Lesson 10 · The question section - Build a DNS Resolver

The question section

A question is a name plus two 16-bit fields - the record type you want and its class. Today you encode a complete question, the part of the message that says what you are actually asking for.

The goal

Encode a question as a QNAME followed by QTYPE and QCLASS.

Start here - the target

TO DO

Scenario: A question is a name plus type and class

Givena question for "www.example.com" of type A (1) and class IN (1)

Whenthe question is encoded

Thenthe result is the QNAME bytes followed by 00 01 (QTYPE A) and 00 01 (QCLASS IN), 21 bytes in all

Andthe last four bytes are 00 01 00 01

Background

The question section is where a query states its request. Each question is a QNAME (the name from lessons 7 and 8) followed by two 16-bit fields: QTYPE, the kind of record you want, and QCLASS, the network class. For internet names the class is always IN, value 1, and the most common type is A, an IPv4 address, also value 1. So a question for www.example.com A IN is the 17-byte name plus 00 01 00 01, twenty-one bytes total.

That is the entire question format. QTYPE reuses the same numbering as record types

you will meet AAAA (28), NS (2), CNAME (5), MX (15), TXT (16), and SOA (6) in chapter four - and a query simply names the one it wants. With a header and a question you now have both halves a real query packet needs; the next lesson snaps them together.

Make it work

const (
  TypeA  uint16 = 1
  ClassIN uint16 = 1
)
func encodeQuestion(name string, qtype, qclass uint16) []byte {
  out := encodeName(name)
  out = append(out, putUint16(qtype)...)
  return append(out, putUint16(qclass)...)
}

CheckpointDONE

You can encode a complete question. Commit and stop here.