Greetings. The definition of "recursive resolver" has been problematic
both in RFC 7719 and in draft-ietf-dnsop-terminology-bis. Section 6 of
draft-ietf-dnsop-terminology-bis defines a bunch of terms about servers,
including "recursive mode" and "recursive resolver". The current text
Recursive mode: A resolution mode of a server that receives DNS
queries and either responds to those queries from a local cache
sends queries to other servers in order to get the final answers
to the original queries. Section 2.3 of [RFC1034] describes this
as "The first server pursues the query for the client at another
server". A server operating in recursive mode may be thought of
as having a name server side (which is what answers the query)
a resolver side (which performs the resolution function).
operating in this mode are commonly called "recursive servers".
Sometimes they are called "recursive resolvers". While strictly
the difference between these is that one of them sends queries to
another recursive server and the other does not, in practice it
not possible to know in advance whether the server that one is
querying will also perform recursion; both terms can be observed
in use interchangeably.
Recursive resolver: A resolver that acts in recursive mode. In
general, a recursive resolver is expected to cache the answers it
receives (which would make it a full-service resolver), but some
recursive resolvers might not cache.
That is, "recursive mode" is only barely defined in RFC 1034, and
"recursive resolver" is defined almost trivially.
Can these be improved on? This is one of the core ideas in the DNS
protocol and it seems a bit weird that we don't have a crisp set of
definitions. If there is more text from RFCs to quote, that would
possibly be a big help.
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