Linux Fundamentals

1
Linux Fundamentals
Courtesy of Justin Peltier, Peltier Associates -
2005
2
Linux Commands

• Basic Commands
• cp instead of copy
• mv instead of ren
• ls for list of file
• ls al for list of file including hidden files
  (starting with a .)
• ps ef for a list of processes
• pwd for print directory
• pico to edit files
• Current Directory not in the path value
• Case Sensitive
• Use the tab key for file or directory completion

3
How to run a file

• Linux does not look in the current directory for
  a file
• To run the file foobar
• If you execute
• foobar
• It will not run
• Execute this instead
• ./foobar
• The root of the filesystem is / or root not
  /root these are two different places

4
Unix filesystem

5
Networking Considerations

• Networking as easy as SETUP
• Straight to /etc/sysconfig/networking/devices/
• Changes to networking files possible
• You can use setup or neat in RedHat
• Host files to speed DNS look-ups
• Straight to /etc/hosts to aide testing speed for
  DNS lookups
• About IPtables and IPchains
• Built within the Linux Kernel
• Stateful Firewall with GUI Configurator in
  X-windows
• Not covered in this course but has to be verified
  on your real attack machine
• All about closing unneeded services
• Easiest to configure through GUI setup in RedHat
• Use ntsysv if no GUI is installed

6
Linux File Archiving and Compression

• In Linux, files are packaged and compressed in
  various ways. The common formats used are
• GZIP is a standard compression (for files)
• To compress file use gzip -9 foobar (where 9
  is max)
• To decompress file use gzip -d foobar.gz or
  gunzip foobar.gz
• TAR is a standard archiver used to store a
  directory as a file
• To archive use tar cvf foobar.tar directory or
  file
• To open the archive use tar xvf foobar.tar
• To open a gzip archive use tar zxvf foobar.tar.gz
• BZIP2 is a better compression algorithm
• To compress use bzip2 -z foobar
• To uncompress use bzip2 -d foobar.bz2 or
  bunzip2 foobar.bz
  
• ZIP is like zip for Windows (for files only)
• To compress use zip -9 foobar.zip foobar
  (where 9 is max)
• To decompress use unzip foobar.zip

7
Installing Clean Tools

• Checking the Hash
• Using md5sum before installing a tool protects
  the user from installing possible Trojan Horses
• md5sum OPTION FILE
• Md5sum filename.tar.gz gt filename.md5
• Always verify the signature of software you
  download

Example of verifying MD5 checksums for
sendmail Correct versions73e18ea78b2386b774963c
8472cbd309 sendmail.8.12.6.tar.gzcebe3fa43731b31
5908f44889d9d2137 sendmail.8.12.6.tar.Z8b9c78122
044f4e4744fc447eeafef34 sendmail.8.12.6.tar.sig
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Installing Tools

• Installations
• Each type of file is installed differently
• RPM The install files for RedHat and Mandrake
  Linux
• GCC The compiler for the C and C programming
  languages
• PERL Ready to run as is but may need libraries
  installed
• SHELL Scripts Scripts are not installed but may
  need to be in the paths which are called by
  programs
• Make Program recompiler for configuring to
  specific systems from the GNU C package

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RPM

• RPM
• To install packages
• rpm -i foobar-1.0-1.i386.rpm
• To install the package via ftp
• rpm -i ftp//ftp.redhat.com/pub/redhat/RPMS/foo-1.
  0-1.i386.rpm
• To uninstall the package
• rpm -e foobar
• To upgrade the package
• rpm -U foobar-1.0-1.i386.rpm
• To see help on rpm options
• rpm --help
• To read the online manual page
• man rpm
• To make installation happen regardless of checks
• rpm i foobar-1.0-1.138.rpm --force

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GCC

• GCC
• To compile code
• gcc foobar.c -o foobar
• To see help on gcc options
• gcc --help
• To read the online manual page
• man gcc
• To read the c code
• cat foobar.c more

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SHELL Scripts

• BASH SHELL
• To run scripts
• ./script.sh
• sh ./script.sh
• sh debug ./script.sh
• To see help on script options
• use a text editor like VIM or PICO
• To read the online manual page
• man command
• man bash

12
Make

• Make
• To configure the Makefile before compiling
• ./configure
• To compile the configured Makefile
• make
• To install the binaries
• make install (must be as root)
• To see help on make options
• make --help
• To read the online manual page
• man make

13
PERL

• PERL
• To run perl scripts
• perl foobar.pl
• To read perl scripts
• use a text editor like VIM, PICO
• To get libraries
• use CPAN http//www.cpan.org
• To see help on perl options
• perl --help
• To verify which version of PERL you have
• perl -v
• To read the online manual page
• man perl

14
Your Logon Account

• Logging on and off
• The root account
• Changing your password

15
The Logon Environment

• Logon shell
• Home directory
• Environment variables

16
Linux File Name Conventions

• Up to 256 characters allowed
• No prohibition of the use of spaces
• No formal rules related to file name extensions

17
The File System

• Single tree of directories and subdirectories
• Root directory represented by slash ( / )

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Paths

• Absolute paths
• Relative paths and the current working directory
• Periods, double periods, and tildes

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Working with Directories

• Listing directories
• Long form listing
• Changing the working directory
• Creating subdirectories
• Deleting subdirectories

20
File and Directory Permissions

• Control of access at three levels
• User
• Group
• Everyone (or other)
• Changing permissions

21
Copying, Moving, and Deleting Files

• cp copies files
• mv moves and renames files and directories
• rm deletes files

22
File Content-Listing Commands

• more views text files page by page
• cat prints the contents of files onto terminal
• tac prints file contents in reverse order
• head and tail views the first few or last few
  lines of a file
• nl numbers the lines of a file
• tee writes all output into a file

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Simple Text File Utilities

• wc displays a count of lines, words, and
  characters
• diff reports differences between files
• od displays a binary file in human-readable form
• strings finds printable characters in a binary
  file

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Miscellaneous Commands

• date shows and sets time and date
• w lists logon information about users
• cal provides a monthly calendar
• bc runs a calculator utility