Course contents

Chapter 1: Cisco IOSLesson 4 of 10

DNS on Cisco routers and switches

ip name-server, ip domain-name and static ip host entries, and why a mistyped command makes the router pause.

Beginner · 7 min read

Technical definition

DNS on Cisco devices means the router or switch resolving names to IP addresses itself, so you can type ping www.example.com or telnet to a hostname, and so features such as SSH key generation have the domain name they need.

Three things to set

Example commands and output on this page were written from Cisco IOS documentation and generally described behaviour. They were not captured from a live device, so prompts and details can differ on your platform and version.

DNS commands on IOS
DNS commands on IOSCommand; Purpose. ip domain-lookup: Turns name resolution on (it is on by default) ip name-server 192.0.2.53: Which DNS server to ask; up to six can be listed ip domain-name example.com: Domain appended to short names; required to create SSH keys ip host R2 10.0.0.2: A static name to address entry, for lab use no ip domain-lookup: Stops IOS trying to resolve mistyped commands show hosts: Displays cached and static host entriesCommandPurposeip domain-lookupTurns name resolution on (it is on by default)ip name-server 192.0.2.53Which DNS server to ask; up to six can be listedip domain-name example.comDomain appended to short names; required to create SSH keysip host R2 10.0.0.2A static name to address entry, for lab useno ip domain-lookupStops IOS trying to resolve mistyped commandsshow hostsDisplays cached and static host entries

On small screens, swipe sideways or focus the diagram and use the arrow keys.

The address 192.0.2.53 is from the documentation range; use your own DNS server.
configure terminal ip domain-name example.com ip name-server 192.0.2.53 ip host R2 10.0.0.2 end

The router now resolves other names through 192.0.2.53, and R2 resolves to 10.0.0.2 even without a server.

Testing it

Example output · based on Cisco documentation; exact format varies by platform and software version
R1#ping www.example.com
Translating "www.example.com"...domain server (192.0.2.53) [OK]

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 203.0.113.80, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5)
Example output. The first line shows the router asking the name server and getting an answer. The target address is from the documentation range.

The mistyped-command pause

Example output · based on Cisco documentation; exact format varies by platform and software version
R1#shwo ip route
Translating "shwo"...domain server (255.255.255.255)
% Unknown command or computer name, or unable to find computer address
Example output. With no name server configured, IOS broadcasts the lookup and waits. This is why labs and production devices often have no ip domain-lookup.

Keep lookup on only if the device uses names, for example for NTP or syslog by hostname.

Test yourself

Predict · scenario 1

A router waits many seconds after you type shwo. What turns this off?

Predict · scenario 2

Which command is needed before crypto key generate rsa will work for SSH?

Predict · scenario 3

Which command defines a static name to address mapping on the router?

FAQ

Why does my router pause when I mistype a command?
IOS treats an unknown word as a hostname and tries to resolve it, sending a DNS request to the broadcast address if no server is set. Turn that off with no ip domain-lookup, or press Ctrl+Shift+6 to abort.
Do I need ip domain-name?
For SSH, yes: crypto key generate rsa needs a hostname and a domain name to build the key name. It is also appended to short names you type.
Can a Cisco router be a DNS server?
It can answer simple queries with ip dns server for names defined with ip host, which is mainly for labs. It is not a replacement for a real DNS server.