Lesson 28 · Referrals and glue - Build a DNS Resolver

Referrals and glue

When a server is not authoritative it does not answer - it refers you to servers closer to the name, listing them as NS records with their addresses as glue. Today you read a referral to find the next server to ask.

The goal

Extract the next server address from a referral's NS and glue records.

Start here - the target

TO DO

Scenario: A referral points at the next server via NS and glue

Givena referral response with no answers, an authority NS record naming "a.gtld-servers.net", and an additional (glue) A record giving a.gtld-servers.net the address 192.5.6.30

Whenthe resolver reads the referral for a next server to query

Thenit returns the address "192.5.6.30" (the glue for the referred name server)

Andthe next server comes from matching an authority NS name to an additional-section A record

Background

DNS is a delegated tree, and most servers you ask are not authoritative for the name. Instead of answering, they send a referral: an empty answer section, a set of NS records in the authority section naming servers that are closer to the name, and - crucially - glue in the additional section, which is the A records giving those name servers’ IP addresses. Without glue you would have a chicken-and-egg problem (you would need to resolve the name server’s name to resolve the name), so the parent zone bundles the addresses right in.

Reading a referral is therefore: find an NS record (type 2) in the authority section, decode the server name from its RDATA, and look up that name’s address in the additional section’s glue. Decoding the NS name means resolving its RDATA against the whole message (its pointers are message-relative), so the parsed Message needs to keep the raw response bytes around - add a Raw field that ParseMessage fills, and decode the NS target from m.Raw at the record’s RDATA offset. Here the .com name server a.gtld-servers.net comes with glue 192.5.6.30, so that is the address to query next. This single step - turning a referral into a next hop - is the engine of iterative resolution, which the next lesson wraps in a loop from the root all the way down.

Make it work

// TypeNS = 2. Decoding the NS name needs the whole message, so keep the
// raw response bytes on the Message (a Raw field set in ParseMessage).
func nextServer(m Message) (string, error) {
  for _, ns := range m.Authority {
    if ns.Type != TypeNS { continue }
    nsName, _ := decodeNameRData(m.Raw, ns.RDataOff) // the referred server
    // find that name's glue A record in the additional section
    if a := findGlue(m.Additional, nsName); a != "" { return a, nil }
  }
}

CheckpointDONE

The resolver reads a referral to find the next server. Commit and stop here.