1. Linux+ Guide to Linux Certification, Third Edition
Chapter 14
Troubleshooting, Performance, and Security

2. Objectives Describe and outline good troubleshooting practices
Effectively troubleshoot common hardware- and software-related problems
Monitor system performance using command-line and graphical utilities
Identify and fix common performance problems
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3. Objectives (continued)
Describe the different facets of Linux security
Increase the security of a Linux computer
Outline measures and utilities that can be used to detect a Linux security breach
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4. Troubleshooting Methodology
Figure 14-1: The maintenance cycle
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5. Troubleshooting Methodology (continued)
Monitoring: observing log files and running performance utilities system to identify problems and their causes
Proactive maintenance: minimizing chance of future problems
e.g., perform regular system backups
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6. Troubleshooting Methodology (continued)
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
All maintenance actions should be documented
Troubleshooting procedures: tasks performed when solving system problems
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7. Troubleshooting Methodology (continued)
Figure 14-2: Common troubleshooting procedures
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8. Troubleshooting Methodology (continued)
Two troubleshooting golden rules:
Prioritize problems according to severity
Spend reasonable amount of time on each problem given its priority
Ask for help if you can’t solve the problem
Try to solve the root of the problem
Avoid missing underlying cause
Justify why a certain solution is successful
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9. Resolving Common System Problems
Three categories of problems:
Hardware-related
Software-related
User interface-related
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10. Hardware-Related Problems
Often involve improper hardware or software configuration
SCSI termination
Video card and monitor configuration
All hardware is on Hardware Compatibility List
POST test alerts
Loose hardware connections
Problems specific to the type of hardware
View output of dmesg command
View content of /var/log/boot.log, /var/log/messages
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11. Hardware-Related Problems (continued)
Absence of device drivers prevent OS from using associated devices
dmesg command: displays the hardware that is detected by the Linux kernel
lsusb command: displays a list of USB devices detected by the Linux kernel
lspci command: displays a list of PCI devices detected by the Linux kernel
Compare outputs of commands to output of lsmod to determine if driver module is missing from kernel
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12. Hardware-Related Problems (continued)
Hardware failure can render a device unusable
HDDs most common hardware components to fail
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
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13. Hardware-Related Problems (continued)
If HDD containing / filesystem fails:
Power down computer and replace failed HDD
Reinstall Linux on new HDD
Restore original configuration and data files
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14. 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
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15. Software-Related Problems: Application-Related Problems (continued)
rpm –V command: identify missing files in a package or package dependency
ldd command: display shared libraries used by a program
ldconfig command: updates list of shared library directories (/etc/ld.so.conf) and list of shared libraries (/etc/ld.so.cache)
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16. Software-Related Problems: Application-Related Problems (continued)
Too many running processes
Solve by killing parent process of zombie processes
Filehandles: connections programs make to files
ulimit command: modify process limit parameters in current shell
Can also modify max number of filehandles
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17. Software-Related Problems: Application-Related Problems (continued)
/var/log directory: contains most system log files
Some are hard linked to /var/log directory
If applications stop functioning due to difficulty gaining resources, restart using SIGHUP
Do determine if another process trying to access the same resources attempt to start application in Single User Mode
If resource conflict is the cause of the problem, download newer version of application or application fix
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18. Software-Related Problems: Operating System-Related Problems
Most software-related problems related to OS
X windows, boot loader, and filesystem problems
Problem detecting video card or monitors by the kernel
To isolate problem starting X Windows or gdm:
View /var/log/Xorg.0.log file
Execute xwininfo or xdpyinfo
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19. Software-Related Problems: OS-Related Problems (continued)
LILO problems: place “linear” in, remove “compact” from /etc/lilo.conf file
GRUB problems: typically result of missing files in /boot directory
Ensure Linux kernel resides before 1024th cylinder and lba32 keyword is in configuration file
Eliminates BIOS problems with large HDDs
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20. Software-Related Problems: OS-Related Problems (continued)
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
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21. Software-Related Problems: OS-Related Problems (continued)
If / filesystem is corrupted:
Boot from Fedora installation media and enter System Rescue
At shell prompt within System Rescue:
Use mkfs to recreate the filesystem
Use backup utility to restore original data to the re-created / filesystem
Exit System Rescue and reboot system
Knoppix Linux and BBC Linux: bootable Linux distributions with many filesystem repair utilities
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22. Software-Related Problems: User Interface-Related Problems
Assistive technologies: tools that users can use to modify their desktop experience
Assistive Technologies Preference utility within GNOME Desktop Environment
Preferred Applications to configure Web browser, multimedia player and terminal applications to be opened automatically
Mouse Accessibility to configure speed and click behavior
Keyboard Accessibility to configure keyboard related assistive technologies
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23. Software-Related Problems: User Interface-Related Problems (continued)
Figure 14-3: The Assistive Technologies Preferences utility
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24. Performance Monitoring
Jabbering: failing hardware components send large amounts of information to CPU
Other causes of poor performance:
Software monopolizes system resources
Too many processes
Too many read/write requests to HDD
Rogue processes
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25. Performance Monitoring (continued)
To solve software performance issues:
Remove software from the system
Move software to another Linux system
Add CPU or otherwise alter hardware
Bus mastering: peripheral components perform tasks normally executed by CPU
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26. Performance Monitoring (continued)
To increase performance:
Add RAM
Upgrade to faster HDDs
Disk Striping RAID
Keep CD/DVD drives on a separate HDD controller
Run performance utilities on a regular basis
Record results in a system log book
Eases identification of performance problems
Baseline: measure of normal system activity
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27. Monitoring Performance with sysstat Utilities
System Statistics (sysstat) package: contains wide range of system monitoring utilities
Use yum install sysstat command to install
mpstat (multiple processor statistics) command: displays CPU statistics
Used to monitor CPU performance
Can specify interval and number of measurements rather than displaying average values
%sys should be smaller than %usr and %nice combined
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28. Monitoring Performance with sysstat Utilities (continued)
iostat (Input/Output Statistics) command: measures flow of information to and from disk devices
Displays CPU statistics similar to mpstat
Displays statistics for each disk device on the system
Output includes:
Transfers per second
Number of blocks read and written per second
Total number of blocks read and written for the device
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29. Monitoring Performance with sysstat Utilities (continued)
sar (System Activity Reporter) command: displays various system statistics taken in the last day
Provides more information than mpstat and iostat
By default scheduled to run every 10 minutes
Output logged to a file in /var/log/sa directory
-f option: View statistics from a specific file
Can be used to take current system measurements
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30. Monitoring Performance with sysstat Utilities (continued)
Additional sar options:
-q option: Displays processor queue statistics
runq -sz value: Number of processes waiting for execution on processor run queue
plist -sz value: Indicates number of processes currently running
ldavg values: Represent average CPU load
-W option: Displays number of pages sent to and taken from swap partition
Large number causes slower performance
Add RAM to resolve
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31. Monitoring Performance with sysstat Utilities (continued)
Table 14-1: Common options to the sar command
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32. Other Performance Monitoring Utilities
top command: displays CPU statistics, swap usage, memory usage and average CPU load
free command: displays total amounts of physical and swap memory and their utilizations
Can be used to indicate whether more physical memory is required
vmstat command: displays memory, CPU, and swap statistics
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33. Security
Linux systems typically made available across networks such as the Internet
More prone to security loopholes and attacks
Should improve local and network security
Understand how to detect intruders who breach the system
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34. Securing the Local Computer
Limit access to physical computer itself
Prevent malicious users from accessing files by directly booting the computer with their own device
Server closet: secured room to store servers
Remove floppy, CD, and DVD drives from workstations
Ensure BIOS prevents booting from USB ports
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35. Securing the Local Computer (continued)
Ensure BIOS password is set
Set boot loader password in LILO or GRUB configuration file
Prevents intruder from interacting with boot loader
Limit access to graphical desktops and shells
Exit command-line shell before leaving computer
nohup command: prevents background processes from being killed when parent shell is killed or exited
Lock screen using GNOME or KDE
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36. Securing the Local Computer (continued)
Minimize root user’s time logged in
su (switch user) command: switch current user account to another
Used to switch between root user and regular user
sudo command: perform commands as another user if you have the rights to do that listed in /etc/sudoers file
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37. Protecting Against Network Attacks
Always a possibility that hackers can manipulate a network service by interacting with it in unusual ways
Buffer overrun: program information for a network service altered in memory
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38. Network Security Essentials
Minimize number of running network services
nmap (network mapper) command: scans ports on network computers
User can determine what network services are running
Ensure that services that are not needed are not automatically started when entering the runlevel
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39. Network Security Essentials (continued)
Ensure network service daemons for essential services not run as root user when possible
Ensure that shell listed in /etc/passwd for daemons is set to /sbin/nologin
Hacker will not be able to get BASH shell
New network service versions usually include fixes for known network attacks
Keep network services up-to-date
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40. Network Security Essentials (continued)
TCP wrapper: program that can start a network daemon
Checks /etc/hosts.allow and /etc/hosts.deny files before starting a network daemon
Examine permissions for files and directories associated with system and network services
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41. Configuring a Firewall
netfilter/iptables: used to configure a firewall
Discard network packets according to chains of rules
Chains: specify general type of network traffic to apply rules to
Rules: match network traffic to be allowed or dropped
Three chain types:
INPUT: incoming packets
FORWARD: packets passing through computer
OUTPUT chain: outgoing packets
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42. Configuring a Firewall (continued)
iptables command: creates rules for a chain
Can be based on source IP, destination IP, protocol used, or packet status
Stateful packet filter: Remembers traffic allowed in an existing session and adjust rules appropriately
Easier to use graphical utility to configure firewalls
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43. Table 14-2: Common iptables options
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44. Configuring a Firewall (continued)
Figure 14-4: The Firewall Configuration utility
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45. Configuring SELinux SELinux: Security Enhanced Linux
By default, configured and enabled during Fedora installation
Series of kernel patches and utilities created by NSA
Enforces role-based security
To enable, edit /etc/selinux/config file
Configure SELINUXTYPE option
Reboot and relabel the system
sestatus command: view current SELinux status
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46. Using Encryption to Protect Network Data
Use encryption algorithms to protect data before it is transmitted on a network
Asymmetric encryption: uses a pair of keys uniquely generated on each system
Public key: freely distributed
Private key: used only by the system, never distributed
Can be used to authenticate messages
Digital signature: message that has been encrypted using a private key
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47. Working with SSH
By default, SSH uses RSA to encrypt data and DSA to digitally sign data
System wide RSA and DSA key pairs are generated the first time SSH daemon is started
Tunneling: enclosing network traffic within encrypted SSH packets
SSH identity: used to automatically authenticate to other computers using digital signatures
Manage keys using Password and Encryption Keys utility
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48. Working with SSH (continued)
Figure 14-5: The Passwords and Encryption Keys utility
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49. Working with GPG Open source version of PGP
Each user has a key pair used for encryption and authentication
Authentication uses trust model
Typically uses RSA and DSA key pairs for asymmetric encryption and digital signing
Can manage GPG keys and encrypt data using:
gpg command
Graphical utility such as Passwords and Encryption Keys utility
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50. Detecting Intrusion
Log files can contain information or irregularities indicating an intrusion
Review log files in /var/log associated with network services
At minimum, review system log files associated with authentication
Pluggable Authentication Module (PAM): handles authentication requests by network applications
Log file in /var/log/secure
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51. Detecting Intrusion (continued)
Check /var/log/wtmp log file
Lists users who receive BASH shells
Use who command to view the file
lsof (list open files) command: lists files that are currently being edited
Periodically search for files that have SUID bit set
Tripwire: monitors important files and directories
Intrusion Detection System (IDS): program used to detect intruders on a Linux system
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52. Detecting Intrusion (continued)
Table 14-3: Common Linux Intrusion Detection Systems
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53. Linux+ Guide to Linux Certification, 3e
Summary
Administrators monitor the system, perform proactive/reactive maintenance, and document system information
Common troubleshooting procedures involve:
Isolating and determining the cause of system problems and implementing and testing solutions that can be documented for future use
Invalid hardware settings, absence of device drivers, and hard disk failure are common hardware-related problems
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54. Linux+ Guide to Linux Certification, 3e
Summary (continued)
Software-related problems can be application-related or OS-related
Users can use assistive technologies to modify their desktop experience
System performance is affected by a variety of hardware and software factors
Using performance monitoring utilities to create a baseline is helpful for diagnosing future performance problems
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55. Linux+ Guide to Linux Certification, 3e
Summary (continued)
Securing a Linux computer involves:
Improving local and network security and monitoring to detect intruders
Greatly improve local security by:
Restricting access to the computer and using root account only when required via su and sudo commands
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56. Linux+ Guide to Linux Certification, 3e
Summary (continued)
Reduce chance of network attacks by:
Reducing number of network services, implementing firewalls, SELinux, service updates, encryption, and TCP wrappers, and restricting services from running as root user and permissions on key files
Analyzing log files and key system files and running IDS applications can be used to detect intruders
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