In the preceding chapters, you
learned how to connect your Linux system to a local area network
(LAN) or to the Internet via an Internet service provider (ISP). By
doing so, you were able to access a plethora of services provided by
others, including file transfers via FTP (File Transfer Protocol),
web pages, email, and Telnet. This chapter explains how to set up
several Linux Internet servers, including an FTP server, an Apache
web server, and a DNS server. These applications let you and others
access data on your Linux system via the Internet. These applications
will be most useful if your system is connected to the Internet 24/7.
But, even if your connection is intermittent, you and others can
access the services these applications provide whenever the
connection is active. The chapter also explains how to implement a
basic firewall to help protect your systems from unauthorized access
via the Internet. Finally, the chapter explains how to use Nmap to
test your firewall. Most Internet services are configurable only by
the root user. So, most of the operations in this chapter require
that you be logged in as root, or possess temporary root privileges
as indicated by the keys icon.
If you configured a medium- or high-security firewall during system
installation or thereafter, remote hosts will not be able to connect
to Internet services offered by your host. To permit remote hosts to
access services, you must disable or customize your firewall, as
described in Section 12.5.