I have an oddball connection issue that appears to be related to the routing equipment between the two end points.
I have a server trying to connect to an access control system controller across the country over a WAN. When I sniff the connection between the server and the controller, I see a sequence of SYN packets to the controller service port (> 2000 < 4000) on the controller with no SYN ACK from the controller.
However, if I open a telnet session to the controller on the port where telnet is listening on the controller (> 8000), suddenly the server gets a SYN ACK on the controller service port and the controller comes online.
If I break the connection on the controller service port and try to re- establish it, I just get a sequence of SYN packets with no SYN ACK until I open another telnet session again.
Seems the telnet session is establishing some kind of state that is needed for the connection to occur that cannot be created on the controller service port for some reason.
The controller connects perfectly on the local network. The problem only occurs over the WAN. There are no firewalls between the two end points.