periodically running them from cron. The source distributions of some major applications, like inn or C News, contain such scripts. You only have to tailor them to suit your needs and preferences.
The output from any of your cron jobs should be mailed to an administrative account. By default, many applications will send error reports, usage statistics, or log file summaries to the
However carefully you have configured your site, Murphy's law guarantees that some problem
System Security
Another very important aspect of system administration in a network environment is protecting your system and users from intruders. Carelessly managed systems offer malicious people many targets. Attacks range from password guessing to Ethernet snooping, and the damage caused may range from faked mail messages to data loss or violation of your users' privacy. We will mention some particular problems when discussing the context in which they may occur and some common defenses against them.
This section will discuss a few examples and basic techniques for dealing with system security. Of course, the topics covered cannot treat all security issues you may be faced with in detail; they merely serve to illustrate the problems that may arise. 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 and monitoring use of privileged accounts. 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 to contain both upper- and lowercase numbers, as well as non-alphabetic characters.
When making a service accessible to the network, make sure to give it 'least privilege'; don't permit it to do things that aren't required for it to work as designed. For example, you should make programs setuid to
You might also want to restrict certain services to users from certain hosts, say from your local network. In Chapter 12, we introduce tcpd, which does this for a variety of network applications. More sophisticated methods of restricting access to particular hosts or services will be explored later in Chapter 9.
Another important point is to avoid 'dangerous' software. Of course, any software you use can be dangerous because software may have bugs that clever people might exploit to gain access to your system. Things like this happen, and there's no complete protection against it. This problem affects free software and commercial products alike.[11] However, programs that require special privilege are inherently more dangerous than others, because any loophole can have drastic consequences.[12] If you install a setuid program for network purposes, be doubly careful to check the documentation so that you don't create a security breach by accident.
Another source of concern should be programs that enable login or command execution with limited authentication. The rlogin, rsh, and rexec commands are all very useful, but offer very limited authentication of the calling party. Authentication is based on trust of the calling host name obtained from a name server (we'll talk about these later), which can be faked. Today it should be standard practice to disable the r commands completely and replace them with the ssh suite of tools. The ssh tools use a much more reliable authentication method and provide other services, such as encryption and compression, as well.
You can never rule out the possibility that your precautions might fail, regardless of how careful you have been. You should therefore make sure you detect intruders early. Checking the system log files is a good starting point, but the intruder is probably clever enough to anticipate this action and will delete any obvious traces he or she left. However, there are tools like tripwire, written by Gene Kim and Gene Spafford, that allow you to check vital system files to see if their contents or permissions have been changed. tripwire computes various strong checksums over these files and stores them in a database. During subsequent runs, the checksums are recomputed and compared to the stored ones to detect any modifications.
Chapter 2. Issues of TCP/IP Networking
In this chapter we turn to the configuration decisions you'll need to make when connecting your Linux machine to a TCP/IP network, including dealing with IP addresses, hostnames, and routing issues. This chapter gives you the background you need in order to understand what your setup requires, while the next chapters cover the tools you will use.
To learn more about TCP/IP and the reasons behind it, refer to the three-volume set
Networking Interfaces
To hide the diversity of equipment that may be used in a networking environment, TCP/IP defines an abstract
For each peripheral networking device, a corresponding interface has to be present in the kernel. For example, Ethernet interfaces in Linux are called by such names as
Before being used by TCP/IP networking, an interface must be assigned an IP address that serves as its
