Lesson 05 · Parsing the header - Build a DNS Resolver

Parsing the header

With encoding and flag accessors in hand, you can now read a header off the wire. Today you decode 12 bytes back into a Header struct and prove a full round-trip, closing the chapter with a header you can both write and read.

The goal

Parse 12 bytes into a Header and confirm it round-trips with encoding.

Start here - the target

TO DO

Scenario: Twelve bytes parse back into a header

Giventhe 12 bytes 13 14 01 00 00 01 00 00 00 00 00 00 (a query with ID 0x1314, RD set, QDCOUNT 1)

Whenthe bytes are parsed into a Header

ThenID is 0x1314, QDCOUNT is 1, ANCOUNT/NSCOUNT/ARCOUNT are 0, and the flags word is 0x0100 (RD set)

Andencoding that Header reproduces the original 12 bytes exactly

Background

Reading a header is the mirror of writing one: pull six 16-bit values off the front of the message at offsets 0, 2, 4, 6, 8, 10 and drop them into the struct fields. The only new discipline is a bounds check - a real message might be too short, so refuse anything under 12 bytes rather than reading past the end.

Parsing the query header from today’s bytes gives back exactly the struct that produced them: ID 0x1314, flags 0x0100 (RD set, which the accessors from lesson 4 confirm), QDCOUNT 1, and the rest zero. A round-trip test - parse then re-encode and compare to the original bytes - is the cleanest proof the two halves agree, and it is the pattern you will reuse for names and records. The header is now something you can both build and read.

Make it work

func ParseHeader(b []byte) (Header, error) {
  if len(b) < 12 {
    return Header{}, errShort
  }
  // read six uint16 fields with uint16At at offsets 0,2,4,6,8,10
}

CheckpointDONE

You can parse a header and it round-trips with Encode. Commit and stop here.