Hi all. Thanks for so many replies so quickly. I totally take on board the comments about UDP responses on the same IP not being trivial and probably not being worth it to implement.
However its worth pointing out for the record why its useful here. The situation here is that the RADIUS requests come from load-balanced upstream telco proxies who require two IPs for the RADIUS servers for both resilience and load-balancing. Normally these would be serviced by two physical servers with two real IPs but, when one server is not available, the other can take over by taking the IP as a virtual interface. There are a lot of arguments about whether this is a sensible thing to do etc. however this is what I am trying to implement (and it works for UDP DNS requests with ISC bind). Thanks for all the help on this - I think for now I'll look for a solution outside of the RADIUS software (translation on firewalls etc. most likely) as this appears the correct place to do this kind of thing, Paul - List info/subscribe/unsubscribe? See http://www.freeradius.org/list/users.html
