Linux® Quick Fix

The first is slow response times from the remote server, and the second is a complete lack of connectivity. These symptoms can be caused by:...

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 corr...

It is always a good practice in troubleshooting to be versed in monitoring the status of your NIC from the command line. The following sections introduce a few commands that will be useful. Viewing Your Activated Interfaces The ifconfig command w...

There are times when you lose connectivity with another server that is directly connected to your local network. Taking a look at the ARP table of the server from which you are troubleshooting will help determine whether the remote server's NIC is re...

Whether or not your troublesome server is connected to your local network, it is always a good practice to force a response from it. One of the most common methods used to test connectivity across multiple networks is the ping command. ping sends IC...

An easy way to tell if a remote server is listening on a specific TCP port is to use the telnet command. By default, telnet will try to connect on TCP port 23, but you can specify other TCP ports by typing them in after the target IP address. HTTP us...

The following sections illustrate the use of telnet TRoubleshooting from a Linux box. Note Always remember that many Linux servers have the iptables firewall package installed by default. This is often the cause of many connectivity problems and ...

Sometimes you have to troubleshoot Linux servers from a Windows PC. The telnet commands are the same, but the results are different. Go to the command line and type the same telnet command as you would in Linux. Screen Goes BlankSuccessful Connect...

A much better combination of tools is to use telnet to test your site's TCP port 80 response time in conjunction with data from the Linux curl and wget HTTP utilities. Rapid TCP response times, but slow curl and wget response times, usually point no...

Like curl and wget, netstat can be very useful in helping determine the source of problems. Using netstat with the -an option lists all the TCP ports on which your Linux server is listening, including all the active network connections to and from yo...

Determining Whether iptables Is Running You can easily test whether iptables is running by using the /etc/init.d/iptables script with the status qualifier. If it isn't running you'll get a very short listing of the firewall rules. Here is some sampl...

Another tool for network troubleshooting is the TRaceroute command. It gives a listing of all the router hops between your server and the target server. This helps you verify that routing over the networks in between is correct. The traceroute comma...

It is a good tool to use whenever you suspect there is some intermittent network congestion. You type mTR followed by the target IP address to get output similar to the following: [root@bigboy tmp]# mtr 192.168.25.26 ...

The tcpdump command is one of the most popular packages for viewing the flow of packets through your Linux box's NIC card. It is installed by default on Red Hat/Fedora Linux and has very simple syntax, especially if you are doing simpler types of tro...

The tethereal program is a text version of the graphical Ethereal product that is part of the Fedora Linux RPM suite. The command-line options and screen output mimic that of tcpdump in many ways, but tethereal has a number of advantages. The tether...

Using nslookup to Test DNS The nslookup command can be used to get the associated IP address for your domain and vice versa. The nslookup command is very easy to use; you just need to type the command, followed by the IP address or Web site name you...

You can use nmap to determine all the TCP/IP ports on which a remote server is listening. It isn't usually an important tool in the home environment, but it can be used in a corporate environment to detect vulnerabilities in your network, such as ser...

Sometimes you realize that your system is under a denial-of-service attack. This could be either malicious or simply someone rapidly downloading all the pages of your Web site with the Linux wget command. Symptoms include a large number of establishe...

It is always important to know who has logged into your Linux box. This isn't just to help track the activities of malicious users, but mostly to figure out who made the mistake that crashed the system or blew up Apache with a typographical error in ...

One of the greatest sources of frustration for any systems administrator is to try to isolate whether poor server performance is due to a network issue or problems with an application or database. The worry can be amplified, especially as network ins...

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