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backup media are retained for an extended period, perhaps permanently. At the very least, one full backup from each month should be retained for a year or more. A backup scheme such as this is called a media rotation scheme, because media are continually written, retained for a defined period, and then reused. The media themselves are said to belong to a media pool, which defines the monthly full, the weekly full, and differential or incremental media assignments, as well as when media can be reused. When media with full backups are removed from the pool for long-term storage, new media join the pool, keeping the size of the pool constant. Media may also be removed from the pool if your organization chooses to limit the number of uses media are allowed, assuming that reliability goes down as the number of passes through a tape mechanism increases. Your organization s data storage requirements dictate the complexity of your backup scheme. On systems in which many people frequently update mission-critical data, a conservative and detailed backup scheme is essential. For casual-use systems, such as desktop PCs, only a basic backup scheme is needed, if at all. Backup verification To be effective, backup media must be capable of yielding a successful restoration of files. To ensure this, a backup scheme must also include some kind of backup verification in which recently written backup media are tested for successful restore operations. This could take the form of a comparison of files after the backup, an automated restoration of a select group of files on a periodic basis, or even a random audit of media on a recurring basis. However the verification is performed, it must prove that the media, tape drives, and programming will deliver a restored system. Proof that your backups are solid and reliable ensures that they will be useful in case of data loss. Device Files Before discussing actual backup procedures, a word on so-called device files is necessary. When performing backup operations to tape and other removable media, you must specify the device using its device file. These files are stored in /dev and are understood by the kernel to stimulate the use of device drivers that control the device. Archiving programs that use the device files need no knowledge of how to make the device work. Here are some typical device files you may find on Linux systems: /dev/st0 First SCSI tape drive /dev/ft0 First floppy-controller tape drive, such as Travan drives /dev/fd0 First floppy disk drive /dev/hdd An ATAPI Zip or other removable disk 186 Administrative Tasks (Topic 2.11)
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All of these reasons for creating a backup strategy could be summarized as insurance. Far too much time and effort goes into a computer system to allow random incidents to force repeated work. Backup Concepts and Strategies Most backup strategies involve copying data between at least two locations. At a prescribed time, data is transferred from a source media (such as a hard disk) to some form of backup media. Backup media are usually removable, and include tapes, floppy disks, Zip disks, and so on. These media are relatively inexpensive, compact, and easy to store off-site. On the other hand, they are slow relative to hard disk drives. Backup types Backups are usually run in one of three general forms: Full backup A full, or complete, backup saves all of the files on your system. Depending on circumstances, all files may mean all files on the system, all files on a physical disk, all files on a single partition, or all files that cannot be recovered from original installation media. Depending on the size of the drive being backed up, a full backup can take hours to complete. Differential backup Save only files that have been modified or created since the last full backup. Compared to full backups, differentials are relatively fast because of the reduced number of files written to the backup media. A typical differential scheme would include full backup media plus the latest differential media. Intermediate differential media are superseded by the latest and can be recycled. Incremental backup Save only files that have been modified or created since the last backup, including the last incremental backup. These backups are also relatively fast. A typical incremental backup would include full backup media plus the entire series of subsequent incremental media. All incremental media are required to reconstruct changes to the filesystem since the last full backup. Typically, a full backup is coupled with a series of either differential backups or incremental backups, but not both. For example, a full backup could be run once per week with six daily differential backups on the remaining days. Using this scheme, a restoration is possible from the full backup media and the most recent differential backup media. Using incremental backups in the same scenario, the full backup media and all incremental backup media would be required to restore the system. The choice between the two is related mainly to the tradeoff between media consumption (incremental backup requires more media) versus backup time (differential backup takes longer, particularly on heavily used systems). For large organizations that require retention of historical data, a backup scheme longer than a week is created. Incremental or differential backup media are retained for a few weeks, after which the tapes are reformatted and reused. Full Study Guide101 Maintain an Effective Data Backup Strategy 185
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In the previous code listing, ^D indicates that the user typed Ctrl D on the keyboard, sending the end-of-file character to terminate the at command. Example 2 Run commands that are listed in the file command_list at 9 p.m. two days from now: $ at -f command_list 9pm + 2 days List items in the at queue (root sees all users entries): $ at -l Remove job number 5 from the at queue: $ at -d 5 Using at to schedule jobs for delayed execution, such as while you re asleep or on vacation, is simple and doesn t require creation of a recurring cron entry. Controlling User Access to cron and at In most cases, it is safe to allow users to use the cron and at facilities. However, if your circumstances dictate that one or more users should be prohibited from using these services, two simple authorization files exist for each: cron.allow, cron.deny at.allow, at.deny These files are simply lists of account names. If the allow file exists, only those users listed in the allow file may use the service. If the allow file does not exist but the deny file does, only those users not listed in the deny file may use the service. For cron, if neither file exists, all users have access to cron. For at, if neither file exists, only root has access to at. An empty at.deny file allows access to all users and is the default. Objective 5: Maintain an Effective Data Backup Strategy Regardless of how careful we are or how robust our hardware might be, it is highly likely that sometimes data will be lost. Though fatal system problems are rare, accidentally deleted files or mistakes using mv or cp are common. Routine system backup is essential to avoid losing precious data. There are many reasons to routinely back up your systems: Protection against disk failures Protection against accidental file deletion and corruption Protection against disasters, such as fire, water, or vandalism Retention of historical data Creation of multiple copies of data, with one or more copies stored at off-site locations for redundancy 184 Administrative Tasks (Topic 2.11)
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On most Linux distributions, /etc/crontab contains some standard content to enable the execution of programs and scripts on the minute, hour, week, and month. These arrangements allow you to simply drop executable files into the appropriate directory (such as /etc/cron.hourly), where they are executed automatically. This eliminates cron configuration altogether for many tasks and avoids cluttering the root crontab file with common commands. Using at The cron facility is intended for the execution of commands on a regular, periodic schedule. When you need to simply delay execution of a command or a group of commands to some other time in the future, you should use at. The at facility accepts commands from standard input or from a file. at Syntax at [-f file] time at [options] Description In the first form, enter commands to the at queue for execution at time. at allows fairly complex time specifications. It accepts times of the form HH:MM to run a job at a specific time of day. (If that time is already past, the next day is assumed.) You may also specify midnight, noon,or teatime (4 p.m.), and you suffix a time of day with AM or PM for running in the morning or evening. You can also say what day the job will be run by giving a date in month-day form, with the year being optional, or by giving a date in MMDDYY, MM/DD/YY or DD.MM.YY form. The date specification must follow the time-of-day specification. You can also give times like now + count time-units, where time-units can be minutes, hours, days, or weeks, you can tell at to run the job today by suffixing the time with today, and you can tell it to run the job tomorrow by suffixing the time with tomorrow. If f file is given, commands are taken from the file, otherwise at will prompt the user for commands. In the second form, list or delete jobs from the at queue. Frequently used options d job1 [,job2, …] Delete jobs from the at queue by number (same as the atrm command). l List items in the at queue (same as the atq command). Example 1 Run myprogram once at 6:15 p.m. tomorrow: $ at 6:15pm tomorrow at> myprogram at> ^D Study Guide101 Automate System Administration Tasks 183
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Day of the week (0 through 6 [where 0 is Sunday] or sun through sat) Command (any valid command, including spaces and standard bourne shell syntax) For example, to execute myprogram once per day at 6:15 a.m., use this crontab entry: # run myprogram at 6:15am 15 6 * * * myprogram Lines that begin with the pound sign (#) are comment lines and are ignored by crond. Comments must begin on a new line and may not appear within commands. The asterisks in this crontab are placeholders and match any date or time for the field where they re found. Here, they indicate that myprogram should execute at 6:15 a.m. on all days of the month, every month, all days of the week. Each of the time specifications may be single, list (1,3,5), or range (1-5 or wedfri) entries or combinations thereof. To modify the previous example to execute at 6:15 and 18:15 on the 1st and 15th of the month, use: # run myprogram at 6:15am and 6:15pm on the 1st and 15th 15 6,18 1,15 * * myprogram As you can see, the time specifications are very flexible. Because the cron daemon evaluates each crontab entry when it executes each minute, it is not necessary to restart or reinitialize crond when crontab entries are changed or new files are created. System crontab files In addition to crontab files owned by individual users, crond also looks for the system crontab files /etc/crontab and files in the directory /etc/cron.d. The format for these system crontabs differs slightly from user crontabs. System crontabs have an additional field for a username between the time specifications and the command. For example: # /etc/crontab # run myprogram at 6:15am as root 15 6 * * * root myprogram In this example, myprogram will be executed by cron as the root user. System crontab files located in /etc/cron.d are of the same form as /etc/crontab, including the extra user field. These files are usually associated with some package or service that includes a system crontab. Allowing a collection of files in /etc/ cron.d allows software installation and upgrade procedures to keep the cron configuration up-to-date on an individual package basis. In most cases, however, you won t need to change the crontab files in /etc/cron.d. On the Exam Memorize the sequence of time/date fields used in crontab files. 182 Administrative Tasks (Topic 2.11)
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The cron daemon wakes up every minute and examines all crontab files, executing any commands scheduled for that time. User crontab files To use the cron facility, users do not need to interact directly with the crond daemon. Instead, each system user has access to the cron facility through her crontab file. These files are stored together in a single directory (usually /var/spool/ cron) and are created and maintained using the crontab utility. crontab Syntax crontab [options] Description View or edit crontab files. Frequently used options e Interactively edit the crontab file. Unless otherwise specified in either the EDITOR or VISUAL environment variables, the editor is vi. l Display the contents of the crontab file. r Remove the crontab file. u user Operate on user s crontab file instead of your own. Only root can edit or delete the crontab files of other users. Examples Display the crontab file for user jdoe: # crontab -l -u jdoe Edit your own crontab file: $ crontab -e crontab files use a flexible format to specify times for command execution. Each line contains six fields: minute hour day month dayofweek command These fields are specified as follows: Minute (0 through 59) Hour (0 through 23) Day of the month (1 through 31) Month (1 through 12 or jan through dec) Study Guide101 Automate System Administration Tasks 181
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This might be useful, for example, to watch system activity as an Internet connection is established via modem. To look specifically for messages regarding your mouse, you might use grep: # grep ‘[Mm]ouse’ /var/log/messages Dec 8 00:15:28 smp kernel: Detected PS/2 Mouse Port. Dec 8 10:55:02 smp gpm: Shutting down gpm mouse services: Often, if you are using grep to look for a particular item you expect to find in /var/ log/messages, you will need to search all of the rotated files with a wildcard. For example, to look for all messages from sendmail, you may issue a command like this: # grep ’sendmail:’ /var/log/messages* When you note problems in log files, look at the hostname and sender of the message first, then the message text. In many cases, you will be able to determine what is wrong from the message. Sometimes the messages are only clues, so a broader review of your logs may be necessary. In this case, it may be helpful to temporarily turn on more messaging by using the debug level in /etc/syslog.conf to help yield additional information that can lead you to the problem. Objective 4: Automate System Administration Tasks There is a surprising amount of housekeeping that must be done to keep a complex operating system such as Linux running smoothly. Log file rotation, cleanup of temporary files and directories, system database rebuilds, backups, and other tasks should be done routinely. Clearly such mundane things should be automated by the system, freeing weary system administrators for more interesting work. Fortunately, any system task that can be accomplished without real-time human intervention can be automated on Linux using the cron and at facilities. Both have the ability to execute system commands, which may start any executable program or script, at selectable times. Further, cron and at can execute these commands on behalf of any authorized system user. cron is intended mainly for regularly scheduled recurring activities, and at is most useful for scheduling single commands for execution in the future. cron gets its name from the chron- prefix of chronology (time). Using cron The cron facility consists of two programs:* crond This is the cron daemon. This is the process that executes your instructions. It starts at system initialization time and runs in the background thereafter. crontab This is the cron table manipulation program. This program gives you access to your cron table or crontab file. Each authorized user may have his own crontab file to run commands and processes on a regular basis. * There is no individual program called cron, which is the overall name given to the facility. If you execute man cron however, you will, see the manpage for crond. 180 Administrative Tasks (Topic 2.11)
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Example 1-8: Sample /etc/logrotate.conf File (continued) create 0664 root utmp rotate 1 } /var/log/messages { postrotate /usr/bin/killall -HUP syslogd endscript } This example specifies rotations for two files, /var/log/wtmp and /var/log/messages. Your configuration will be much more complete, automatically rotating all log files on your system. A complete understanding of logrotate configuration is not necessary for LPIC Level 1 exams, but you must be familiar with the concepts involved. See the logrotate manpages for more information. Examining Log Files You can learn a lot about the activity of your system by reviewing the log files it creates. At times, it will be necessary to debug problems using logged information. Since most of the log files are plain text, it is very easy to review their contents with tools such as tail, less, and grep. syslog stores the messages it creates with the following information, separated by (but also including) spaces: Date/time Origin hostname Message sender (such as kernel, sendmail, or a username) Message text Typical messages will look like this: Dec 8 10:41:23 smp kernel: Symbols match kernel version 2.2.5. Dec 8 10:41:23 smp kernel: Loaded 182 symbols from 12 modules. Dec 8 10:50:19 smp kernel: Kernel logging (proc) stopped. Dec 8 10:50:19 smp kernel: Kernel log daemon terminating. In this case, smp is the hostname, and the messages are coming from the kernel. At any time you can review the entire contents of your log files using less: # less /var/log/messages You can then page through the file. This is a good way to become familiar with the types of messages you ll see on your system. To actively monitor the output to your messages file, you could use tail: # tail -f /var/log/messages Study Guide101 Configure and Use System Log Files 179
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If you examine this syslog.conf file, you ll see that nearly all system messages are sent to the /var/log/messages file via the *.info message selector. In this case, the asterisk directs syslog to send messages from all facilities except mail and authpriv, which are excluded using the special none level. The /var/log/messages file is the default system message destination, and you will consult it frequently for information on processes running (or failing to run) and other events on your system. In this example, the low severity level of info is used for the messages file, which logs all but debugging messages. On heavily loaded servers, this may result in an unwieldy file size due to message volume. Depending upon your available disk space, you may choose to save less information by raising the level for the messages file. Log File Rotation Most distributions will install a default syslog configuration for you, including logging to messages and other log files in /var/log. To prevent any of these files from growing unattended to extreme sizes, a log file rotation scheme should be installed as well. The cron system issues commands on a regular basis (usually once per day) to establish new log files; the old files are renamed with numeric suffixes (see Objective 4 for more on cron). With this kind of rotation, yesterday s /var/log/messages file becomes today s messages.1, and a new messages file is created. The rotation is configured with a maximum number of files to keep, and the oldest log files are deleted when the rotation is run. The utility that establishes the rotation is logrotate. This privileged command is configured using one or more files, which are specified as arguments to the logrotate command. These configuration files can contain directives to include other files as well. The default configuration file is /etc/logrotate.conf. Example 1-8 depicts an example logrotate.conf file. Example 1-8: Sample /etc/logrotate.conf File # global options # rotate log files weekly weekly # keep 4 weeks worth of backlogs rotate 4 # send errors to root errors root # create new (empty) log files after rotating old ones create # compress log files compress # specific files /var/log/wtmp { monthly 178 Administrative Tasks (Topic 2.11)
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the log file. A single period separates the facility from the level, and together they comprise the message selector. The asterisk (*) can be used to describe all facilities or all levels. action The action directive is arguably misnamed. It represents the destination for messages that correspond to a given selector (facility.level). The action can be a filename (including the full pathname), a hostname preceded by the @ sign, or a comma-separated list of users or asterisk (this means all logged-in users will be included). The action is to send the message to the specified destination. For example, if you wanted to create a separate log file for activity reported by the scripts you write, you might include a line like this in /etc/syslog.conf: # Define a new log file for the local5 facility local5.* /var/log/local5 You could then use the logger utility to write messages to the facility from your shell script:* $ logger -p local5.info “Script terminated normally” The message Script terminated normally would be placed into /var/log/local5, along with a timestamp and the hostname that sent the message. Example 1-7 contains an example /etc/syslog.conf file. Example 1-7: Sample /etc/syslog.conf File # Log everything except mail & authpriv of level info # or higher to messages *.info;mail.none;authpriv.none /var/log/messages # The authpriv file has restricted access. authpriv.* /var/log/secure # Log all the mail messages in one place. mail.* /var/log/maillog # Everybody gets emergency messages *.emerg * # Save boot messages also to boot.log local7.* /var/log/boot.log On the Exam If you re not yet familiar with syslog, spend some time with it, modifying /etc/ syslog.conf and directing messages to various files. An understanding of syslog is critical because so many programs depend on it. Study Guide101 * syslog must be restarted or signaled to reinitialize before the new log file is created. Configure and Use System Log Files 177
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