The Resolver Walks Down the Tree

Core Question

Core Question

How does a resolver get from the root to an answer, and what does each hop actually decide?

Outcome

Outcome

By the end of this session, the learner should be able to:

• describe iterative resolution as a loop, not a lookup
• explain what a zone knows directly versus what it delegates
• explain why the walk picks the most specific matching delegation
• name the three ways the walk can end without an answer

Read Order

Read Order

1. Read Zone
2. Read zone_covers()
3. Read pick_delegation()
4. Read walk_from_root()
5. Run examples/dns/session_02_walkthrough.py
6. Trace www.example.com by hand: which zone answers, and which zones only refer?

Read It Like Code

Read It Like Code

Zone(
    name,
    answers,
    delegations,
)

Fields That Matter

Fields That Matter

Walk Loop

Walk Loop

start at root
  zone has (qname, qtype)?        -> answer, stop
  a delegation covers qname?      -> descend to the most specific child
  neither?                        -> dead end, stop
repeat, bounded by max_steps

Reading Lens

Reading Lens

The important move in this session is to stop thinking "the resolver asks DNS" and start asking:

• which zone was consulted at each step?
• did that zone answer, or did it refer?
• which condition ended the loop?

Toy Model Boundary

Toy Model Boundary

Real iterative resolution sends packets to nameserver addresses and handles glue records. This lesson models each vantage point as a Zone object in a dict so the loop structure is readable without sockets.

max_steps plays the role that packet budgets and QNAME minimization limits play in real resolvers: the walk must be bounded.

Code Landmarks

Code Landmarks

zone_covers()

One suffix comparison. This is the entire meaning of "the name is inside this zone's subtree".

pick_delegation()

When several children could cover the name, the longest match wins. The same "most specific wins" instinct you know from routing tables appears here.

walk_from_root()

The main reading target. One loop, three exits: answer, no matching delegation, missing zone. The path list is the trace a debugging engineer wishes dig +trace output made this explicit.

Failure Questions

Failure Questions

Use the source file to answer these:

1. What does the walk return when a zone has no answer and no matching delegation?
2. What does stopped_because say when a delegation points to a zone nobody serves?
3. Why does the walk for ftp.example.com still reach example.com before failing?
4. What prevents the loop from running forever?
5. Why does the root zone cover every name?

Walkthrough

Walkthrough

Run this:

PYTHONPATH=src python3 examples/dns/session_02_walkthrough.py

The walkthrough prints the zone path for each scenario, including a broken delegation.

Done When

Done When

The learner can say all of the following without looking at notes:

• "Resolution is a loop that descends one delegation per iteration."
• "A zone either answers or refers. The walk only ends on an answer or a dead end."
• "The most specific delegation that covers the name is the one that gets followed."

References

References

• RFC 1034 Section 4.3.2
• RFC 1034 Section 5.3.3
• RFC 8499 (zone, delegation, referral terminology)

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