Is Host Cache required for RADIUS?

The Host Cache was designed for RADIUS in order to not perform an access/auth exchange with the RADIUS server for each query: when the query comes from an already seen client (same RADIUS idenfier) the answer from the RADIUS server is available from the host cache. This was critical when both were designed because the access/auth exchange was synchronous (i.e. blocking until the answer is received) and single threaded (i.e. blocking the whole DHCP service). Perhaps it is less true today but the host cache is in memory when RADIUS exchanges are over the network so far slower, and the Host Cache also handles negative answers so covers (excepting for the bug described in #3269) all cases.

The Host Cache has a second function for RADIUS: when the RADIUS server returns an address (vs a pool name which is translated into a client class name directly added to the query object) a host entry for this reserved address is inserted in the Host Cache. The idea is the host lookup will be able to find it. This is not essential: the host entry can be attached to the callout handle associated to the query and got back latter as the current code does for the [re]selected subnet.