Linux Cookbook, 2nd Edition

Chapter 21. Troubleshooting Networks

Figuring out networking problems is just like any troubleshooting. Know your network, know how to use basic tools well, and be patient and systematic.

In this chapter we learn how to use ping, FPing, Nmap, httping, arping, and mtr to test connectivity, map networks, find rogue services, test website performance, find duplicate IP addresses, and find routing bottlenecks.

Diagnostic Hardware

If you find yourself stuck with mysterious unlabeled Ethernet and phone cabling, get yourself an Ethernet/telephone cable test and tone tracker. There are many that cost under $100. These come in two pieces: one emitter and one receiver. This goes fast with two people, one at each end of a cable. When you find both ends of a cable, label it and move on. You can do it alone, but it is faster with two people.

Multimeters are useful for a lot of jobs, such as finding shorts and opens, testing for continuity and attenuation, determining whether a wire is terminated correctly, testing electric outlets, and testing computer power supplies and motherboards. Adafruit is a great site to find excellent tutorials on using a multimeter and learning electronics.

Keep a few spare parts, if you can. Sometimes it is faster to swap out a network interface, cable, or switch to find a defective piece of hardware.

21.1 Testing Connectivity with ping

Problem

Some services or hosts on your network are not accessible or have intermittent failures. You want to figure out if ...