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12/15/2007 -- When the time comes to fix a networking problem, the good news is that there are plenty of sources to which you can turn. The bad news? Some of these tools and solutions can be quite costly, while others are more suited for hobbyists than administrators. This month, we'll look at some free tools you can use to help solve a networking problem in Linux.
Start with the Basics
A problem with a workstation's networking could be a problem with the workstation itself. To see if this is the case, one of the first things you should do is take a quick look at the system's metrics. You can do this from the command line using tools such as top and uptime, or use one of the graphical tools that come standard with most Linux distributions.
Figure 1 shows the System Monitor in Ubuntu (System --> Administration --> System Monitor) as resource utilization is displayed.
[Click on image for larger view.] |
| Figure 1. System Monitor can offer a quick view of the system's key components, including networking. |
After you're sure the system is working well, you can turn to the standard TCP/IP toolbox and use the tools it offers. Here are some of them:
| lookup/nslookup |
A utility for interactively querying name servers. |
| netstat |
Displays network status information. |
| ping |
Displays echo messages to show whether or not a host can be reached. |
| traceroute |
Shows the path taken to reach a host. |
Often, a graphical interface can be used to interact with these command-line tools. Figure 2, for example, shows the interface available in Ubuntu (System --> Administration --> Network Tools).
[Click on image for larger view.] |
| Figure 2. The standard TCP/IP networking tools can be accessed through a graphical interface in Ubuntu. |
Other useful tools include:
| arp |
Shows the entries in the Address Resolution table. |
| dig |
Shows DNS entries. |
| dmesg |
Print out the boot-up messages. |
| host |
A utility for looking up host name information on DNS. |
| hostname |
Shows, or allows you to set, the system hostname. |
| ifconfig |
Displays the TCP/IP configuration parameters. |
| route |
Displays the routing table. |
Turn to the Web
A number of diagnostics can be accessed through the Internet, though some sites offer nothing more than the same old tools in slightly different packaging.
Others, however, offer features -- for example, simplicity or data aggregation -- that you can't get anywhere else. The table below lists some of the best:
| Bandwidth Place Speed Test |
Test the speed of your Internet connection. This can be useful in showing if there's a bottleneck -- you may think you're getting access at 5 Mbps, but due to slower devices between you and the outside world, the results may only be 2 Mbps. |
| IPinfo Security Portal |
See the address the workstation gives to the outside world (whether you're using a unique address, NAT, etc.). You can click on the links to find other information, like the city where your server is located, IPWhois, and so on. |
| Domain Tools' "My IP Information" |
Similar to the information at IPinfo, but everything is returned on a single page. |
| Mail Relay Testing |
Useful for testing a mail relay for abuse. |
A word of caution: Run tools from reputable sites only. When you run diagnostics on your machine from a Web site you're not familiar with, you risk putting your trust in a site that may not be managed by the most reliable of individuals ("Sure, I'll be more than glad to offer you a tool to scan your ports, and then I'll record the results and keep the log file..."). Returning your IP address back to you is one thing; looking for vulnerabilities is quite another.
My general rule of thumb -- and it's open to much debate -- is that I don't mind running a simple diagnostic from such a site (and giving the person who runs it the benefit of the doubt), but I won't run an executable that requires downloading.
Then What?
The tools and utilities listed in this column offer software that allow you to locate/diagnose a networking problem. Once you identify a problem with these tools, the solution is generally one of configuration.
If you use the tools listed in this column and still can't diagnose a networking problem, then you need to turn to hardware tools for assistance. Protocol analyzers and other similar tools can shed light where software alone can't. And that's a topic for a future discussion.
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