Lesson 27 · Following a CNAME chain

Following a CNAME chain

When a name is an alias, the answer is a CNAME pointing at another name you must look up in turn. Today you follow that chain through the transport until you reach the address.

The goal

Resolve a name that is a CNAME by re-querying its target until an A record is found.

Start here - the target

TO DO

Scenario: A CNAME answer is chased to its address

Givena scripted transport where querying "www.example.com" returns a CNAME answer pointing to "target.example.net", and querying "target.example.net" returns an A record 203.0.113.5

Whenthe resolver looks up the A record for "www.example.com"

Thenit follows the CNAME and returns "203.0.113.5"

Andif the first response already contained the A record, it is returned without another query

Background

Many names are aliases. Ask for www.example.com and the answer might be a CNAME record saying “the canonical name is target.example.net” - not the address you wanted. To finish the job the resolver must take that target name and look it up in turn, repeating until it gets an A record. A chain can be several hops (www to a load balancer to a CDN host), so loop rather than handle a single alias.

Guard the loop with a small cap so a mischievous chain of CNAMEs pointing in a circle cannot spin forever - the same defensive instinct as the pointer-loop guard. When the answer already has the A record for the name (no alias in the way), return it immediately without a second query. Chasing CNAMEs is the first bit of real resolution logic - the resolver making follow-up queries based on what it learns - and it is a warm-up for the bigger loop ahead: walking the delegation from the root.

Make it work

func (r *Resolver) lookupA(name string) (string, error) {
  for i := 0; i < maxCNAME; i++ {
    msg, _ := r.ask(nextID(), name)
    if a := findA(msg.Answers, name); a != "" { return a, nil }
    if cn := findCNAME(msg.Answers, name); cn != "" {
      name = cn; continue // chase the alias
    }
  }
}

CheckpointDONE

The resolver follows a CNAME chain to an address. Commit and stop here.