Dns query

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

A DNS query is a request for DNS information, such as the network address associated with a domain name.

A DNS query requests DNS information, commonly translating a domain name such as www.google.com into an IP address. Before sending one, a device checks DNS caches maintained by the operating system and applications. If no matching entry exists, it queries its configured DNS server, receives a response, and stores the mapping temporarily for reuse. section 3.1.2 section 3.3

Follow the operational path and exam-relevant behavior:

• Send queries to the DNS server on port 53; standard queries commonly use UDP, while TCP is also supported for certain responses. section 3.1.2 section 3.3
• Use a recursive query from the client when a definite IP address or an error is required; the DNS server performs the resolution. section 3.2.2 section 3.3
• Learn the DNS server address through DHCP or manual configuration, then cache successful name-to-address mappings. section 3.1.2

On Cisco IOS, enable client lookups with ip domain lookup, enabled by default, and configure resolvers with ip name-server ip-address. Specify up to six servers; IOS tries another when the first fails. To provide DNS service locally, configure ip dns server and upstream resolvers with ip name-server. The router then answers client queries from its cache or forwards them upstream. section 3.2.1 section 3.2.2

Verify Cisco IOS name resolution

Before configuring a resolver, ping google.com fails with an unrecognized host message. After configuring `ip name-server 8.8.8.8`, the output shows `domain server (8.8.8.8) [OK]`, resolves the name to an IP address, and completes the ping successfully. Configure `ntp server time1.google.com` to demonstrate that IOS can resolve a service name for NTP. section 3.2.1

Figure 3.2 DNS name resolution. (1) PC1 sends a DNS query to its DNS server (8.8.8.8) asking for the IP address of www.google.com. (2) The DNS server replies, informing PC1 of the IP address. (3) PC1 communicates with the web server via HTTPS (for example, to retrieve a web page).

Trace the complete exchange: PC1 queries its DNS server, receives the address for www.google.com, and then connects to the web server using HTTPS. section 3.1.2

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes

Explains caching, recursive resolution, transport choices, and Cisco IOS configuration while deferring detailed TCP-versus-UDP conditions.

Examples

Uses www.google.com resolution, Cisco ping output, IOS host mappings, and time1.google.com NTP configuration.

What it assumes

Requires a reachable DNS server address learned through DHCP or manual configuration.

What you can do after

Configure and troubleshoot hostname resolution on Cisco IOS using ip name-server, show output, and test commands.

How it connects

Connects DNS queries with DNS cache, DNS server, DHCP, recursive queries, HTTPS, and NTP.

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Related concepts

• DNS serveruses · A DNS server is a network service that answers domain-name queries by returning corresponding DNS records or obtaining them from other DNS servers.
• DNS cachecontrasts with · A DNS cache is temporary storage of domain-name resolution results used to answer subsequent lookups without repeating upstream DNS queries.
• IP addressproduces · An IP address is a numerical address used by the Internet Protocol to identify a network interface and support packet delivery across networks.
• random forestproduced by · A random forest is an ensemble model that combines predictions from multiple decision trees built with randomized data samples, feature selections, or both.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 2 Advanced Networking and Security. The section links point to the parts of the book it draws on.