[SR-Users] How to encode domain in username for Asterisk REGISTER forwarding (was: How to configure Kamailio + Asterisk (on same server) to route between several disjoint networks?)

[Alex Villací­s Lasso]

On the previous issue (disjoint networks), I eventually settled for binding Asterisk on localhost:5080, and using rtpproxy to route media to the networks as required. The only snag is that I had to write a script to rewrite the Kamailio configuration in 
order to take current IPs into account.

Now, for the same project. What I want now is to be able to express multiple domains with an Asterisk backend. That is: given pbx.company1.com and pbx.company2.com, two DNS addresses that point to the same server, I want to have separate accounts 
bob at pbx.company1.com and bob at pbx.company2.com, with separate registration and accounting, without collisions.

I know about the Kamailio "domain" module, and I have enabled WITH_MULTIDOMAIN and filled the kamailio.domain table for it to work. Now the authentication is taking domains into account.

Next, I want to express two non-colliding registrations for Asterisk. What comes to mind is username rewrite - Kamailio sees bob at company1.com, and forwards a REGISTER to Asterisk using bob_company1_com as the SIP username. I am looking right now at the 
WITH_ASTERISK block that uses the uac module:

# Forward REGISTER to Asterisk
route[REGFWD] {
         if(!is_method("REGISTER"))
         {
                 return;
         }
         $var(rip) = $sel(cfg_get.asterisk.bindip);
         $uac_req(method)="REGISTER";
         $uac_req(ruri)="sip:" + $var(rip) + ":" + $sel(cfg_get.asterisk.bindport);
         $uac_req(furi)="sip:" + $au + "@" + $var(rip) + ":5060";
         $uac_req(turi)="sip:" + $au + "@" + $var(rip) + ":5060";
         $uac_req(hdrs)="Contact: <sip:" + $au + "@"
                                 + $sel(cfg_get.kamailio.bindip)
                                 + ":" + $sel(cfg_get.kamailio.bindport) + ">\r\n";
         if($sel(contact.expires) != $null)
                 $uac_req(hdrs)= $uac_req(hdrs) + "Expires: " + $sel(contact.expires) + "\r\n";
         else
                 $uac_req(hdrs)= $uac_req(hdrs) + "Expires: " + $hdr(Expires) + "\r\n";
         uac_req_send();
}
#!endif

The above is the non-encoding version. What is the best way to modify it to do a domain encoding in the username? Or, if a better solution exists, what is it?