Failure Has Flavors

Core Question

Core Question

NXDOMAIN, SERVFAIL, and a timeout all mean "no IP address". Why does a resolver treat one as an answer and the others as reasons to try again?

Outcome

Outcome

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

• classify one server attempt into Answered, NXDOMAIN, SERVFAIL, REFUSED, or Timeout
• explain why NXDOMAIN stops the fallback while SERVFAIL does not
• describe server fallback as an ordered loop with an early exit
• explain what the resolver reports when every server fails differently

Read Order

Read Order

1. Read ServerAttempt
2. Read classify_attempt()
3. Read is_final()
4. Read try_servers()
5. Run examples/dns/session_07_walkthrough.py
6. For each scenario, predict answered_by and tried before looking

Read It Like Code

Read It Like Code

ServerAttempt(
    server,
    timed_out,
    rcode,
    records,
)

Fields That Matter

Fields That Matter

Decision Flow

Decision Flow

classify one attempt:
  timed_out          -> Timeout
  rcode NXDOMAIN     -> NameError
  rcode SERVFAIL     -> ServerFailure
  rcode REFUSED      -> Refused
  otherwise          -> Answered

is it final?
  Answered, NameError   -> yes, stop the fallback
  everything else       -> no, try the next server

Reading Lens

Reading Lens

The important move in this session is to stop reading every failed lookup as "DNS is down" and start asking:

• is this verdict about the name or about the server?
• which attempt in the list produced the final outcome?
• how many servers were tried before the loop stopped?

Toy Model Boundary

Toy Model Boundary

Real resolvers add retry timers, exponential backoff, and per-server reputation. This lesson keeps the attempts as a pre-built tuple so the only logic under study is classification and the stop/continue decision.

The attempts tuple plays the role of "what each server would say if asked" — the loop decides how far down the list the resolver actually gets.

Code Landmarks

Code Landmarks

classify_attempt()

Timeout is checked before rcode, because a timed-out attempt has no rcode worth reading.

is_final()

The single most important line in the file. NXDOMAIN is grouped with Answered, not with the failures — an authoritative "this name does not exist" is information about the name, and asking another server cannot change it.

try_servers()

An ordered loop with an early exit. Note what happens when nothing is final: the result carries the last non-final kind and an empty answered_by.

Failure Questions

Failure Questions

Use the source file to answer these:

1. Why does the NXDOMAIN scenario stop after one attempt even though the second server has records?
2. Which classification comes back for an attempt with both timed_out=True and rcode="SERVFAIL", and why?
3. When every server fails, which kind does the result carry?
4. Why is REFUSED retryable here while NXDOMAIN is not?
5. What does an empty answered_by tell you that the kind alone does not?

Walkthrough

Walkthrough

Run this:

PYTHONPATH=src python3 examples/dns/session_07_walkthrough.py

The walkthrough runs five server lists and prints who answered and how many servers were tried.

Done When

Done When

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

• "NXDOMAIN is an answer about the name. SERVFAIL and timeout are complaints about a server."
• "Fallback is an ordered loop that stops on the first final outcome."
• "Retrying a different server can fix a server problem, never a name problem."

References

References

• RFC 1035 Section 4.1.1 (RCODE)
• RFC 2308 Section 2.1 (name errors)
• RFC 4697 (observed resolver misbehavior, including retry storms)

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