DNS zone transfers occur when a secondary name server (slave) checks the Start of Authority (SOA) record from the primary server. The serial number in the SOA indicates the version of the zone file.

If:

Primary SOA serial > Secondary SOA serial

→ Zone transfer is initiated by the secondary server to update its data.

From CEH v13:

Module 3: DNS Enumeration

Topic: Zone Transfers and SOA Logic

CEH v13 Study Guide states:

“A secondary name server periodically queries the primary name server for changes. If the serial number in the SOA record is higher on the primary, it indicates a zone update, and a zone transfer is triggered.”

Incorrect Options:

B: Reversed condition

C/D: Restarting services doesn’t necessarily trigger a transfer

E: TTL expiration affects caching, not zone transfer logic