DNS software may need to operate in environments where the root servers or other servers are unavailable due to network connectivity or other problems. In this situation, DNS name servers and resolvers MUST continue to provide service for the reachable part of the name space, while giving temporary failures for the rest.
Although the DNS is meant to be used primarily in the connected Internet, it should be possible to use the system in networks which are unconnected to the Internet. Hence implementations must not depend on access to root servers before providing service for local names.