Sources of Network Slowness
NIC duplex and speed incompatibilities
An overloaded server at the remote end of the connection
Misconfigured DNS (Covered in Chapter 18, "Configuring DNS" and Chapter 19, "Dynamic DNS")
Sources of a Lack of Connectivity
All sources of slowness can become so severe that connectivity is lost. Additional sources of disconnections are:
The remote server or an application on the remote server being shut down.
We discuss how to isolate these problems and more in the following sections.
Doing Basic Cable and Link Tests
Your server won't be able to communicate with any other device on your network unless the NIC's "link" light is on. This indicates that the connection between your server and the switch/router is functioning correctly.
In most cases a lack of link is due to the wrong cable type being used. As described in Chapter 2, "Introduction to Networking", there are two types of Ethernet cables crossover and straight-through. Always make sure you are using the correct type.
Other sources of link failure include:
The cables are bad.
The switch or router to which the server is connected is powered down.
The cables aren't plugged in properly.
If you have an extensive network, investment in a battery-operated cable tester for basic connectivity testing is invaluable. More sophisticated models in the market will be able to tell you the approximate location of a cable break and whether an Ethernet cable is too long to be used.
- 1 Introduction
- 1.1 Sources of Network Slowness
- 1.2 Sources of a Lack of Connectivity
- 1.3 Doing Basic Cable and Link Tests
- 2 Testing Your NIC
- 2.1 Viewing Your Activated Interfaces
- 2.2 Viewing All Interfaces
- 2.3 Testing Link Status from the Command Line
- 2.4 Viewing NIC Errors
- 2.5 netstat Error Output
- 3 How to See MAC Addresses
- 4 Using ping to Test Network Connectivity
- 5 Using telnet to Test Network Connectivity
- 6 Testing Web sites with the curl and wget Utilities
- 7 The netstat Command
- 8 The Linux iptables Firewall
- 9 Using traceroute to Test Connectivity
- 9.1 Sample traceroute Output
- 9.2 Possible traceroute Messages
- 9.3 traceroute Time Exceeded False Alarms
- 9.4 traceroute Internet Slowness False Alarm
- 9.5 traceroute Dies At The Router Just Before The Server
- 9.6 Always Get a Bidirectional traceroute
- 9.7 ping and traceroute Troubleshooting Example
- 9.8 traceroute Web sites
- 9.9 Possible Reasons For Failed Traceroutes
- 10 Using MTR To Detect Network Congestion
- 11 Viewing Packet Flows with tcpdump
- 12 Viewing Packet Flows with tshark
- 13 Basic DNS Troubleshooting
- 14 Using nmap
- 15 Using netcat to Test Network Bandwidth
- 16 Determining the Source of an Attack
- 17 Who Has Used My System?