If you are in need of a CAT5 crossover cable, but can’t find one then it’s very easy to make one providing you have the right tools.

This is what you need:
Length of CAT5 (or CAT5e / 6) cable
Wire cutters
RJ45 connectors
RJ45 crimper

The drawing below shows you what each end of the crossover cable should look like:

The cable on the right of the drawing is the crossover end, notice that pins 1 & 3 have been crossed over, and pins 2 & 6 have also been swapped over. If you were to make a straight through cable, you obviously would make it the same both ends. In fact, it doesn’t matter which colours are used on each pin but we’re sticking to standards and good practice!

Tip: If you haven’t got the tools, but you need a quick crossover cable then cut the wire 6 inches from one of the ends, and solder the wires together ensuring you’ve swapped 1/3 and 2/6. Some may frown at this, but it works and it’s a good quick fix.

Why do you need a crossover cable?

The transmit wires (pins 1 and 2) on one computer need to be connected to the Receive wires (pins 3 and 6) on the other computer. If you use a hub/switch, it performs this ‘crossover’ automatically. Also bear in mind that if you’re connecting two hubs together you’ll also need a crossover cable. Most hubs/switches do have a small switch next to one of the ports, enabling you to use a straight through cable.

Checking kernel & CentOS version

To check which kernel version you are running use uname -r

# uname -r

To find out which CentOS (or RedHat) release is installed have a look at /etc/redhat-release

# cat /etc/redhat-release

Checking Disk Space

To check disk space used / free use the df command and it will display each volume. The -h switch turns the bytes reading into GB to make it more readable.

# df -h

If you want to check the size of a particular directory then use:

# du -sh /

Configuring DNS Settings

To configure which DNS servers your Linux server will use you need to edit /etc/resolv.conf

# vi /etc/resolv.conf

Just add each name server as a new line as in the following example:

nameserver
nameserver

Changing the hostname

You’ll find the hostname of the server in two places: /etc/hosts and /etc/sysconfig/network.

# vi /etc/hosts

and

# vi /etc/sysconfig/network

Show Processor & Memory Usage

This is equivilant to the Windows Task Manager, and will show each process and the consumed CPU and RAM usage. Very useful!

# top

Show Processor Information

This command will display the processor type and speed.

# cat /proc/cpuinfo

Search command history for old commands you have forgotten!

Use the history command with grep to find previously entered commands.

# history | grep command

Compiling sendmail.mc

# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

Updating virtusertable.db from /etc/mail/virtusertable

# makemap hash /etc/mail/virtusertable.db < /etc/mail/virtusertable

Saving IPTABLES to /etc/sysconfig/iptables

# iptables-save > /etc/sysconfig/iptables