Here’s a Kamailio question I posed to the mailing list the other day:

I’m working on a scenario with a Kamailio box with a private IP, with a public IP 1:1 NATed to it (but the VM does not see the public IP on the NICs).

When forwarding requests to some hosts I want to set the Via address to be the public IP, but when forwarding requests to other hosts I want to leave the Via address to the internal IP address.

If I set the Advertise parameter in the bind config, this sets the Via to the advertised IP, but I’m seeing that address used even when communicating with hosts on the private IP.

Of course if the IP was on the VM itself I could use $fs or force_send_socket, but that only works if I’ve got the public IP bound, which I can’t do.

Is there a simple way to set / override what IP gets baked into the Via header?

Where you’ve got multiple IPs on your box, you can include the advertise paramter to override the IP you show, for example if you have the IP 100.64.253.251 on your NIC, but you’ve NATed the traffic and instead want to show 1.2.3.4 you can set this in the general config:

listen=tcp:100.64.253.251:5061 advertise 1.2.3.4
listen=udp:100.64.253.251:5061 advertise 1.2.3.4

But then every message Kamailio forwards, will contain the 1.2.3.4 address int he Via header. For my scenario, this didn’t work, as I wanted to only use the 1.2.3.4 IP when communicating with hosts outside of the RFC1918 address space (only conditionally use that address).

Because of this I couldn’t use the advertise option, but I found the set_advertised_address() function to use in my routing logic, where I set the advertised address just on the given routes I care about:

#General Config:
listen=tcp:100.64.253.251:5061 


#My routing blocks that go to hosts outside RFC1918 address space:
                rtpengine_manage("media-address=1.2.3.4");
                set_advertised_address("1.2.3.4:5060");
                msg_apply_changes();

So now any packets by default will have a Via of 100.64.253.251 but when I want to I can set the Via to the 1.2.3.4 public IP (and I do the same in RTPengine).