[Serusers] Allowing incoming calls from trusted peers

[Cameron Beattie]

I want to allow peering with specific providers and am not sure of the best 
way to do this. At the moment I have the following in ser.cfg:
        if (method=="INVITE" && !allow_trusted()) {
                if (!proxy_authorize("","subscriber")) {
                        proxy_challenge("","0");
                        break;
                } else if (!check_from()) {
                        sl_send_reply("403", "Use From=ID");
                        break;
                };

                consume_credentials();
        };

SER sends a 407 back to any server trying to send a call to my server.

As I understand it allow_trusted only allows specification of trusted IP 
addresses. I'm not clear what it is checking though. Is it the IP address of 
the host provided in the URI, or From, or something else?

What if I want to validate based on domain name? What are the risks of the 
following approach?
        if (method=="INVITE" && !is_from_local()) {

Any help appreciated.

Cameron