CDN DNS & Domain Management 3 — Questions and Answers
Question 1: What is the function of the DNS SOA (Start of Authority) record's 'Refresh' field?
- How often secondary nameservers check for zone updates (Correct answer)
- The minimum TTL for negative responses
- Time before a zone is considered expired
- How long a failed zone transfer is retried
Correct answer: How often secondary nameservers check for zone updates
The Refresh field tells secondary nameservers how frequently to poll the primary for zone changes.
Question 2: A CDN customer wants to use their own branded domain (white-label) for the CDN edge URL. Which DNS record type enables this without changing the apex domain?
- CNAME (Correct answer)
- A
- NS
- PTR
Correct answer: CNAME
A CNAME aliases a subdomain (e.g., cdn.example.com) to the CDN's edge hostname, enabling white-label delivery.
Question 3: Why can't a CNAME record be placed at the zone apex (root domain) in standard DNS?
- RFC 1034 prohibits CNAME coexistence with other record types such as SOA and NS at the same node (Correct answer)
- CNAME records do not support IPv6
- The apex domain cannot be resolved by recursive resolvers
- CDN providers block apex CNAMEs for security reasons
Correct answer: RFC 1034 prohibits CNAME coexistence with other record types such as SOA and NS at the same node
RFC 1034 states a CNAME cannot share a node with other records; since the apex needs SOA/NS, a CNAME there is invalid.
Question 4: What technology do many CDNs offer to work around the CNAME-at-apex limitation?
- ANAME / ALIAS records (flattened CNAME) (Correct answer)
- Wildcard A records
- Split-horizon DNS
- DNSSEC chain of trust
Correct answer: ANAME / ALIAS records (flattened CNAME)
ANAME/ALIAS records resolve the CNAME target server-side and return its IPs as an A record, bypassing the apex restriction.
Question 5: Which DNS query type asks the resolver to return all available record types for a given name?
- ANY (QTYPE=255) (Correct answer)
- TXT
- SOA
- AXFR
Correct answer: ANY (QTYPE=255)
A DNS query with QTYPE=255 (ANY) requests all record types, though many resolvers now limit responses due to amplification abuse.
Question 6: In the context of CDN DNS management, what is 'split-horizon' (split-brain) DNS?
- Returning different DNS responses to internal vs. external clients for the same hostname (Correct answer)
- Using two separate CDN vendors simultaneously
- Splitting a zone file across multiple nameservers
- A BGP technique for routing around outages
Correct answer: Returning different DNS responses to internal vs. external clients for the same hostname
Split-horizon DNS serves different records based on the requester's network, commonly used to route internal traffic off the CDN.
Question 7: What does a high TTL on a CDN CNAME record negatively impact during a CDN provider migration?
- DNS propagation speed — old IPs stay cached longer across the internet (Correct answer)
- TLS certificate issuance time
- HTTP/2 multiplexing efficiency
- Anycast path selection accuracy
Correct answer: DNS propagation speed — old IPs stay cached longer across the internet
High TTLs mean resolvers cache the old CNAME target longer, delaying traffic migration to the new CDN provider.
CDN DNS & Domain Management 3
What is the function of the DNS SOA (Start of Authority) record's 'Refresh' field?