TCP connection stuck in SYN_RECV state despite ACK received, Linux 2.6.18, embedded, ARM

My client cannot connect to my protocol port (TCP) after some network glitches, even though all other protocols (telnet/HTTP/FTP) work fine.

netstat shows that my server is listening and tcpdump on the server shows all 3 packets are exchanged:

18:29:16.578964 IP 10.9.59.10.3355 > 10.9.43.131.5084: S 2602965897:2602965897(0) win 65535 <mss 1460,nop,nop,sackOK>

18:29:16.579107 IP 10.9.43.131.5084 > 10.9.59.10.3355: S 3464857909:3464857909(0) ack 2602965898 win 5840 <mss 1460,nop,nop,sackOK>

18:29:16.579284 IP 10.9.59.10.3355 > 10.9.43.131.5084: . ack 1 win 65535

But somehow netstat -t shows the connection still in SYN_RECV, as if the ack is not seen by the TCP state machine. I have to restart my server to get it to work.

syncookie is not enabled, and I know from client code behavior and tcpdump that there is no SYN flooding.

Help much appreciated.

Solution 1:

The connection is in SYN_RECV state because the kernel has received a SYN packet for a port that is in LISTENING mode, but the other end did not reply with ACK.

Check if the ACK is received by the server by running the capture on the server. Is the capture taken on the client or on the server?

Solution 2:

This can happen if the listener has set the DEFER_ACCEPT option on the socket and is not yet ready to accept the connection.