How can I set krt_realm in the kernel protocol by gw (nexthop) value?

[Eric Wheeler]

Hello all,

I would like to put routes in realms for traffic accounting based on the 
peer IP provided by a BGP route server on an exchange.  I was thinking I 
could do something like this, but bird gives errors like "syntax error, 
unexpected KRT_REALM":

        filter gw_realms {
                if gw = 1.1.1.1 then return 1;
                if gw = 2.2.2.2 then return 2;
                if gw = 3.3.3.3 then return 3;
                return 0;
        }

        protocol kernel kernel_ipv4 {
            ipv4 {
                krt_realm filter peer_realms;
            };
        }


What is a proper way to dynamically set the krt_realm attribute per gw?

Thank you for your help!

--
Eric Wheeler