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System services are the services that your server will run, and be used. These are what makes your server a server basically, and what brings you customers. Whether your customers be paying users (such as in an ISP), or they be donation users (users that give little or no money towards your server), they are still important. They are the reason that you want a server, so it is important to think of them. Linux offers *MANY* services, here are some of the more common services:

* Shell Services (The Secure SHell (SSH), Telnet, Remote Shell, etc)
* Mail Services (POP3, SMTP)
* Web Services (Web Hosting)
* DNS Services (Domain Name Services)
* FTP Services
* Talk Services (via Talked, enables users logged in to chat with each other)
* Kerberos Services (secure authentication techniques)

Linux offers many other services, of which will be described in detail later. Most of the services lie in the /etc/init.d directory, which is actually a set of start up scripts, but as most services are started at boot, so the /etc/init.d directory would be a good place to start looking for services installed…..you can also try tel netting to known service ports, which are listed as follows:

Telnet: 23

SMTP: 25

HTTP: 80

DNS: 53

SSH: 22

Talk: 517

Kerberos: 750

FTP: 21

If you are running the service, you will get a message that looks like:

/home/ares> telnet localhost.localdomain 22
Trying 192.122.209.42…
Connected to localhost.localdomain.
Escape character is ‘^]’.
SSH-1.5-1.2.20

Then you are running the service that is reported. Also, you can download the portscanner nmap from from insecure.org. It is available in RPM formats, as well as .tar and .tgz formats. Once installed, you can invoke it by using the following syntax:

nmap localhost.localdomain
Nmap will then output all the open ports on your server. This is also good for scanning other servers on your network, to see what services they are running. If you see a service, such as Sendmail (SMTP service) running, and you don’t need it, I recommend removing it, via commenting out the lines in /etc/init.d/sendmail, or by just issuing the following command:

/etc/init.d/sendmail stop

Then un-installing it, with the package manager for your distribution (i.e. rpm for Red Hat Linux). For instance, to un-install a rpm package, use the following command:

rpm -e packagename

To see a list of all rpm packages, use the following command:

rpm -qa | more

This will send the output to the more command, as we have covered earlier. If a package gives you a dependency error, you will need to un-install the dependency first, or you may force the un-install with the -f option (*NOT* recommended). You can use the deselect program in Debian Linux to remove the unwanted package, simply type:

deselect

To bring the deselect menu up. This is more user-friendly than the rpm utility, if you like interfaces that is. Now, we will move on to specific services, the first on the list is the apache web server.

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