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38. DNS (Domain Name System)

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Purpose of DNS

  • DNS (Domain Name System) is used to resolve human-readable domain names (like google.com) into IP addresses.
  • Machines (such as PCs) do not use names; they use IP addresses (IPv4 or IPv6).
  • Domain names are much easier for humans to use and remember compared to IP addresses.
    • Example: What is the IP address of youtube.com?
  • When you type youtube.com into a web browser, your device queries a DNS server for the IP address of youtube.com.
  • The DNS servers your device uses can be:
    • Manually configured.
    • Learned automatically through DHCP (Dynamic Host Configuration Protocol).

Basic Functions of DNS

Viewing Local IP Configuration

  • Use the ipconfig /all command to display the local IP configuration of the current device.

Using nslookup

  • The nslookup command shows the IP address information for a given DNS entry (domain name).

DNS Packet Capture (Wireshark)

  • The results of using commands like ipconfig and nslookup can be captured and analyzed with Wireshark.

Managing DNS Cache

  • ipconfig /displaydns shows the DNS cache stored on the device.

  • ipconfig /flushdns clears the DNS cache, forcing the device to fetch fresh DNS information.

Hosts File

  • Devices can use a hosts file to map domain names to IP addresses locally, bypassing DNS.

  • For Windows, the hosts file is located here:

Configuring DNS in Cisco IOS

  • Devices on a network don’t need DNS to be configured on routers to use DNS:
    • Routers forward DNS requests like any other packet.
  • However, a Cisco router can be configured as a DNS server, although this is not common practice.
    • Typically, Windows or Linux servers act as internal DNS servers.
  • Cisco routers can also function as DNS clients.

DNS Commands in Cisco IOS

  • To configure a Cisco router as a DNS server, use the following commands:

    • ip dns server — Enables DNS server functionality on the router.
    • ip host <hostname> <ip address> — Manually assigns a hostname to an IP address.

  • To view the list of manually configured hostnames, use the show hosts command.

  • You can also configure DNS name resolution using the following commands:

    • ip name-server <ip> — Specifies the DNS server(s) the router will use for DNS lookups.
    • ip domain lookup — Enables DNS lookup on the router.

DNS Command Review

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