DNS: It is used to translate human readable names to IP address. For redundancy, there can be multiple DNS server, generally, there are two servers. Primary DNS Server holds the master copy of DNS record, and the secondary DNS Server holds the redundant copy of DNS record. Secondary DNS server may request for DNS update from primary DNS server. Full This transfer can be Full transfer or asynchronous zone transfer (AXFR) or Incremental zone transfer (IXFR).
DNS server with DNS Zone transfer enabled is used. If a server is vulnerable to DNS zone transfer, then it can return a list of all subdomains available on that server

Domain Name System (DNS) Basics
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DNS Domain Name System hierarchy
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