from Matthias Urlichs, who wrote an ISDN driver for Linux and FreeBSD. For this driver, he integrated some of the BSD networking code in the Linux kernel. That project, too is no longer being worked on.
There has been a lot of rapid change in the Linux kernel networking implementation, and change is still the watchword as development continues. Sometimes this means that changes also have to occur in other software, such as the network configuration tools. While this is no longer as large a problem as it once was, you may still find that upgrading your kernel to a later version means that you must upgrade your network configuration tools, too. Fortunately, with the large number of Linux distributions available today, this is a quite simple task.
The Net-4 network implementation is now quite mature and is in use at a very large number of sites around the world. Much work has been done on improving the performance of the Net-4 implementation, and it now competes with the best implementations available for the same hardware platforms. Linux is proliferating in the Internet Service Provider environment, and is often used to build cheap and reliable World Wide Web servers, mail servers, and news servers for these sorts of organizations. There is now sufficient development interest in Linux that it is managing to keep abreast of networking technology as it changes, and current releases of the Linux kernel offer the next generation of the IP protocol, IPv6, as a standard offering.
Where to Get the Code
ftp.kernel.org/pub/linux/kernel/v2.x/ x ftp.kernel.org/pub/linux/kernel/v2.y/ yMaintaining Your System
The absolute minimum in maintenance is to check system and per-application log files regularly for error conditions and unusual events. Often, you will want to do this by writing a couple of administrative shell scripts and 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 root account. This makes sense only if you log in as root frequently; a much better idea is to forward root 's mail to your personal account by setting up a mail alias as described in Chapter 19, Getting Exim Up and Running or Chapter 18, Sendmail.
However carefully you have configured your site, Murphy's law guarantees that some problem will surface eventually. Therefore, maintaining a system also means being available for complaints. Usually, people expect that the system administrator can at least be reached via email as root, but there are also other addresses that are commonly used to reach the person responsible for a specific aspect of maintenence. For instance, complaints about a malfunctioning mail configuration will usually be addressed to postmaster, and problems with the news system may be reported to newsmaster or usenet. Mail to hostmaster should be redirected to the person in charge of the host's basic network services, and the DNS name service if you run a name server.
System Security
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 root or some other privileged account only when necessary. Also, if you want to use a service for only a very limited application, don't hesitate to configure it as restrictively as your special application allows. For instance, if you want to allow diskless hosts to boot from your machine, you must provide Trivial File Transfer Protocol (TFTP) so that they can download basic configuration files from the /boot directory. However, when used unrestrictively, TFTP allows users anywhere in the world to download any world-readable file from your system. If this is not what you want, restrict TFTP service to the /boot directory.