No system with dial-in lines or public access to terminals is absolutely secure. You can make a system as secure as possible by changing the Superuser password frequently and choosing passwords that are difficult to guess. Do not tell anyone who does not absolutely need to know the Superuser password. You can also encourage system users to choose difficult passwords and to change them periodically.
By default, passwords on Red Hat Linux use MD5 (page 1042 ) hashing, which makes them more difficult to break than passwords encrypted with DES (page 990 ). It makes little difference how well encrypted your password is if you make it easy for someone to find out or guess what it is.
A password that is difficult to guess is one that someone else would not be likely to think you would have chosen. Do not use words from the dictionary (spelled forward or backward); names of relatives, pets, or friends; or words from a foreign language. A good strategy is to choose a couple of short words, include some punctuation (for example, put a ^ between them), mix the case, and replace some of the letters in the words with numbers. If it were not printed in this book, an example of a good password would be C&yGram5 (candygrams). Ideally you would use a random combination of ASCII characters, but that would be difficult to remember.
You can use one of several excellent password-cracking programs to find users who have chosen poor passwords. These programs work by repeatedly encrypting words from dictionaries, phrases, names, and other sources. If the encrypted password matches the output of the program, then the program has found the password of the user. A program that cracks passwords is crack . It and many other programs and security tips are available from CERT (www.cert.org ), which was originally called the Computer Emergency Response Team. Specifically look at www.cert.org/tech_tips .
Make sure that no one except Superuser can write to files containing programs that are owned by root and run in setuid mode (for example, mail and su ). Also make sure that users do not transfer programs that run in setuid mode and are owned by root onto the system by means of mounting tapes or disks. These programs can be used to circumvent system security. One technique that prevents users from having setuid files is to use the nosuid flag to mount , which you can set in the flags section in the fstab file. Refer to "fstab : Keeps Track of Filesystems " on page 469 .
The BIOS in many machines gives you some degree of protection from an unauthorized person modifying the BIOS or rebooting the system. When you set up the BIOS, look for a section named Security. You can probably add a BIOS password. If you depend on the BIOS password, lock the computer case. It is usually a simple matter to reset the BIOS password by using a jumper on the motherboard.
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