Recipe 12.10. Troubleshooting Network Connectivity Problems

Problem

You want to troubleshoot network connectivity problems. This is often necessary if a client is experiencing slow logins or network-based failures when accessing resources.

Solution

First and above all else, make sure your network adapter is working. Generally there should be a flashing green light to indicate the adapter is connected and transmitting data.

After you've checked the hardware, you can run several command line tools to aid in troubleshooting connectivity issues. A good first step is to ping the target host, which can tell you if the remote host is reachable and how long it takes to reach it:

> ping <HostNameOrIP>

Here are some of the status messages you can receive from ping:

Reply

The host was reachable.

Request timed out

The target host either did not respond or there is no host configured with the corresponding IP address. You may also see this message if there is a lot of network latency between the two endpoints. You can work around this by using the -w option with ping and specify the number of milliseconds to wait for each reply.

Unknown host

If you used a DNS name in the ping command, this indicates that the DNS name was not resolvable by the DNS client.

Destination unreachable

The ICMP traffic could not reach the network of the target host. This is often due to a routing problem on an intermediate router, a router being down, or a firewall blocking ICMP.

If you've pinged a host and the request timed out or the host was unreachable, a good tool to try next is tracert, which attempts to trace a route from the source computer to the destination computer.

> tracert <HostNameOrIP>

This command shows you the path your data takes to get to the destination. If there are connectivity problems with a remote host, this command shows where along the path to the host the problem occurs.

If everything checks out, next run the netdiag command on the target system. netdiag provides a wealth of information about various network settings configured on the system along with information about DNS, Kerberos, and Active Directory connectivity. Use the /debug option to view detailed output. If you suspect authentication (Kerberos) to be a potential issue, run the kerbtray utility to ensure you have functioning Kerberos tickets.

If you are still having network problems, a good last step is to look at the network traffic to see if you can spot any obvious errors being transmitted. See Recipe 12.11 for more information.

See Also

MS KB 169790, "How to Troubleshoot Basic TCP/IP Problems," MS KB 321708, "HOW TO: Use the Network Diagnostics Tool (Netdiag.exe) in Windows 2000," MS KB 219289, "Description of the Netdiag/fix Switch," MS KB 314067, "How to troubleshoot TCP/IP connectivity with Windows XP," and MS KB 325487, "How to troubleshoot network connectivity problems"