I have a Cisco ASA 5505 that functions as my primary firewall and a Mitel 5000 controller behind it. I have two external phone users that have been connecting through the firewall with no issues for six months until about two weeks ago. I am now seeing the following log entry on the phone trying to connect to the Mitel Controller.
|6||May 16 2014||14:52:52||302014||188.8.131.52||6915||192.168.20.2||6801||Teardown TCP connection 1203584 for outside:184.108.40.206/6915 to inside:192.168.20.2/6801 duration 0:00:00 bytes 0 TCP Reset-I|
My phones are designed to work with the Mitel 5000 and Mitel 3300 phone controllers. The 5000 will only use port 6800 for call control, while the 3300 will use 6801 (Secured Minet), 6802 (Minet SSH), and if those fail, port 6800 (Minet Unsecured). When the phones initiate a connection, they try 6801 first. If 6801 is unavailable, the phone controller adds the RST flag to the ACK packet. When the phone sees the RST flag, it is supposed to reset and use the next port (6802). The same process happens again for port 6802, then the phone knows to try 6800. The problem is that the ASA sees the RST flag now and terminates the connection at the firewall. Therefore, the phones never see the RST flag, and continue to try the connection with port 6801.
I have tried to use the TCP State Bypass feature to correct the situation, but the log shows that the connection is still being terminated immediately by the firewall. I am a novice when it comes to configuring the ASA. Any help would be greatly appreciated, as the company that I bought the phone system from is out of troubleshooting options. I do not think that I have made any changes to the firewall around this time. I have packet captures and logs from my ASA and I have wireshark data on the inside of my network. I need to figure out how to configure the ASA so that it ignores the RST flag and sends the packet back to the source.
Any help would be greatly appreciated!