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| 7.4 |  | Troubleshooting TCP/IP Application Layer Problems |
| 7.4.5 |  | Troubleshooting DNS problems |

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:

1. Ping the destination by name and look for an error message indicating the name could not be resolved.
2. If working on 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

3. 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.

4. Verify that the name server can be pinged using its IP address. If the ping fails, then the problem is at a lower layer.
5. 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.
6. 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).