Cisco CCNA (200-301)IP ServicesMedium
A network administrator is configuring a Cisco router to forward DNS queries from internal clients to a public DNS server at 8.8.8.8. The internal clients are configured to use the router's internal interface IP address as their DNS server. Which command should be configured on the router to act as a DNS relay agent?
- Aip domain lookup
- Bip dns server
- Cip dns proxy
- Dip name-server 8.8.8.8
Show answer & explanationAnswer & explanation
Correct answer: B. ip dns server
To configure a Cisco router to act as a DNS relay agent (or DNS server for local clients), the 'ip dns server' command must be enabled globally. This allows the router to accept DNS queries from clients and then forward them to the configured upstream DNS servers (defined by 'ip name-server').
Why the other options are wrong
- A. This command enables DNS lookup functionality on the router, primarily for the router itself to resolve hostnames, not to serve DNS to clients.
- C. There is no 'ip dns proxy' command in Cisco IOS for this specific function; 'ip dns server' serves this purpose.
- D. This command specifies the upstream DNS server(s) the router will use for its own lookups and for forwarding client queries, but it doesn't enable the router to *act* as a DNS server itself.
Cisco IOS DNS Server Functionality
Cisco routers can be configured to act as DNS relay agents or caching DNS servers for local clients, forwarding requests to external DNS servers.
- Enabled with 'ip dns server' command.
- Upstream servers defined by 'ip name-server'.
- Router caches resolved entries to improve performance.
Memory trick: To be a DNS 'server' for clients, the router must have its 'server' function turned ON.