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Linux System Administrator

Linux Administrator Overview

Using Linux involves much more than merely sitting down and turning on the machine. Often you hear talk of a “steep learning curve” but that discouraging phrase can be misleading. Linux is quite different from the most popular commercial operating systems in a number of ways.

While it is no more difficult to learn then other operating system are, it is likely to seem very strange even to the experienced administrator of other systems. In addition, the sophistication of a number of parts of the Red Hat distribution has increased by an order of magnitude, so even an experienced Linux administrator is likely to find much that is new and unfamiliar.

Fortunately, there are new tools designed to make system administration easier than ever before. Make no mistake: Every computer in the world has a system administrator. It may be – and probably is – true that the majority of system administrators are those who decided what software and peripherals were bundled with the machine when it was shipped. That status quo remains because the majority of users who acquire computers for use as appliances probably do little to change the default values. But the minute a user decides on a different wallpaper image or adds an application that was acquired apart from the machine itself, he or she has taken on the role of system administration.

The title of system administrator brings with it some responsibilities. No one whose computer is connected to the Internet, for instance, has been immune to the effects of poorly administered systems, as demonstrated by the distributed denial of service (DDoS) and email macro virus attacks that have shaken the online world in recent years. The scope of these acts of computer vandalism (in some cases, computer larceny) would have been greatly reduced if system administrators had a better understanding of their duties. Linux system administrators are likely to understand the necessity of active system administration more than those who run whatever came on the computer, assuming that things came properly configured form the factory. The user or enterprise that decides on Linux has decided, also, to assume the control that Linux offers, and the responsibilities that this entails.

By its very nature as a modern, multi user operating system, Linux requires a degree of administration greater than that of less robust, home- market systems. This means that even if you use just a single machine connected to the Internet by a dial – up modem – or not even connected at all – you have the benefits of the same system employed by some of the largest businesses in the world, and will do many of the same things that IT professionals employed by those companies are paid to do. Administering your system does involve a degree of learning, but it also means that in setting up and configuring your own system you gain skills and understanding that raise you above mere “computer user” status. The Linux system administrator does not achieve that mantle by purchasing a computer but by full control of what the computer does and how it does it.

You may end up configuring a small home or small office network of two or more machines, perhaps including ones that are not running Linux. You may be responsible for a business network of dozens of machines. The nature of system administration in Linux is surprisingly constant, no matter how large or small your installation. It merely involves enabling and configuring features you already have available.

By definition, the Linux system administrator is the person who has “root” access, which is to say the one who is the system’s “super user”(or root user). A standard Linux user is limited to whatever he or she can do with the underlying engine of the system. But the root user has unfettered access to everything – all user accounts, their home directories, and the files therein; all system configurations; and all files on the system. A certain body of thought says that no one should more easily and safely through other, more specific means, which we discuss in due course. Because the system administrator has full system privileges, your first duty is to knowwhat you’re doing, lest you break something.

Note: By definition, the Linux system administrator can be anyone who has “root” access – anyone who has root access is the system’s “super user”.

The word duty implies a degree of drudgery; in fact, it’s a manifestation of the tremendous flexibility of the system measured against the responsibility to run a tight organization. These duties do not so much constrain you, the system administrator, as free you to match the job to the task. Let’s take a brief look at them.

Installing and Configuring Linux Servers

When you hear the word server to describe a computer, you probably think of a computer that offers some type of service to clients. The server may provide file or printer sharing, file transfer protocol (FTP) or web access, or email- processing tasks. Don’t think of a server as a standalone workstation, think of it as a computer that specifically performs these services for many users.

In the Linux world, the word server has a broader meaning than what you might be used to. For instance, the standard Red Hat graphical user interface (GUI) requires a graphical layer called XFree86. This is a server. It runs even on a standalone machine with one user account. It must be configured. (Fortunately, Red Hat has made this a simple and painless part of installation on all but the most obscure combinations of video card and monitor; gone are the days of anguish as you configure a graphical desktop.)

xfree86

Likewise, printing in Linux takes place only after you configure a print server. Again, this has become as easy as to be nearly trivial.

In certain areas the client-server nomenclature can be confusing, though. While you cannot have a graphical desktop without an X server, you can have remote web access without running a local web server, remote FTP access without running a local FTP server, and email capabilities without ever starting a local mail server. You may well want to use these servers, all of which are included in Red Hat; then again maybe not. Whenever a server is connected to other machines outsider. You want your users to have easy access to the things they need, but you don’t want to open up the system you’re administering to the whole wide world.

Note: whenever a server is connected to machines outside your physical control, security issues arise. You want users to have easy access to the things they need but you don’t want to open the system you’re administering to the whole wide world.

Linux distributions used to ship with all imaginable servers turned on by default. Just installing the operating system on the computer would install and configure — with default parameters — all the services available with the distribution. This was a reflection of an earlier, more innocent era in computing when people did not consider vandalizing other people’s machines to be good sportsmanship. Unfortunately, the realities of this modem, more dangerous world dictate that all but the most essential servers remain turned off unless specifically enabled and configured. This duty falls to the system administer. You need to know exactly which servers you need and how to employ them, and to be aware that it is bad practice and a potential security nightmare to enable services that the system isn’t using and doesn’t need. Fortunately, the following pages show you how to carry out this aspect of system administration easily and efficiently.

Installing and Configuring Application Software

Although it is possible for individual users to install some applications in their home directories—drive space set aside for their own files and customizations — these applications may not be available to other users with-out the intervention of the user who installed the program or the system administrator. Besides, if an application is to be used by more than one user, it probably needs to be installed higher up in the Linux file hierarchy, which is a job that only the system administrator can perform. (The administrator can even decide which users may use which applications by creating a “group” for that application and enrolling individual users in that group.)

New software packages might be installed in /opt if they are likely to be upgraded separately from the Red Hat distribution itself. Doing this makes it simple to retain the old version until you are certain that the new version works and meets your expectations. Some packages may need to go in /usr / src or even usr if they are upgrades of packages installed as part of upgrades Red Hat. (For instance, there are sometimes security s of existing packages.) The location of the installation usually matters only if you compile the application from sourced code; if you use a Red Hat Package Manager (RPM) application package, it automatically goes where it should.

red-hat-linux_package_installation

Configuration and customization ac age of applications is to some extent at the user’s discretion, but not entirely, “Skeleton” configurations — administrator-determined default configurations — set the baseline for user employment of applications. If there are particular forms, for example, that is used through. out an enterprise, the system administrator would set them up or at least make them available by adding them to the skeleton configuration. The same applies to configuring user desktops and in even deciding what applications should appear on user desktop menus. For instance, your company may not want to grant users access to the games that ship with modern Linux desktops. You may also want to add menu items for newly installed or custom applications. The system administrator brings all this to pass.

Creating and Maintaining User Accounts

Not just anyone can show up and log on to a Linux machine. An account must be created for each user and—you guessed it—no one but the system administrator can do this. That’s simple enough. But there’s more. It involves decisions that either you or your company must make. You might want to let users select their own passwords, which would no doubt make them easier to remember but which probably would be easier for a malefactor to crack. You might want to assign passwords, which is more secure in theory but increases the likelihood that users will write them down on a conveniently located scrap of paper — a risk if many people have access to the area where the machine(s) is located.

You might decide that users must change their passwords periodically — something you can configure Red Hat Enterprise Linux to prompt users about.

What happens to old accounts? Suppose that someone leaves the company. You probably don’t want that person to gain access to the company’s network, but you also don’t want to delete the account wholesale, only to discover later that essential data resided nowhere else.

To what may specific users have access? It might be that there are aspects of your business that make Web access desirable, but you don’t want everyone spending their working hours surfing the Web. If your system is at home, you may wish to limit your children’s access to certain Web sites.

These and other issues are part of the system administrator’s duties in man- aging user accounts. Whether the administrator or his or her employer establishes policies governing accounts, these policies should be delineated—preferably in writing for a company — for the protection of all concerned.

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Backing Up and Restoring Files

Until computer equipment becomes infallible, until people lose the desire to harm others’ property, and — truth be told — until system administrators become perfect, there is considerable need to back up important files so that the system can be u and running again with minimal disruption in the event of hardware, security, or administration failure. Only the system administrator may do this. (Because of its built-in security features, Linux doesn’t allow even user to back up their own to removable disks.)

It’s not enough to know that performing backups is your job. You need to formulate a strategy for making sure your system is not vulnerable to catastrophic disruption. This is not always obvious. If you have a high-capacity tape drive and several good sets of restore disks, you might make a full system backup every few days. If you are managing a system with scores of users, you might find it more sensible to back up user accounts and system configuration files, figuring that re-installation from the distribution CDs would be quicker and easier than getting the basics off a tape archive. (Don’t forget about applications you install separately from your Red Hat distribution, especially those involving heavy customization.)

Once you decide what to back up, you need to decide how frequently to perform backups, whether to maintain a series of incremental backups – adding only files that have changed since the last backup- or multiple full backups, and when these backups should be performed. Do you trust an automated, unattended process? If you help determine which equipment to use, do you go with a redundant array of independent disks (RAID), which is to say multiple hard drives all containing the same data as insurance against the failure of any one of them, in addition to other backup system? (A RAID is not enough because hard drive failure is not the only means by which a system can be brought to a halt.)

You don’t want to become complacent or foster a lackadaisical attitude among users. Part of your strategy should be to maintain perfect backups without ever needing to resort to them. This means encouraging users to keep multiple copies of their important files in their important files in their home directories so that you won’t be asked to mount a backup to restore a file that a user corrupted. (If your system is a standalone on then, as your won system administrator, you should make a habit of backing up your configuration and other important files.)

Restoring files from your backup media is no less important than backing them up in the first place. Be certain you can restore your files if the need arises by testing your restore process at least once during a noncritical time. Periodically testing your backup media is also good idea.

Chances are good that even if you work for a company, you’ll be the one making these decisions. Your boss just wants a system that runs perfectly, all the time. Backing up is only part of the story, however. You need to formulate a plan for bringing the system back up after a failure. A system failure could be caused by any number of problems, either related to hardware of software (application, system configuration) trouble, and could range from a minor inconvenience to complete shutdown.

Hardware failures caused by improper configuration can be corrected by properly configuring the device. Sometimes hardware failures are caused by the device itself, which typically requires replacing the device. Software failures caused by improperly configured system files are usually corrected by properly configuring those files. An application can cause the system to fail for many reasons, and finding the root of the problem may require a lot of research on the part of the administrator.

If you are the administrator of servers and workstations for a business, you should have a disaster recovery plan in place. Such a plan takes into account the type of data and services provided and how much fault tolerance your systems require, that is, how long your systems could be down and what effect that would have on your company’s ability to conduct business. If you require 100 percent fault tolerance, meaning your systems must be online 24/7, disaster recovery may be unnecessary in some circumstances as your systems never go down and there is no disaster from which to recover. Most organizations, though, cannot afford such a high level of fault tolerance; they are willing to accept less stringent standards.

Based on the level of fault tolerance you require, Your disaster recovery plan should list as many possible failures as You can anticipate and detail the steps required to restore your systems.

TIP Backing up is only part of the story. You need to formulate a disaster recovery plan to bring your system back up in the event of a failure.

Monitoring and Tuning Performance

The default installation of Red Hat Enterprise Linux goes a long way toward capitalizing on existing system resources. There is no “one size fits all” configuration, however. Linux is infinitely configurable, or close to it.

On a modern standalone system, Linux runs pretty quickly. If it doesn’t, there’s something wrong — something the system administrator can fix. Still, you might want to squeeze one last little bit of performance out of your hardware — or a number of people might be using the same file server, mail server, or other shared machine, in which case seemingly small improvements in system performance add up.

System tuning is an ongoing process aided by a variety of diagnostic and monitoring tools. Some performance decisions are made at installation time, while others are added or tweaked later. A good example is the use of the hdparm utility, which can increase throughput in IDE drives considerably, but for some high-speed modes a check of system logs shows that faulty or inexpensive cables cam in combination with hdparm, produce an enormity of non-destructive but system-slowing errors.

Proper monitoring allows you to detect a misbehaving application that consumes more resources than it should or fails to exit completely upon closing. Through the use of system performance tools, you can determine when hard-ware such as memory, added storage, or even something as elaborate as a hardware RAID — should be upgraded for more cost-effective use of a machine in the enterprise or for complicated computational tasks such as three dimensional rendering.

Possibly most important, careful system monitoring and diagnostic practices give you a heads up when a system component is showing early signs of failure, so that you can minimize any potential downtime. Combined with the resources for determining which components are best supported by Fedora Core and Red Hat Enterprise Linux, performance monitoring can result in replacement components that are far more robust and efficient in some cases.

In any case, careful system monitoring plus wise use of the built-in configurability of Linux allows you to squeeze the best possible performance from your existing equipment, from customizing video drivers to applying special kernel patches or simply turning off unneeded services to free memory and processor cycles.

TIP:To squeeze the best performance from your equipment, monitor your system carefully and use Linux’s built-in configurability wisely.

Configuring a Secure System

If there is a common thread in Linux system administration, it is the security of the computer and data integrity.

What does this mean? Just about everything. The system administrator’s task, first and foremost, is to make certain that no data on the machine or network is likely to become corrupted, whether by hardware or power failure, mis-configuration or user error (to the extent that the latter can be avoided), or malicious or inadvertent intrusion from elsewhere. This means doing all the tasks described throughout this chapter, and doing them well, with a full understanding of their implications.

No one involved in computing has failed to hear of the succession of increasingly serious attacks on machines connected to the Internet. For the most part, these attacks have not targeted Linux systems. That doesn’t mean Linux systems have been entirely immune, either to direct attack or to the effects of attacks on machines running other operating systems. In one distributed denial of service (DDoS) attack aimed at several major online companies,

For instance, many “zombie” machines — those that had been exploited so that the vandals could employ thousands of machines instead of just a few —were *running Linux that had not been patched to guard against a well-known security flaw. In the various Code Red attacks during the summer of 2001, Linux machines themselves were invulnerable, but the huge amount of traffic generated by this worm infection nevertheless prevented many Linux machines from accomplishing much Web-based work for several weeks, so fierce was the storm raging across the Internet. And few email users have been immune from receiving at least some Sir Cam messages — nonsensical messages from strangers with randomly selected files attached from their machines. While this infection did not corrupt Linux machines per se, as it did those running MS Windows, anyone on a dial-up Internet connection who had to endure downloading several megabytes of infected mail each day would scarcely describe himself or herself as unaffected by the attack.

Depending on how a Linux machine is connected, and to what; the sensitivity of the data it contains; and the uses to which it is put, security can be as simple as turning off unneeded services, monitoring the Red Hat security mailing list to make sure that all security advisories are followed, regularly using system utilities to keep the system up to date, and otherwise engaging in good computing practices to make sure that the system runs robustly. It’s almost a full-time job, involving levels of security permissions within the system and systems to which it is connected; elaborate firewalls to protect not just Linux machines but machines that, through their use of non-Linux software, are far more vulnerable; and physical security — making sure that no one steals the machine itself!

For any machine connected to another machine, security means hardening against attacks and making certain that no one else uses your machine as a platform for launching attacks against others. If you run Web, FTP, or mail servers, it means giving access to only those who are entitled to it, while locking out everyone else. It means making sure that passwords are not easily guessed and not made available to unauthorized persons. It means that disgruntled former employees no longer have access to the system and that no reauthorized person may copy files from your machines.

Security is an ongoing process. The only really secure computer is one that contains no data, is unplugged from networks and power supplies, has no keyboard attached, and resides in a locked vault. While this is theoretically true, it implies that security diminishes the usefulness of the machine.

In the chapters that follow, you learn about the many tools that Red Hat pro-ides to help you guard against intrusion, even to help you prevent intrusion do non-Linux machines that may reside on your network. Linux is designed om the beginning with security in mind. In all your tasks you should main-in that same security awareness.

TIP: Your job as system administrator is to strike the right balance between maximum utility and maximum safety, all the while bearing in mind that confidence in a secure machine nothing about the machine’s security tomorrow.

Using Tools to Monitor Security

People who, for purposes of larceny or to amuse themselves, like to break into computers – they’re called crackers – are a clever bunch. If there is vulnerability in a system, they will find it.

Fortunately, the Linux development community is quick to find potential exploits and to crate ways of slamming the door shut before crackers can enter. Fortunately, too, Red Hat is diligent in making available new, patched versions of packages in which potential exploits have been found. Your first and best security tool, therefore, is making sure that whenever a security advisory is issued, you download and install the repaired package. This line of defense can be annoying but it is nothing compared to rebuilding a compromised system.

As good as the bug trackers are, sometimes their job is reactive. Preventing the use of your machine for nefarious purpose and guarding against intrusion are, in the end, your responsibility alone. Red Hat equips you with tools to detect and deal with unauthorized access of many kinds.

As this book unfolds, you’ll learn how to install and configure these tools and how to make sense of the warnings they provide. Pay careful attention to those sections and do what they say. If your machine is connected to the Internet, you will be amazed at the number of attempts made to break into your machine. You’ll be struck by how critical the issue of security is.

For indepth understanding of Linux click on

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