7. Stub Resolver Considerations

Although not strictly required to do so by the protocol, most DNS queries originate from stub resolvers. Stub resolvers, by definition, are minimal DNS resolvers that use recursive query mode to offload most of the work of DNS resolution to a recursive name server. Given the widespread use of stub resolvers, the DNSSEC architecture has to take stub resolvers into account, but the security features needed in a stub resolver differ in some respects from those needed in a security-aware iterative resolver.

Even a security-oblivious stub resolver may benefit from DNSSEC if the recursive name servers it uses are security-aware, but for the stub resolver to place any real reliance on DNSSEC services, the stub resolver must trust both the recursive name servers in question and the communication channels between itself and those name servers. The first of these issues is a local policy issue: in essence, a security-oblivious stub resolver has no choice but to place itself at the mercy of the recursive name servers that it uses, as it does not perform DNSSEC validity checks on its own. The second issue requires some kind of channel security mechanism; proper use of DNS transaction authentication mechanisms such as SIG(0) ([RFC2931]) or TSIG ([RFC2845]) would suffice, as would appropriate use of IPsec. Particular implementations may have other choices available, such as operating system specific interprocess communication mechanisms. Confidentiality is not needed for this channel, but data integrity and message authentication are.

A security-aware stub resolver that does trust both its recursive name servers and its communication channel to them may choose to examine the setting of the Authenticated Data (AD) bit in the message header of the response messages it receives. The stub resolver can use this flag bit as a hint to find out whether the recursive name server was able to validate signatures for all of the data in the Answer and Authority sections of the response.

There is one more step that a security-aware stub resolver can take if, for whatever reason, it is not able to establish a useful trust relationship with the recursive name servers that it uses: it can perform its own signature validation by setting the Checking Disabled (CD) bit in its query messages. A validating stub resolver is thus able to treat the DNSSEC signatures as trust relationships between the zone administrators and the stub resolver itself.