DNS

Table of Contents

• View DNS Server Info[Windows]
• DNS Cache
• DNS in Cisco IOS

A client by the name of Bob wants to access youtube.com on his laptop. He types in the URL in his web browser, chrome; and hit enter. The browser issues a DNS query to google’s DNS server asking for the IP. It is then resolved accordingly.

Domain Name System or DNS for short is an important protocol in resolving human-readable names to an IP address.

• names easier for humans
• machines use IPv4 or IPv6 addresses
• DNS server is responsible for the conversion
• 8.8.8.8 is Google’s public DNS server
• A records maps IPv4
• AAAA records maps IPv6

View DNS Server Info[Windows]

ipconfig /all          #Display IP configurations
nslookup youtube.com   #DNS lookup info
ping youtube.com       #Connectivity test

Cisco routers always forward DNS lookup queries as DNS clients. But they can be also configured to be a DNS server.

DNS Cache

Saves unnecessary traffic due to DNS queries look up.

• Windows host files – alternative to DNS

ipconfig /displaydns #display dns cache entries on host
ipconfig /flushdns   #flush dns resolver cache on host 

DNS in Cisco IOS

In most cases, you do not have to configure DNS on a Cisco router. Usually, a DNS server is used.

Below is an example of how you can configure DNS on a cisco router in packet tracer on router R1.

R1(config)#ip name-server 1.1.1.1    #CloudFlare DNS server 
R1(config)#ip host R1 192.168.0.254  #Entry for R1
R1(config)#ip host PC1 192.168.0.1   #Entry for PC1
R1(config)#ip host PC2 192.168.0.2   #Entry for PC2
R1(config)#ip host PC3 192.168.0.3   #Entry for PC3