivio <sbrivio@redhat.com>]

Reported by somebody on IRC: if the server has considerable latency,
it might happen that the client retries sending SYN segments for the
same flow while we're still in a TAP_SYN_RCVD, non-ESTABLISHED state.

In that case, we should go with the blanket assumption that we need
to reset the connection on any unexpected segment: RFC 9293 explicitly
mentions this case in Figure 8: Recovery from Old Duplicate SYN,
section 3.5. It doesn't make sense for us to set a specific sequence
number, socket-side, but we should definitely wait and see.

Ignoring the duplicate SYN segment should also be compatible with
section 3.10.7.3. SYN-SENT STATE, which mentions updating sequences
socket-side (which we can't do anyway), but certainly not reset the
connection.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tcp.c b/tcp.c
index 7787381..51ad692 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1920,6 +1920,9 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 
 	/* Establishing connection from tap */
 	if (conn->events & TAP_SYN_RCVD) {
+		if (th->syn && !th->ack && !th->fin)
+			return 1;	/* SYN retry: ignore and keep waiting */
+
 		if (!(conn->events & TAP_SYN_ACK_SENT))
 			goto reset;
 
-- 
@@ -1920,6 +1920,9 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 
 	/* Establishing connection from tap */
 	if (conn->events & TAP_SYN_RCVD) {
+		if (th->syn && !th->ack && !th->fin)
+			return 1;	/* SYN retry: ignore and keep waiting */
+
 		if (!(conn->events & TAP_SYN_ACK_SENT))
 			goto reset;
 
-- 
2.43.0