[Serusers] Nathelper natping-interval malfunction.

[Younger Wang]

I am testing Nathelper module. It works well delivering SIP messages.
However, natping-interval function seems malfunction.

The NAT scenario is simulated by a microsoft ICS. There is a Cisco ATA
behind NAT with internal address 192.168.0.2 and it's mapped to external
address 192.17.1.22. Ser is listening on external address 192.17.1.173. I
added "modparam("nathelper", "natping-interval", 5) " to the nathelper.cfg
file.

When ATA registered to Ser, I noticed 192.168.0.2:5056(UDP) is mapped to
192.17.1.22:1025(UDP). Register is successful and call could be established
between internal ATA and external ATA(one way audio).  It's expected that
Ser should send a UDP packet to 192.17.1.22:1025(UDP) every 5 seconds.
However, the UDP packet is actually sent to 192.17.1.22:260(UDP). I wonder
what's the problem?

ATA-------------------->NAT------------------------>Ser
   From:192.168.0.2:5060          From:192.17.1.22:1025
   To: 192.17.1.173:5060           To: 192.17.1.173:5060

ATA                                NAT<-------------------------Ser
                                                  From: 192.17.1.173:5060
                                                  To: 192.17.1.22:260

ATA                                NAT-------------------------->Ser
                                                   Destination Unreachable