In a SIP Register message should the From header contain a resolvable host.domain?

Question

I have this problem: In kamailio I serve multiple domains that are not actually true Internet domains. A specific pbx, when registering, has a configuration that asks for the host and domain of the registering user, and puts that in the From header along with the username:

From: <sip:username@host.domain>

But the problem is that it first tries to resolve the hostname.domain, and if it is not resolvable it does not even try to register. Is this behaviour conforming to the SIP standard?

Answer 1

No it's not conforming if it is doing a lookup on the From header hostname. From the RFC (section 8.1.1.3):

...it is very important that the From URI not contain IP addresses or the FQDN of the host on which the UA is running, since these are not logical names.

Furthermore the SIP RFC gives an example of a valid From header as sip:thisis@anonymous.invalid where the hostname is not even a valid DNS hostname.

In most cases the From header URI is to identify the person making the request as part of the SIP challenge response authentication. In some cases SIP servers will disregard the hostname part of the From URI assuming that the authentication realm is some default value.

Answer 2

It will be depending on SIP server's implementation.

Add "host.domain" in /etc/hosts