Transcript OOS Chapter 8

TROUBLESHOOT LINUX SYSTEM
CHAPTER 8
8.1 Troubleshoot methodology
The maintenance cycle
 Monitoring: Observing system areas for problems or
irregularities
 Proactive maintenance: Minimizing chance of future
problems
 e.g., perform regular system backups
 Reactive maintenance: Correcting problems when they
arise
 Documenting solutions
 Developing better proactive maintenance methods
 Documentation: System information stored in a log book for
future references
 Troubleshooting procedures: Tasks performed when solving
system problems
Common troubleshooting procedures
 Two troubleshooting golden rules:
 Prioritize problems according to severity
 Spend reasonable amount of time on each problem given its priority
 Try to solve root of problem
 Avoid missing underlying cause
 Justify why a certain solution is successful
 Two categories of problems:
 Hardware-related
 Software-related
8.2 Hardware-Related Problems
Hardware-Related Problems
 Often involve improper hardware or software configuration
 SCSI termination
 Video card and monitor configuration
 POST test alerts
 Loose hardware connections
 IRQ or I/O address conflicts
 View output of dmesg (control or print the kernel ring buffer) command
 Absence of device drivers prevent OS from using associated
devices
 Kudzu program: Detect and install support for new hardware
 If hardware device not detected, device driver must be
configured manually
 HDDs most common device to fail
 Good idea to use RAID
The kudzu welcome screen
Configuring new hardware using kudzu
• If HDD containing partitions mounted on
noncritical directories fails:
– Power down computer and replace failed
HDD
– Boot Linux system
– Use fdisk to create partitions on replaced
HDD
– Use mkfs to create filesystems
– Restore original data
– Ensure /etc/fstab has appropriate entries to
mount filesystems
• If HDD containing / filesystem fails:
– Power down computer and replace failed
HDD
– Reinstall Linux on new HDD
– Restore original configuration and data files
• You should update your package every
time you made changes to your system
(hardware/software).
• You can run PUP, yum, apt-get, or GUI
based synaptic package manager to do
the update.
8.3 Software-Related Problems:
Application-Related Problems
 Missing program libraries/files, process restrictions, or
conflicting applications
 Dependencies: Prerequisite shared libraries or packages required
for program execution
 Programs usually check at installation
 Package files may be removed accidentally
• rpm –V command: Identify missing files in
a package or package dependency
• ldd command: Display shared libraries
used by a program
• ldconfig command: Updates
/etc/ld.so.conf and /etc/ld.so.cache files
• /etc/ld.so.conf file: List of directories
containing shared libraries
• /etc/ld.so.cache file: Contains location of
shared library files
• compressor/decompressor (codec) file:
Contains rules to compress or
decompress multimedia information
 ulimit command: Modify process limit parameters in current
shell
 Can also modify max number of filehandles
 /var/log directory: Contains most system log files
 If applications stop functioning due to difficulty gaining
resources, restart using SIGHUP / SIGHKILL
8.4 Software-Related Problems:
Operating System-Related Problems
 Most software-related problems related to OS
 Boot loader, filesystem, serial device problems
 LILO problems: Place “linear” in, remove “compact”
from /etc/lilo.conf file
 GRUB problems: Typically result of missing files in /boot
directory
 mkbootdisk command: Create a boot floppy diskette
 If filesystem on partition mounted to noncritical directory
becomes corrupted:
 Unmount filesystem
 Run fsck command with –f (full) option
 If fsck command cannot repair filesystem, use mkfs command
to re-create the filesystem
 Restore filesystem’s original data
 If / filesystem is corrupted:
 Boot from first Red Hat Fedora installation CD
 Type “linux rescue” at welcome screen
 Enter shell for Linux system on CD
 Create new / filesystem via mkfs command
 Restore original data to re-created / filesystem
 Reboot system
Lost root password
 First, you have to reboot into recovery mode.
If you have a single-boot (Ubuntu is the only operating
system on your computer), you may have to press the
Escape key during bootup in order to see the boot menu.
If you have a dual-boot (Ubuntu is installed next to
Windows, another Linux operating system, or Mac OS
X; and you choose at boot time which operating system
to boot into), the boot menu should appear without the
need to press the Escape key.
From the boot menu, select recovery mode, which is
usually the second boot option.
After you select recovery mode and wait for all the boot-up
processes to finish, you'll be presented with a few options. In
this case, you want the Drop to root shell prompt option so
press the Down arrow to get to that option, and then press
Enter to select it.
 The root account is the ultimate administrator and can do
anything to the Ubuntu installation (including erase it), so please
be careful with what commands you enter in the root terminal.
 Once you're at the root shell prompt, if you have forgotten your
username as well, type
ls /home
 You should then see a list of the users on your Ubuntu
installation.
 To reset the password, type passwd username
 username is the username you want to reset.
 You'll then be prompted for a new password. When you type the
password you will get no visual response acknowledging your
typing.Your password is still being accepted.
 Just type the password and hit Enter when you're done.You'll be
prompted to retype the password. Do so and hit Enter again.
 Now the password should be reset. Type
to the recovery menu.
exit
to return
 After you get back to the recovery menu, select resume
normal boot, and use Ubuntu as you normally would—only
this time, you actually know the password!