stream: tcp: Handle retransmitted SYN with TSval

For connections that use TCP timestamps for which the first SYN packet
does not reach the server, any replies to retransmitted SYNs will be
tropped.

This is happening in StateSynSentValidateTimestamp, where the timestamp
value in a SYN-ACK packet must match the one from the SYN packet.
However, since the server never received the first SYN packet, it will
respond with an updated timestamp from any of the following SYN packets.

The timestamp value inside suricata is not being updated at any time
which should happen. This patch fixes that problem.

This problem was introduced in 9f0294fadca3dcc18c919424242a41e01f3e8318.

Fixes: #4649 - https://redmine.openinfosecfoundation.org/issues/4649
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

diff --git a/src/stream-tcp.c b/src/stream-tcp.c
index eea743d3c9bf129c89bafc24004145ca6a58c9c3..cf05d498e2da76b5afb5a476a0bb8ae111f9d47c 100644 (file)
--- a/src/stream-tcp.c
+++ b/src/stream-tcp.c
@@ -1636,6 +1636,24 @@ static int StreamTcpPacketStateSynSent(ThreadVars *tv, Packet *p,
                     "ssn->client.last_ack %"PRIu32"", ssn,
                     ssn->client.isn, ssn->client.next_seq,
                     ssn->client.last_ack);
+        } else if (PKT_IS_TOSERVER(p)) {
+            /*
+             * On retransmitted SYN packets, the timestamp value must be updated,
+             * to avoid dropping any SYN+ACK packets that respond to a retransmitted SYN
+             * with an updated timestamp in StateSynSentValidateTimestamp.
+             */
+            if ((ssn->client.flags & STREAMTCP_STREAM_FLAG_TIMESTAMP) && TCP_HAS_TS(p)) {
+                uint32_t ts_val = TCP_GET_TSVAL(p);
+
+                // Check whether packets have been received in the correct order (only ever update)
+                if (ssn->client.last_ts < ts_val) {
+                    ssn->client.last_ts = ts_val;
+                    ssn->client.last_pkt_ts = p->ts.tv_sec;
+                }
+
+                SCLogDebug("ssn %p: Retransmitted SYN. Updated timestamp from packet %" PRIu64, ssn,
+                        p->pcap_cnt);
+            }
         }
 
         /** \todo check if it's correct or set event */