Build it: the concierge who walks (a recursive resolver)

The concierge who walks the chain of referrals for you. In DNS Resolution you watched the walk with dig +trace. Here you build the walker, and the sticky notes it keeps.

Before you start. You need DNS Resolution (the concierge, referrals, glue, TTL), DNS on the wire (the bytes of a question and an answer) and Write the telegram yourself (you wrote a question by hand and read a name back out of a reply). This lesson joins them: the stub of 0009, grown into a program that starts knowing four addresses and ends up able to answer for any name. Python 3, no packages. A different build, From Scratch’s DNS server, is the other half of DNS: that one answers questions for a zone, and this one asks them.

The idea in 30 seconds

A resolver is born knowing only the root hints, a short list of root server addresses, and learns everything else by following referrals downward: root, then the registry, then the zone’s own servers.
Each answer is one of four things: the answer, an alias to chase, a “no” to remember, or a referral to follow with the glue addresses it came with.
Only glue for the names a referral itself listed should be believed; a resolver that trusts any extra address in a reply can be told lies for names it never asked about.
A reply is a stranger’s letter: a random 16-bit tag, a connected socket and a random source port make it hard to forge, but nothing here is encrypted or signed.
A cache keeps answers for their TTL, remembers which servers run each zone so later lookups skip the root, and remembers “no such name” for the smaller of the SOA’s TTL and its minimum field (RFC 2308).

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