The best way to run Nmap is to use the SYN Stealth scan, with OS Detection and Version Probe turned on. You need to be root to enable the first two options (they are on by default when you are root), but it is well worth it. When you run Nmap (click the Scan button), it tests every port on your machine and checks whether it responds. If it does respond, Nmap queries it for version information and then prints its results onscreen.
The output lists the port numbers, service name (what usually occupies that port), and version number for every open port on your system. The information Nmap shows you should not be a surprise. If there is something open that you do not recognize, a hacker might have placed a backdoor on your system to allow easy access.
You should use the output from Nmap to help you find and eliminate unwanted services. The fewer services that are open to the outside world, the more secure you are.
Protecting Your Machine
After you have disabled all the unneeded services on your system, what remains is a core set of connections and programs that you want to keep. However, you are not finished yet: You need to clamp down your wireless network, lock your server physically, and put scanning procedures in place (such as Tripwire and promiscuous mode network monitors).
Securing a Wireless Network
Wireless networking, although convenient, can be very insecure by its very nature because transmitted data (even encrypted data) can be received by remote devices. Those devices could be in the same room; in the house, apartment, or building next door; or even several blocks away. Extra care must be used to protect the actual frequency used by your network. Great progress has been made in the past couple of years, but the possibility of a security breech is increased when the attacker is in the area and knows the frequency on which to listen. It should also be noted that the encryption method used by more wireless NICs is weaker than other forms of encryption (such as SSH) and should not be considered as part of your security plan.
Always use OpenSSH-related tools, such as ssh or sftp, to conduct business on your wireless LAN. Passwords are not transmitted as plain text, and your sessions are encrypted. Refer to Chapter 15, "Remote Access with SSH," to see how to connect to remote systems with ssh.
The better the physical security is around your network, the more secure it will be (this applies to wired networks as well). Keep wireless transmitters (routers, switches, and so on) as close to the center of your building as possible. Note or monitor the range of transmitted signals to determine whether your network is open to mobile network sniffing — now a geek sport known as war driving. (Linux software is available at http://sourceforge.net/project/showfiles.php?group_id=57253.) An occasional walk around your building not only gives you a break from work, but can also give you a chance to notice any people or equipment that should not be in the area.
Keep in mind that it takes only a single rogue wireless access point hooked up to a legitimate network hub to open access to your entire system. These access points can be smaller than a pack of cigarettes, so the only way to spot them is to scan for them with another wireless device.
Passwords and Physical Security
The next step toward better security is to use secure passwords on your network and ensure that users use them as well, especially the root password. If the root password on just one machine is cracked, the whole network is in trouble. For somewhat more physical security, you can force the use of a password with the LILO or GRUB bootloaders, remove bootable devices such as floppy and CD-ROM drives, or configure a network- booting server for Fedora. This approach is not well supported or documented at the time of this writing, but you can read about one way to do this in Brieuc Jeunhomme's Network Boot and Exotic Root HOWTO, available at http://www.tldp.org/HOWTO/Network-boot-HOWTO/. You can also read more about GRUB and LILO in Chapter 36, "Kernel and Module Management."
Also, keep in mind that some studies show that as many as 90% of network break-ins are by current or former employees. If a person no longer requires access to your network, lock out access or, even better, remove the account immediately. A good security policy also dictates that any data associated with the account first be backed up and retained for a set period of time to ensure against loss of important data. If you are able, remove the terminated employee from the system before he leaves the building.
Finally, be aware of physical security. If a potential attacker can get physical access to your system, getting full access becomes trivial. Keep all servers in a locked room, and ensure that only authorized personnel are given access to clients.
Configuring and Using Tripwire
Tripwire is a security tool that checks the integrity of normal system binaries and reports any changes to syslog or by email. Tripwire is a good tool for ensuring that your binaries have not been replaced by Trojan horse programs. Trojan horses are malicious programs inadvertently installed because of identical filenames to distributed (expected) programs, and they can wreak havoc on a breached system.
Fedora does not include the free version of Tripwire, but it can be used to monitor your system. To set up Tripwire for the first time, go to http://www.tripwire.org/, and then download and install an open-source version of the software. After installation, run the twinstall.sh script (found under /etc/tripwire) as root like so:
# /etc/tripwire/twinstall.sh
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The Tripwire site and local passphrases are used to
sign a variety of files, such as the configuration,
policy, and database files.
Passphrases should be at least 8 characters in length
and contain both letters and numbers.
See the Tripwire manual for more information.
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Creating key files...
(When selecting a passphrase, keep in mind that good passphrases typically
have upper- and lowercase letters, digits, and punctuation marks, and are
at least 8 characters in length.)
Enter the site keyfile passphrase:
You then need to enter a password of at least eight characters (perhaps best is a string of random madness, such as 5fwkc4ln) at least twice. The script generates keys for your site (host) and then asks you to enter a password (twice) for local use. You are then asked to enter the new site password. After following the prompts, the (rather extensive) default configuration and policy files (tw.cfg and tw.pol) are encrypted. You should then back up and delete the original plain-text files installed by Fedora's RPM package.
To then initialize Tripwire, use its --init option like so:
# tripwire --init