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.
