Lsof doesn’t always come by default on some Linux distributions, but it is a handy tool to have. The latest release of Lsof is always available via anonymous FTP from ftp://vic.cc.purdue.edu/pub/tools/unix/lsof.

Know when and how to properly use networking utilities to troubleshoot a networking problem.

Network hardware problems

When troubleshooting networking problems, don’t forget to check your hardware systems — they are just as important as the computer networking software. Networking can be interrupted at many points, ranging from a faulty router port to a bad ethernet cable. Knowing how to examine and test your hardware for network connectivity problems is an important skill — and is just as important as your networking software knowledge.

Cabling

One of the most common sources of network connectivity problems is the network cabling. Defective cables, crossed wires, broken jacks, and cut cabling all contribute from minor to major effects on the network.

Basic, twisted-pair Ethernet cabling, which simply consists of four pairs of twisted wires, is very susceptible to damage and improper installation. If the administrator is making his or her own cables, it can be very easy to mismatch wires or not crimp the end connectors properly. The easiest way to avoid mistakes is to use commercially purchased cables. However, remember that these cables may also have defects.

Coaxial network cabling is more impervious to damage, and is typically used in industrial environments where the cables are in greater danger of being damaged. However, improper terminations or split or cut cabling can quickly bring down this type of network because it’s using a bus technology for network communications, and any break in one part of the cabling will bring down the entire network.

When troubleshooting networking problems, ensure that the problem is not simply the cabling. You can easily test the cabling by exchanging the cable with a known good one. This very short test can save you a lot of time if your problem is simply a faulty cable.

Switches and routers

The source of network problems can often be traced back to a main switch or hub. A faulty port can often project thousands of network packets onto the network, thus causing large scale, general network congestion.

In most cases, a faulty port won’t work at all, and if you trace your problems back to the port, you can simply plug it into another port to establish connectivity. Other, more serious problems with the main networking equipment include overall equipment malfunction, misconfiguration, or even just loss of power.