I had an issue with one of my SIP trunk yesterday and had to do some packet sniffing on the wan side.
My setup was as follow:
Cable modem <->Mikrotik<->PaBx
The cable modem is setup as bridge and provide to the Mikrotik via DHCP a routable wan IP address (lets say 22.214.171.124).
However, the cable modem has also a non routable IP address (lets say 192.168.100.1) to access its dashboard.
So I added to the Mikrotik WAN interface a static IP address of 192.168.100.10 to be able to access this webpage.
My PaBx has a local LAN address (lets say 192.168.200.20)
Now when my PaBX send a REGISTER SIP request, on the LAN side, it shows (in the packet content) as coming from 192.168.200.20.
But on WAN side, after the SIP algo, instead of showing coming from the 126.96.36.199 address, it shows (again in the packet content) as coming from the 192.168.100.10 address.
If I remove the 192.168.100.10 address from the WAN interface, the SIP packet is properly sent to the WAN side with the 188.8.131.52 address.
How can I force the WAN address to use the 184.108.40.206 address for all traffic that is not going local? As the 220.127.116.11 address is obtained by DHCP, the preferred source field is not available in the dynamic 0.0.0.0/0 route. But again it is not a problem with the packet header, just the SIP content.
Is it a SIP algo problem?
For now I am just disabling the local non routable address on the WAN side. But I would like to learn how to avoid that issue in the future.