Lesson 13 · Following a pointer - Build a DNS Resolver

Following a pointer

A compressed name is read by jumping to the offset the pointer names and decoding the labels found there. Today you extend the name decoder to follow a single pointer, and to report the right consumed length despite the jump.

The goal

Decode a name that is a pointer by following it to an earlier offset.

Start here - the target

TO DO

Scenario: A pointer name decodes to the name at its target

Givena message holding the name www.example.com at offset 12, and the two bytes C0 0C at offset 40

Whena name is decoded starting at offset 40

Thenthe name is "www.example.com"

Andonly 2 bytes were consumed at offset 40 (the pointer itself), not the length of the name it points to

Background

Following a pointer is a jump: when the decoder sees a pointer byte, it reads the target offset, decodes the name sitting there, and uses that as the rest of the current name. In the simplest case the whole name is just a pointer - the two bytes C0 0C at offset 40 mean “the name here is whatever is at offset 12,” so decoding at offset 40 yields www.example.com by reading the labels stored back at offset 12.

The subtlety that trips people up is the consumed length. The name lives at offset 12, but at offset 40 you only spent the two pointer bytes - so the caller advancing through the message at offset 40 must move forward by 2, not by the seventeen bytes of the name it borrowed. A pointer always ends the current name (there is nothing after it), so returning a consumed length of 2 the moment you follow one keeps the outer parser aligned. Partial names and pointer chains build on exactly this.

Make it work

func decodeName(b []byte, off int) (string, int) {
  i := off
  for b[i] != 0 {
    if isPointer(b[i]) {
      // jump to the target; the name there IS this name today
      name, _ := decodeName(b, pointerTarget(b[i], b[i+1]))
      return name, (i + 2) - off // consumed only the 2 pointer bytes
    }
    // ...otherwise read a normal label
  }
}

CheckpointDONE

You can follow a single pointer to decode a name. Commit and stop here.