DNS/IP NameServer Command
DNS behaviors of a
Cisco router are important topics for both the CCNA exam and real-world
production networks, and you probably didn't know there were so many
DNS details before you began studying for the exam! In this tutorial,
we'll look at the ip name-server command and its proper usage.
When a command is mistyped on a Cisco router, the default behavior of
the router is to attempt to resolve it via DNS. First, the router looks
for an IP Host table on the local router to perform this resolution –
that’s what the “translating” word in the output is referring to. If
there’s no IP Host table or the IP Host table doesn’t contain an entry
for what you typed, the router will send a broadcast in an attempt to
resolve this name through a remote DNS server. To prevent this
broadcast, enter the global command no ip domain-lookup. Of course, to
use DNS to resolve hostnames, ip domain-lookup would have to be
reenabled if it’s been turned off.
R2#contin
Translating "contin"...domain server (255.255.255.255)
% Unknown command or computer name, or unable to find computer address
A command is mistyped as “contin”. The Cisco router’s default behavior
is to resolve this entry locally via an IP Host table, which isn't
present on the router. A broadcast is then sent out to find a DNS
server to perform the name resolution. The DNS lookup attempt must time
out before the configuration can continue.
R2#conf t
R2(config)#no ip domain-lookup
R2#contin
Translating "contin"
% Unknown command or computer name, or unable to find computer address
With “no ip domain-lookup” configured, the router doesn’t attempt to
find a remote DNS server. It sees there is no local resolution
configured and almost immediately sends a message to the console that
the name can’t be resolved.
R2#conf t
R2(config)#ip domain-lookup
R2(config)#ip name-server 10.1.1.1
R2#contin
Translating "contin"...domain server (10.1.1.1)
A DNS server is installed on the network with the IP address 10.1.1.1.
DNS lookup is reenabled with the command ip domain-lookup, and the IP
address of the DNS server is specified with the ip name-server command.
It's just that easy to tell a Cisco router exactly where the DNS server
is!
