REF: AI Tools/OpenAI/ChatGPT
You’re thinking very carefully about how authority and recursion interact in DNS, and you’re spot on to distinguish between root/TLDs and other domain servers. Let me unpack this clearly:
Subdomains and DNS Servers — Iterative vs Recursive
1. Authoritative Servers (for domains & subdomains)
- Every domain can delegate parts of its namespace to child servers.
- Example:
example.comhas an authoritative server.shop.example.comcould have its own DNS server.us.shop.example.comcould delegate further.
Each of these servers is authoritative for its zone.
👉 Their primary job: return authoritative records (A, MX, CNAME, etc.) for their zone, or referrals if they delegate to a subdomain.
2. Do Authoritative Servers Perform Recursion?
- By design (in the DNS model): Authoritative servers are not required to do recursion.
- Most authoritative servers are configured not to provide recursive lookups, because:
- They’re optimized for answering only about their zone.
- Recursion adds workload and caching responsibilities.
- Mixing roles (authoritative + recursive) can create security risks (e.g., cache poisoning).
- But technically: Nothing prevents a DNS server from being configured as both authoritative and recursive. Some smaller organizations or ISPs do this, though best practice is to separate the roles.
3. Why Root & TLDs Must Be Iterative Only
- As you said:
- Root and TLDs cannot be recursive — it would be unsustainable at global scale.
- They just give referrals back (iteration).
4. Subdomains Can Delegate Too
Let’s take your example further:
- A query for
us.shop.example.com:- Resolver asks root → referral to
.com. - Resolver asks
.com→ referral toexample.com. - Resolver asks
example.com→ referral toshop.example.com. - Resolver asks
shop.example.com→ referral tous.shop.example.com. - Finally,
us.shop.example.comauthoritative server returns the IP.
- Resolver asks root → referral to
At each step, the servers gave iterative referrals, not recursive answers.
👉 The recursive behavior happened only at the resolver (ISP/public DNS).
5. Can Subdomain Servers Be Recursive?
- They can, but usually aren’t.
- Example:
dns.shop.example.comcould be configured as both authoritative (forshop.example.com) and recursive (to fetch answers for clients). - But best practice is:
- Keep authoritative servers iterative-only.
- Use dedicated recursive resolvers (like ISP or Google DNS).
This separation keeps the DNS system clean, scalable, and secure.
✅ So the rule of thumb for your blog post:
- Root and TLDs → Always iterative-only.
- Authoritative servers (for domains/subdomains) → Typically iterative-only, but can technically be recursive if configured (not recommended).
- Resolvers (ISP/public) → Perform recursion on behalf of clients.
Would you like me to write this up as a section titled “Can Authoritative DNS Servers Be Recursive?” so your blog readers understand why it’s possible but rare?
