Lesson 01 · A 16-bit value on the wire

A 16-bit value on the wire

DNS is a byte format, and almost every field in it is a 16-bit number stored big-endian - most significant byte first. Today you build the two tiny helpers that put a 16-bit value into two bytes and read it back, the foundation every later lesson reuses.

The goal

Write a 16-bit integer to two big-endian bytes and read it back.

Start here - the target

TO DO

Scenario: A 16-bit value round-trips through two big-endian bytes

Giventhe 16-bit value 1234 (which is 0x04D2)

Whenit is written to two bytes big-endian

Thenthe bytes are 0x04 then 0xD2

Andreading those two bytes back gives 1234, and reading 0xFF 0xFF gives 65535

Background

Everything in a DNS message is packed into bytes in a fixed order, and the single most common unit is the 16-bit unsigned integer stored big-endian: the high byte comes first, the low byte second. The message identifier, every section count, the record type, the class, and the data length are all 16-bit big-endian values. So the very first thing to build is a reliable way to move a 16-bit number onto two bytes and back off them.

The value 1234 is 0x04D2 in hex, so its high byte is 0x04 and its low byte is 0xD2. Writing high-then-low and reading hi<<8 | lo is the whole idea. It is deliberately tiny, but you will call these two helpers in nearly every lesson from here on, so getting the byte order right now saves a great deal of confusion later.

Make it work

// big-endian: high byte first, low byte second
func putUint16(v uint16) []byte {
  return []byte{byte(v >> 8), byte(v)}
}
func uint16At(b []byte, off int) uint16 {
  // combine the two bytes back into one value
  return uint16(b[off])<<8 | uint16(b[off+1])
}

CheckpointDONE

You can move a 16-bit value to and from the wire. Commit and stop here.