I need to understand where from packets received. Now I use something like 

If $si == "1.2.3.4" {
 xlog("L_INFO","bla bla bla");
}

But I need to check source server not only by IP and PORT, but at Domain too
For example 

if (some_pseudovariable=="pbx.server.com"){
 xlog("L_INFO","bla bla bla");
}

I can use $fu for example because for some packets it includes domain name of kamailio (i think details not important but this situations can be)
Does kamailio have some mechanisms to do that? I searched it at cookbook but not found anything.

Thank you