DNS name resolution can fail even when IP connectivity works properly. To
troubleshoot this problem, use one of the following methods to determine if DNS
is resolving the name of the destination:
- Ping the destination by name and look for an error message indicating the
name could not be resolved.
- If working on a Windows NT/2000/XP or a UNIX machine, use nslookup
<fully-qualified domain name> to perform a DNS lookup on the
destination. If it is successful, the address of the host should be displayed:
unix% nslookup www.somedomain.com
Server:
localhost
Address: 127.0.0.1
Non-authoritative answer:
Name:
www.somedomain.com
Address: 10.1.1.1
If nslookup fails, the output should be similar to the following:
unix% nslookup www.somedomain.com
Server:
localhost
Address: 127.0.0.1
*** localhost cannot find www.notvalid.com: Non-existent
host/domain
- Verify the name of the DNS server that should be used to help resolve the
name. This can be found in different places on each operating system. If unsure
of how to find it, consult the device manual. The following describes the
instructions for several common platforms:
- On a Cisco router, type show run and look for the
name-server.
- On Windows 9x and Windows Me, use winipcfg.exe.
- On Windows XP, 2000, or NT, use ipconfig.exe.
- On a UNIX platform, type cat /etc/resolv.conf at a
command prompt.
- Verify that the name server can be pinged using its IP address. If the ping
fails, then the problem is at a lower layer.
- Verify that names can be resolved within the local domain. For example, if
a host is host1.test.com, the names of other hosts, such as host2.test.com, in
the test.com domain should resolve to an IP address.
- Verify that one or more domain names outside the local domain can be
resolved. If names from all domains except that of the destination can be
resolved, it is possible there is a problem with the DNS for the destination
host. Contact the administrator of the destination device.
If names within the local domain or a large number of external domains
cannot be resolved, contact the DNS administrator, since there may be a problem
with the local DNS (or the local host could be using the wrong domain server).