# Troubleshooting

Invariably, troubles and misconfigurations creep into networks. New devices get connected and added to a network. Old devices are removed, and something seemingly unrelated breaks. Troubleshooting is really a test in discerning patterns.

My favored method for solving problems is to start with the simplest elements, verifying correct operation and proceeding to the next layer or element until I have isolated the problem element. If you are lucky, you'll know from a symptom where the problem is likely to be, but more often, you'll have to start at the bottom of the networking hierarchy, and verify each other layer.

## Introduction to Troubleshooting

The first thing to consider whenever somebody reports a strange networking problem is any recent change. What has changed recently in the network? Have any new machines been added? Is the user using a service which was recently decommissioned? Did a machine (firewall, mail server, DNS resolver) recently reboot? Did all of the services restart?

## Troubleshooting at the Ethernet Layer

## Troubleshooting at the IP Layer

## Handling and Diagnosing Routing Problems

## Identifying Problems with TCP Sessions

## DNS Troubleshooting