Lesson 25 · Parsing a full response

Parsing a full response

A response is a header, the echoed question, and three record sections. Today you parse the whole message into one struct, composing every parser you have built into a single Message you can inspect.

The goal

Parse a complete response message into a Message with its questions and record sections.

Start here - the target

TO DO

Scenario: A whole response parses into header, question, and answers

Givena response with ID 0x1314, QR and RA set, QDCOUNT 1 and ANCOUNT 1: the question www.example.com A IN at offset 12, then one A answer (name pointer C0 0C, type A, class IN, TTL 300, RDATA 5D B8 D8 22)

Whenthe message is parsed

Thenit has 1 question ("www.example.com", A, IN) and 1 answer whose A address is "93.184.216.34"

Andthe answer's name is "www.example.com", resolved through its compression pointer to offset 12

Background

A response has the same shape as a query but fuller: the 12-byte header, then QDCOUNT questions (the server echoes back what you asked), then three record sections - answer (ANCOUNT), authority (NSCOUNT), and additional (ARCOUNT). Parsing it is pure composition of what you already have: parse the header, decode each question (a name plus QTYPE and QCLASS), then call parseRecords three times with the three counts, threading the cursor forward through the whole message.

The one thing to get right is that everything shares one byte buffer, because compression pointers in the records reach back to the question’s name at offset 12. That is why the answer’s C0 0C resolves to www.example.com - the parser passes the full message to every name decode. With a Message struct in hand you can now ask real questions of a reply: did it succeed, what are the answers, is there a referral? Those are the next lessons.

Make it work

type Message struct {
  Header                          Header
  Questions                       []Question
  Answers, Authority, Additional  []RR
}
// parse header, then QDCOUNT questions, then the three sections by count
// using parseRecords for each, threading the cursor through

CheckpointDONE

You can parse a complete response into a Message. Commit and stop here.