Lesson 07 · Encoding a QNAME - Build a DNS Resolver

Encoding a QNAME

A full domain name is a sequence of labels ending in a zero byte - the root label. Today you encode a whole name by splitting it on dots, encoding each label, and terminating with that zero.

The goal

Encode a dotted name into labels terminated by the zero root label.

Start here - the target

TO DO

Scenario: A dotted name encodes to labels plus a zero terminator

Giventhe name "www.example.com"

Whenit is encoded as a QNAME

Thenthe result is the 17 bytes 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00

Andthe two-label name "example.com" encodes to 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00, ending in the zero root label

Background

A complete name in the question section is called a QNAME, and it is just the labels from lesson 6 laid end to end, followed by a single zero byte. That trailing zero is the root label - a label of length zero - and it marks the end of the name the same way every name in the DNS tree ultimately traces up to the unnamed root. So www.example.com is three labels (www, example, com) then 00, seventeen bytes in all.

Splitting the string on dots and encoding each piece is the whole job; the zero terminator is what a parser watches for to know the name is done. Notice the two-label case example.com ends the same way - labels, then 00. Every name you ever encode, from a single label to a long fully-qualified name, finishes with that zero root label.

Make it work

func encodeName(name string) []byte {
  var out []byte
  for _, label := range strings.Split(name, ".") {
    out = append(out, encodeLabel(label)...)
  }
  return append(out, 0x00) // the zero-length root label ends every name
}

CheckpointDONE

You can encode a whole domain name as a QNAME. Commit and stop here.