Why DNS NS Records Point to Hostnames Instead of IP Addresses: Technical Deep Dive


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When examining the standard DNS query output for NS records, we encounter a seemingly circular dependency:

; <<>> DiG 9.10.3-P4-Ubuntu <<>> NS facebook.com
;; QUESTION SECTION:
;facebook.com.          IN  NS

;; ANSWER SECTION:
facebook.com.       86400   IN  NS  a.ns.facebook.com.
facebook.com.       86400   IN  NS  b.ns.facebook.com.

The DNS specification (RFC 1034 and RFC 1035) mandates this design for several critical reasons:

  • IP Flexibility: Hostnames allow name servers to change IP addresses without requiring global DNS updates
  • Load Balancing: Multiple A records can be associated with a single NS hostname
  • Anycast Support: The same NS hostname can resolve to different IPs in different network locations

Here's how modern resolvers handle this efficiently:

# First query: Get NS records
$ dig +short NS facebook.com
a.ns.facebook.com.
b.ns.facebook.com.

# Second query: Get IPs for NS (with glue records)
$ dig +short a.ns.facebook.com
129.134.30.12

DNS servers actually provide IP hints (glue records) in the ADDITIONAL section when returning NS records:

;; AUTHORITY SECTION:
facebook.com.       3600    IN  NS  a.ns.facebook.com.

;; ADDITIONAL SECTION:
a.ns.facebook.com.  3600    IN  A   129.134.30.12

Modern DNS implementations mitigate the extra lookup through:

  • Glue record caching at resolvers
  • Prefetching of NS record IPs
  • Parallel query optimization

Consider this problematic scenario that would occur with IP-based NS records:

# Hypothetical broken implementation
facebook.com. IN NS 129.134.30.12

# When the IP changes, all resolvers worldwide would need
# to update their caches simultaneously - impossible!

When examining DNS resolution through tools like dig, a fundamental question arises: why do NS records return hostnames rather than direct IP addresses? This architectural choice becomes evident when querying major domains:

dig ns google.com +short
ns1.google.com.
ns2.google.com.
ns3.google.com.
ns4.google.com.

The primary reason stems from avoiding circular dependencies in DNS resolution. If NS records contained IP addresses:

  • Changing IPs would require updating all parent zones
  • DNS glue records would become obsolete
  • IP changes would immediately break resolution

Here's what actually happens during resolution:

  1. Query root servers for TLD NS records
  2. Get hostnames of authoritative nameservers
  3. Resolve these hostnames through additional queries
# Complete resolution example
dig +trace facebook.com

; <<>> DiG 9.16.1 <<>> +trace facebook.com
;; Received 525 bytes from 8.8.8.8#53(8.8.8.8) in 12 ms

facebook.com.     172800  IN  NS  a.ns.facebook.com.
facebook.com.     172800  IN  NS  b.ns.facebook.com.
;; Received 117 bytes from 192.5.5.241#53(f.root-servers.net) in 48 ms

a.ns.facebook.com.  300  IN  A  129.134.30.12
;; Received 63 bytes from 129.134.30.12#53(a.ns.facebook.com) in 24 ms

This design is formalized in several RFCs:

  • RFC 1034 (Domain Names - Concepts and Facilities)
  • RFC 1035 (Domain Names - Implementation and Specification)
  • RFC 2181 (Clarifications to the DNS Specification)

While the extra lookup seems inefficient, modern DNS implementations optimize this through:

# Caching NS records locally
dig +norecurse @8.8.8.8 facebook.com

# Pre-fetching glue records
dig +additional facebook.com NS

The indirection provides important security benefits:

  • IP addresses can change without breaking the DNS hierarchy
  • DNSSEC validation works more effectively
  • Reduces attack surface for DDoS

For developers working with DNS programmatically, understanding this design is crucial when implementing resolvers or troubleshooting DNS issues.