You can learn more about configuring inetd and xinetd from the article Configuring inetd.conf securely and from the xinetd home page. When adding your own service names and ports to /etc/services, remember to check first that they don't conflict with the assigned port numbers.

The netfilter home page is a good place to start learning more about iptables and the Linux packet filter. Until you're comfortable building your own ruleset, you might want to use an existing script for this. We recommend gShield.

Useful security tools include Tripwire, one of the most popular intrusion detection packages, and Psionic Technologies' PortSentry, which is a cross between intrusion prevention and detection. (The LinuxWorld article How to stop crackers with PortSentry offers advice on installation and configuration.) Finally, be sure to become familiar with Wietse Venema's TCP Wrappers, which allow monitoring of and control over connections to your system. You can view the TCP wrappers README online (it's also available in /usr/share/doc/tcp_wrappers-7.6). Authenticating users can be much easier with Pluggable Authentication Modules (also known as PAM).

Is your network wide open? Consider trying these two utilities for checking the vulnerability (or "openness") of your system: netcat is a simple UNIX utility that reads and writes data across network connections, using TCP or UDP protocol; nmap is a utility for network exploration or security auditing. Specifically, nmap scans ports to determine what's open.

Password checkers and other tools that will test how easy it is to guess your passwords (and those of your users) include John the Ripper from the Openwall Project, built for just this purpose. You may also want to try a comprehensive checker like SAINT.

These security sites should be among the most-visited by any systems administrator: CERT is a federally-funded center operated by Carnegie Mellon University. They study Internet security and vulnerabilities, publish security alerts, and research other security issues. BugTraq, hosted by Security Focus, is a full-disclosure moderated mailing list for the detailed discussion and announcement of computer security vulnerabilities. Even if you aren't particularly interested in this aspect of administration, a subscription to this list can be quite valuable, as simply scanning subject lines may alert you to vulnerabilities on your own systems that you might otherwise discover much later, or not at all.

Some more security sites we recommend highly for getting a better grip on the security of your Linux machines are the Linux Security HOWTO, O'Reilly's Security Page, and of course developerWorks' Security zone (although it has a greater emphasis on secure programming practices, it does also feature administrative security).

Printing and spooling resources you'll want to check out include the LPRng print spooler home page and the Spooling Software overview from the Printing HOWTO. Of course, the Printing HOWTO itself is a valuable resource, as is LinuxPrinting.org.

For help with specific printers, consult the Serial HOWTO. Also the USB guide offers valuable information on (you guessed it) USB printers.

Samba is a great help in heterogeneous networks. When setting up printing for this kind of environment, you will want to check out the Samba home page as well as the Samba HOWTO, with good printer sharing details.

The two printer filters we discussed were Magicfilter and Apsfilter. Remember that both need a conversion program (we recommend Ghostscript), as they do not do conversion themselves. If you are using the latter filter, you may also find the Apsfilter handbook to be quite useful.

In addition, we recommend the following general resources for learning more about Linux and preparing for LPI certification in particular:

You'll find a wealth of guides, HOWTOs, FAQs, and man pages at http://www.linuxdoc.org. Be sure to check out Linux Gazette and LinuxFocus as well.

The Linux Network Administrator's guide, available from Linuxdoc.org's "Guides" section, is a good complement to this series of tutorials -- give it a read! You may also find Eric S. Raymond's Unix and Internet Fundamentals HOWTO to be helpful.

In the Bash by example article series on developerWorks, learn how to use bash programming constructs to write your own bash scripts. This series (particularly parts 1 and 2) are excellent additional preparation for the LPI exam:

• Bash by example, Part 1: Fundamental programming in the Bourne-again shell
• Bash by example, Part 2: More bash programming fundamentals
• Bash by example, Part 3: Exploring the ebuild system

The Technical FAQ for Linux Users by Mark Chapman is a 50-page in-depth list of frequently-asked Linux questions, along with detailed answers. The FAQ itself is in PDF (Acrobat) format. If you're a beginning or intermediate Linux user, you really owe it to yourself to check this FAQ out. The Linux glossary for Linux users, also from Mark, is also excellent.

If you're not too familiar with the vi editor, you should check out Daniel's tutorial on Vi. This developerWorks tutorial will give you a gentle yet fast-paced introduction to this powerful text editor. Consider this must-read material if you don't know how to use vi.

For more information on the Linux Professional Institute, visit the LPI home page.