Lesson 26 · RCODE and truncation - Build a DNS Resolver

RCODE and truncation

Before trusting a response's answers, a resolver must read its status - the RCODE and the truncation bit. Today you turn a failed or truncated reply into a clear error instead of silently returning empty answers.

The goal

Reject NXDOMAIN and truncated responses, and return answers only on success.

Start here - the target

TO DO

Scenario: Status flags decide success, failure, or truncation

Givenfour parsed responses: one with RCODE 0 and an A answer, one with RCODE 3 (NXDOMAIN), one with RCODE 2 (SERVFAIL), and one with the TC bit set

Wheneach is checked for usable answers

Thenthe RCODE 0 response yields its answers, and the RCODE 3 response returns an NXDOMAIN error

Andthe RCODE 2 response returns a generic server-failure error, and the TC response returns a truncation error (signaling the reply did not fit and would need a retry over TCP)

Background

A response can carry a status as well as data, and a resolver must read it before using the answers. The RCODE (the low nibble of the flags word from lesson 4) reports the outcome: 0 is success, 3 is NXDOMAIN - the name genuinely does not exist - and other non-zero codes are failures like SERVFAIL. Returning an NXDOMAIN as if it were merely an empty answer would be a lie, so map a non-zero RCODE to a clear error.

The TC (truncated) bit is the other guard. UDP responses are size-limited, and when an answer will not fit the server sends what it can and sets TC to say “there is more - ask again over TCP.” Since our transport is UDP-style, a truncated reply is incomplete, so the resolver reports a truncation error rather than trusting a partial answer. Reading these two status signals is what separates a resolver that handles the real world from one that only works on perfect replies. Now the resolver can tell success from failure, it can start chasing multi-step answers.

Make it work

func (m Message) result() ([]RR, error) {
  if m.Header.Flags&flagTC != 0 {
    return nil, errTruncated
  }
  switch rcode(m.Header.Flags) {
  case 0: return m.Answers, nil
  case 3: return nil, errNXDomain
  // other non-zero codes: a generic server-failure error
  }
}

CheckpointDONE

The resolver honors RCODE and truncation. Commit and stop here.