proxy authorization based on NAS IP?

[ST Wong (ITSC)]

Hi all,

We're using freeradius 2.1.1 and are looking for proxy authorization
like following:

1. NAS1 (192.168.10.1) : accepts realmA and realmB, and proxy to remote
radius serverA
2. NAS2 (192.168.10.2) : accepts realmB and realmC, and proxy to remote
radius serverB
3. NAS3 (192.168.10.3) : accepts all realms except realmA, realmB, and
realmC, and proxy to remote radius serverC
4. All other NAS and realm combinations are rejected.

We're studying unlang but have no idea where to add the checking.
Would anyone please help?  Sorry for the newbie question...  

Thanks a lot.
/ST Wong