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3       Understanding Linux Services

In previous chapters we have touched on some of the services that a Linux system provides and the ports that those services communicate through. In this chapter we will provide an overview of the various communication related services. This information will make it easier to make an informed decision as to whether these are services you want to have running on your Linux system and, therefore, potentially accessible to the outside world. 

3.1      Web Server httpd – Port 80

The httpd service is the Hyper Text Transfer Protocol Deamon. If you plan to host your own web site on your Linux system you will need to activate this service. Without out it your web server will not serve any web pages.

http work through port 80 so you will need to make sure that you have this port open on your Firewall and configured to forward requests to the IP address of the Linux system on your network that is running the web server.

3.2   Remote Login - telnet – Port 25 

The telnet service allows users to log into the Linux system from outside. For example you may want to be able to log into your Linux system to perform tasks when you are outside your office or home. You can also use telnet to log into one computer from another on the same network.

The telnet service communicates through port 21. Security experts now advise against the use of telnet these days. Telnet transmits data in plain readable text, which is readily intercepted by hackers leaving vital information (including login and password information) exposed to interception. These days SSH (Secure Shell) is recommended instead.

3.3   Secure Remote Login - ssh – Port 22

Rather like the telnet service the ssh (Secure Shell)  service allows users to log into the Linux system from outside. The difference being that ssh uses an encryption mechanism to product the information being passed over the network thereby preventing others from capturing your login and password information.

The ssh service communicates through port 21.

3.4      File Transfer - ftp – Port 21

FTP is short for File Transfer Protocol and is the protocol for exchanging files over the Internet. FTP is most commonly used to download a file from a server using the Internet or to upload a file to a server. FTP uses port 21 so if you think you or others will need to transfer files to or from your Linux system make sure port 21 is configured correctly on your Firewall. 

The vsftp (very secure ftp) server is recommended since it is more secure than the standard ftp server. It also considered to smaller and faster.

3.5      Mail Transfer - SMTP – Port 25

SMTP is short for Simple Mail Transfer Protocol and is  a protocol for sending e-mail messages between servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another. The messages can then be retrieved with an e-mail client such as Evolution, KMail, or Balsa.

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