dns over tls with unbound on openwrt

Tony Finch dot at dotat.at
Thu Feb 13 14:08:21 UTC 2020

Elmar Stellnberger via Unbound-users <unbound-users at lists.nlnetlabs.nl> wrote:

> What is the difference between recursive and forward DNS?

I make a distinction which is a bit more pedantic than usual...

Recursion is about the kinds of queries a server is willing to answer: if
the server sets the RA bit (recursion available) in its responses and is
therefore willing to answer RD (recursion desired) queries. The effect is
that the server will obtain a complete answer and won't return referrals.

This is independent of how the server gets the answers. It can perform
iterative resolution (making queries with RD=0 and chasing referrals) or
it can send recursive queries to another recursive server - which is
called forwarding.

According to this model, saying a server is recursive doesn't imply
anything about whether it forwards queries or does its own iterative
resolution. But usually when a server is described as recursive, that
implies it does iterative resolution.

The way I relate "recursion" in the DNS sense to its usual meaning, is
when one resolver asks another resolver to answer a query on its behalf,
it's a bit (wave hands vigorously) like a recursive call from one function
to another function. (In the DNS case depth of recursion is determined by
the forwarding topology, rather than reducing the complexity of the query
as one would expect from functional recursion.)

What makes the terminology extra confusing is that iterative resolution is
about traversing a tree-shaped namespace (which has a recursive flavour)
and iterative resolution gets explicitly recursive when the resolver has
to resolve a nameserver address in order to follow a referral.

So my rationalizaion is mostly in vain, because it isn't really possible
to relate the DNS uses of recursion and iteration to their non-DNS

f.anthony.n.finch  <dot at dotat.at>  http://dotat.at/
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