Lesson 09 · Decoding a QNAME - Build a DNS Resolver

Decoding a QNAME

To read any response you must turn wire-format labels back into a dotted name. Today you decode a QNAME by walking its length-prefixed labels until the zero terminator, and report how many bytes you consumed.

The goal

Decode a length-prefixed name back to a dotted string and its byte length.

Start here - the target

TO DO

Scenario: Length-prefixed labels decode to a dotted name

Giventhe 17 bytes 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00

Whenthe name is decoded starting at offset 0

Thenthe name is "www.example.com" and 17 bytes were consumed

Andthe bytes 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 decode to "example.com" with 13 bytes consumed

Background

Decoding reverses the encoder: read a length byte, take that many characters as a label, and repeat until you hit the zero terminator. Joining the labels with dots rebuilds www.example.com. The one subtlety is that callers need to know how many bytes the name occupied, because in a real message the type and class fields (and the next record) come right after it - so return the consumed length alongside the string.

Count carefully: the seventeen bytes of www.example.com include the final 00, so the consumed length is 17, and example.com consumes 13. This consumed-length discipline is what lets a parser advance cleanly from one field to the next, and you will lean on it constantly once records start stacking up. Today’s decoder assumes every name is spelled out in full; chapter three adds the compression pointers that let a name borrow bytes from earlier in the message.

Make it work

func decodeName(b []byte, off int) (string, int) {
  var labels []string
  i := off
  for b[i] != 0 { // stop at the zero root label
    n := int(b[i]); i++
    labels = append(labels, string(b[i:i+n]))
    i += n
  }
  return strings.Join(labels, "."), (i + 1) - off // +1 for the terminator
}

CheckpointDONE

You can decode a QNAME and report its byte length. Commit and stop here.