How would you propagate routing or metric changes to host after it have done the lookup?That doesn't have to be the case. Today, hosts already perform a host/domain name to IP address mapping. You wouldn't take a significant performance hit by looking up the addresses learnt from the DNS in the routing table and tag them with some kind of routing metric. And if you have access to routing at this stage, you can see which exit each addresss takes so which source prefix should be appropriate.