Hi Henning
what about just using the approach in the documentation and example cfg, take the from domain as realm for the challenge?
This is what I am doing now, tentatively...
The Issue with this, is that we have an SBC in front of Kamailio, that uses the R-URI domain of the initial INVITE or REGISTER for routing to the desired registrar / proxy.
So using $rd in kamailio seemed more straight forward to me at that time as I know, existing customers setups will have this domain set correctly when migrating from our actual platform to kamailio. I'm not sure if every customer will send a correct $fd but that has to be determined when testing migration.
Mit freundlichen Grüssen
-Benoît Panizzon-