DNS

Day 38

DNS

DNS (Domain Name System) is used to resolve human-readable names to IP addresses. Devices do not use names, they use addresses. When you type the name of a website into your browser, your device asks a DNS server for the IP address of the website.

The hosts can automatically learn the address of the DNS server via DHCP. To see the basic IP configuration info on Windows, enter the command ipconfig from the command prompt. For more detailed output, use ipconfig /all. To ask a DNS server for an IP address of the specific website, use the command nslookup on Windows. To ping a device with a specific number of pings on Windows, use the command ping followed by an IP address, option -n, and the number of pings to send.

DNS A records are used to map names to IPv4 addresses, and AAAA records - for IPv6. DNS uses both TCP and UDP. Standard DNS messages use UDP. TCP is used for DNS messages greater than 512 bytes. The port used is 53.

Cache

Devices save the DNS server's responses to a local DNS cache so they don't have to query the server every time. To see the DNS cache on Windows, enter the command ipconfig /displaydns. To clear the DNS cache, use the command ipconfig /flushdns.

Cisco IOS

There is no need to configure DNS on routers for hosts to use DNS. Routers just forward DNS messages like any other packets. However, a Cisco router can be configured as a DNS server. If an internal DNS server is used, usually it's a Windows or Linux server. A Cisco router can also be configured as a DNS client. To configure a router as a DNS server:

  1. Configure the router to act as a DNS server - ip dns server.

  2. Configure a list of hostname/IP address mappings - ip host followed by the hostname and IP address.

  3. Configure a DNS server that the router will query if the requested record isn't in its host table - ip name-server followed by the DNS server IP address.

  4. Enable the router to perform DNS queries - ip domain lookup. It is enabled by default.

The last two commands shown above are used to configure the router as a DNS client so the router acts as a DNS client in case it doesn't have an entry for the specified name.

You can also configure the default domain name with the command ip domain name followed by the domain name. The domain name will be automatically appended to any hostnames without a specified domain.

The command to view the DNS table on a router is show hosts.

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