Choose the DNS server setting that will prevent an existing host and its IP address from...
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Choose the DNS server setting that will prevent an existing host and its IP address from being known.
Show answer & explanation
Zone transfers (DNS zone transfers) are requests to replicate DNS records from a DNS server. Unrestricted zone transfers allow attackers to enumerate all hosts and their IP addresses in a domain (DNS zone enumeration). Controlling the accessing source of zone transfers restricts who can perform these transfers, effectively preventing unauthorized disclosure of host and IP address mappings. Options A, C, and D relate to mail relay security, CGI execution restrictions, and web server directory listing—none of which directly control DNS host/IP visibility.
Step-by-step Derivation:
The question asks: what DNS setting prevents hosts and their IPs from being known? This is a security question about DNS enumeration. Zone transfers (AXFR queries) allow full DNS record dumps, exposing all hosts in a domain. By controlling zone transfer access (restricting it to authorized secondary DNS servers only), you prevent attackers from discovering host-IP mappings. Option B directly addresses DNS security; the others address unrelated server security concerns (SMTP relay, CGI, web directory listing).