Workshop

Name Game
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Without name services, very little works. Although you can usually rule out a name service problem by
trying the same operation by IP address rather than name, name services that fail to work on the server
end can be tricky.
It’s important to realize that existing connections will continue to work; it’s only new ones that will
typically fail.
A UNIX host typically performs a name lookup to whatever its name resolution host is after the
connection is established in order to get the symbolic identity of the caller. Therefore, if name services
have a problem, this can cause a domino effect that causes problems for other services.
I see a lot of problems in the field that are DNS related, even though some of them don’t seem to be at
first. For example, after a name server dies on a given UNIX host, Telnet sessions to that host can take a
long time to show a login prompt. That is, even though the connection is accepted, and the netstat
–an output shows there’s a connection, something prevents the login prompt from being issued.
That something is the DNS server. The Telnet server is configured to look up all connecting Telnet
addresses after they connect; although the name server is dead, the Telnet service keeps trying before it
issues a login prompt. Depending on the operating system and Telnet implementation, this can result in
long delays.
nslookup
The tool for checking name services is called nslookup. As the name implies, it can contact a name
server (hence the ns in nslookup) and look up information from it. If you know your UNIX host is
running named and should be answering name queries from itself and others, you can type this:
nslookup hostname hostname
You should get a response. If you get an error message, the named that runs on hostname is likely
down.
Note that you can use nslookup on any host, not just the one that you’re logged into.