Lesson 18 · AAAA records - Build a DNS Resolver

AAAA records

An AAAA record holds an IPv6 address - sixteen bytes read as eight 16-bit groups. Today you format those groups into the colon-separated hex form, the IPv6 counterpart to the A record.

The goal

Convert a 16-byte AAAA record RDATA into a colon-separated IPv6 string.

Start here - the target

TO DO

Scenario: Sixteen bytes of RDATA become an IPv6 address

Givenan AAAA record whose RDATA is 20 01 0d b8 00 00 00 00 00 00 00 00 00 00 00 01

Whenthe RDATA is formatted as eight 16-bit groups in lowercase hex, each without leading zeros, joined by colons

Thenthe result is "2001:db8:0:0:0:0:0:1"

AndRDATA of all zero bytes formats as "0:0:0:0:0:0:0:0"

Background

The AAAA record is the IPv6 address record, and its RDATA is sixteen bytes

an IPv6 address is 128 bits. Read them as eight 16-bit groups and print each in lowercase hex, dropping leading zeros within a group, joined by colons. So 2001:0db8:...:0001 prints as 2001:db8:0:0:0:0:0:1.

Real IPv6 notation also collapses the longest run of zero groups into a ::, but that shortening is a display convenience with fiddly rules, so we deliberately skip it and emit all eight groups every time - a fully valid, unambiguous form that is trivial to produce and to test. Like the A record, an AAAA record’s RDATA has a fixed length (sixteen bytes), so a real parser rejects any other length; we keep this formatter simple and trust the RDLENGTH framing to hand it the right sixteen. The four bytes of A and the sixteen of AAAA are the two address records; the next records swap raw bytes for names.

Make it work

func formatAAAA(rdata []byte) string {
  var groups []string
  for i := 0; i < 16; i += 2 {
    g := uint16(rdata[i])<<8 | uint16(rdata[i+1])
    groups = append(groups, fmt.Sprintf("%x", g)) // lowercase, no pad
  }
  return strings.Join(groups, ":")
}

CheckpointDONE

You can turn an AAAA record into an IPv6 string. Commit and stop here.