A and CNAME records & loop detection
Follow DNS records
Lesson 13 of 20 · 11 min · Module 4: Names and the web
DNS turns names into addresses. Some names are aliases: a CNAME record says "look up this other name instead".
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By the end, you’ll be able to…
- Distinguish A and CNAME records
- Follow a chain with a loop
- Protect a loop against cycles
The idea
An A record holds an IPv4 address. A CNAME points to another name, which may itself be a CNAME. A resolver follows the chain until it reaches an A record, and it must stop if the chain loops.
records = {"www.example.test": ("CNAME", "example.test"), "example.test": ("A", "203.0.113.10")}
print(records["www.example.test"])01Aliases are common
www.example.com is often a CNAME to example.com, and content networks add more hops. Following the chain is routine work for every resolver.
02Unpack each record
kind, value = records[name] splits the pair. An A record ends the search; a CNAME replaces name with its target and the loop continues.
kind, value = ("CNAME", "shop.test")
print(kind, value)CNAME shop.test03Remember where you have been
A misconfiguration such as a → b → a would make a naive loop run forever. A set of visited names detects the cycle on the first repeat.
Your turn
resolve(name, records). records maps a name to ("A", address) or ("CNAME", target). Follow CNAMEs and return the final address. Raise LookupError if a name is missing or if the chain visits the same name twice.What the checks look for
- Returns an A record directly
- Follows a chain of CNAMEs
- Raises LookupError for a missing name
- Raises LookupError for a CNAME loop
Take this with you
Follow CNAMEs to an A record, and track visited names so a loop cannot trap you.
Go a little further
Return the whole chain of names along with the final address, which is useful when debugging DNS.