Another very important aspect of system administration in a network environment is protecting your system and users from intruders. We will mention some particular problems when discussing the context they may occur in, and some common defenses against them. This section will discuss a few examples and basic techniques in dealing with system security. Therefore, reading a good book on security is an absolute must, especially in a networked system.
System security starts with good system administration.
This includes checking the ownership and permissions of all vital files and directories, monitoring use of privileged accounts, etc. The COPS program, for instance, will check your file system and common configuration files for unusual permissions or other anomalies. It is also wise to use a password suite that enforces certain rules on the users' passwords that make them hard to guess. The shadow password suite, for instance, requires a password to have at least five letters, and contain both upper and lower case numbers and digits. For example, you should make programs setuid to root or some other privileged account only when they really need this.