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(Perl: Lesson 12)

{ Policy Part 5: Operating System Resource Permission Checking }

Section 0. Background Information
  1. What is Operating System Resource Checking?
    • The Operating System Resources (OSR)s refers vital system files and directories that should be safe guarded from regular users.
  1. Getting Perl
    • For the purposes of these perl lesson, I will be using a perl package that comes standard on Backtrack, Ubuntu and most flavors of Linux and Unix.
    • However, if you are using Windows, instead of a Linux, Unix or MAC operating system, you still have options.
  2. Pre-Requisite
  3. Lab Notes
    • In this lab we will do the following:
      1. We will download a basic perl program that interrogates the operating system resources files and directories.
      2. The program will provide extraction examples using split.

  4. Legal Disclaimer
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    • In addition, this is a teaching website that does not condone malicious behavior of any kind.
    • Your are on notice, that continuing and/or using this lab outside your "own" test environment is considered malicious and is against the law.
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Section 1. Login to BackTrack
  1. Start Up VMWare Player
    • Instructions:
      1. Click the Start Button
      2. Type Vmplayer in the search box
      3. Click on Vmplayer

     

  2. Open a Virtual Machine
    • Instructions:
      1. Click on Open a Virtual Machine

     

  3. Open the BackTrack5R1 VM
    • Instructions:
      1. Navigate to where the BackTrack5R1 VM is located
      2. Click on on the BackTrack5R1 VM
      3. Click on the Open Button

     

  4. Edit the BackTrack5R1 VM
    • Instructions:
      1. Select BackTrack5R1 VM
      2. Click Edit virtual machine settings

     

  5. Edit Virtual Machine Settings
    • Instructions:
      1. Click on Network Adapter
      2. Click on the Bridged Radio button
      3. Click on the OK Button

     

  6. Play the BackTrack5R1 VM
    • Instructions:
      1. Click on the BackTrack5R1 VM
      2. Click on Play virtual machine

     

  7. Login to BackTrack
    • Instructions:
      1. Login: root
      2. Password: toor or <whatever you changed it to>.

     

  8. Bring up the GNOME
    • Instructions:
      1. Type startx

 

Section 2. Bring up a console terminal
  1. Start up a terminal window
    • Instructions:
      1. Click on the Terminal Window

     

  2. Obtain the IP Address
    • Instructions:
      1. ifconfig -a
    • Note(FYI):
      • My IP address 192.168.1.111.
      • In your case, it will probably be different.

 

Section 3. Download scan_system_resources.pl
  1. Make a student directory
    • Instructions:
      1. cd /home/student
      2. mkdir -p perl_lessons/
      3. cd perl_lessons/

     

  2. Download scan_system_resources.pl
    • Instructions:
      1. wget http://www.computersecuritystudent.com/UNIX/PERL/lesson12/scan_system_resources.pl.TXT
      2. mv scan_system_resources.pl.TXT scan_system_resources.pl
      3. chmod 700 scan_system_resources.pl
      4. perl -c scan_system_resources.pl

     

Section 4. Analyze The Code
  1. SheBang Directive
    • Instructions:
      1. vi scan_system_resources.pl
      2. :set nu
      3. Press the <Enter> key
    • Note(FYI):
      • Line 1: #!/usr/bin/perl
        • #! - is called the SheBang Directive.  SheBang is an interpreter directive that tells Linux to load the following program.
        • /usr/bin/perl - is the Perl Interpreter.  SheBang tells the program loader to run the Perl Interpreter.
      • Line 4: chomp(my $PRINT = $ARGV[0]);
        • $ARGV is a special perl array to receive command line arguments.
        • $PRINT will be used to tell the program to output data to the screen.
      • Line 7:  my $dir = "/home/student/perl_lessons";
        • Set the $dir variable to the directory location of the program.
      • Line 10: $log = "$dir/POLICY-A04.txt";
        • Set the $log variable to the output file name.
      • Line 12: open(LOG,">$log") || die "Cannot Open Filename: $!";
        • Open the log file POLICY-A04.txt.  LOG is the filehandle name. The greater than operator (>) mean to write to output.
      • Line 15: &scan_password;
        • Execute the subroutine scan_password.
      • Line 17: close(LOG);
        • Close the filehandle LOG, which is log file POLICY-A04.txt.

     

  2. Explaining Lines 19 through 32
    • Instructions:
      1. Arrow down to line 19
    • Note(FYI):
      • Line 19 through 32: sub print_it
        • This subroutine has two actions: (1) Print to Output (line 31), and (2) Print to Screen (Lines 25 to 28).
      • Line 22: chomp(my $tmp = $_[0]);
        • Assign variable $tmp to first parameter.
      • Line 25 - 28: if($PRINT eq "-v")
        • If $PRINT is equal to -v, then print output to screen.
      • Line 31: print LOG "$tmp\n";
        • Print output to the filehandle LOG, which means print to the output file POLICY-A04.txt.

     

  3. Explaining Lines 34 through 57
    • Instructions:
      1. Arrow down to line 34
    • Note(FYI):
      • Line 34: sub get_permission
        • The get_permission subroutine returns the OCTAL permission for a file or directory.
      • Line 37: chomp(my $tmp = $_[0]);
        • #Assign variable $tmp to first parameter in sub routine.
      • Line 41: chomp(my $PERM_PAIR = `stat $tmp | grep Access | grep Uid | awk '{print \$2}'`);
        • Use the Unix/Linux Utility "stat" to display the information about the file or directory.
        • Use grep to hone in on the line that contain the OCTAL value.
        • Use awk to cut out second field.
        • Note: stat --printf %a <filename> can be used to grab the OCTAL value without manipulation, but this solution is for learning purposes.
      • Line 44: $PERM_PAIR =~ s/\(|\)//g;
        • Remove both the left "(" and right ")" parenthesis.
      • Line 47: my($OCTAL,$PERM) = split(/\//,$PERM_PAIR);
        • Use split to extract the OCTAL and ASCII values from the $PERM_PAIR variable using the forward slash "/" as the delimiter.
      • Line 49-56: if($OCTAL eq "")
        • If the OCTAL permission is equal to nothing, THEN return "NA".
        • If the OCTAL permission is NOT equal to nothing, then return it.

     

  4. Explaining Lines 59 through 79
    • Instructions:
      1. Arrow down to line 59
    • Note(FYI):
      • Line 39: sub scan_system_resources
        • This sub routine will interrogate each file or directory for a required permission setting provided in the %SYSTEM_RESOURCES HASH.
      • Line 65-79: my $SYSTEM_RESOURCES = ();
        • %SYSTEM_RESOURCES or $SYSTEM_RESOURCES is a HASH.
        • This hash store the required OCTAL permission setting for each directory.
        • E.g., The directory /usr has a required OCTAL permission setting of 0750.

     

  5. Explaining Lines 82 through 91
    • Instructions:
      1. Arrow down to line 82
    • Note(FYI):
      • Line 51: foreach my $OSR (sort keys %SYSTEM_RESOURCES)
        • Check each file or directory contained in the %SYSTEM_RESOURCES hash using the foreach loop.
        • $OSR is the actual file or directory.
      • Line 85: chomp($OSR)
        • Remove hard return or end of line characters from the $OSR variable.
      • Line 88: chomp($EXISTS = `ls -ld $OSR 2>/dev/null`);
        • Check to see if the file or directory is exists.

     

  6. Explaining Lines 94 through 117
    • Instructions:
      1. Arrow down to line 94
    • Note(FYI):
      • Line 94-97: if($EXISTS eq "")
        • If the file does not exist, then print violation.
      • Line 98-118: else
        • If the file does exist, then move to next clause.
      • Line 101: my $CURRENT_PERMISSION = &get_permission($OSR);
        • Get permission of the file or directory.
      • Line 104-107: if($CURRENT_PERMISSION eq "NA")
        • If the current permission is not avaliable, then print violation.
      • Line 108-112: elsif($CURRENT_PERMISSION > $REQUIRED_PERMISSION)
        • If the current permission is greater than the required permission, then print violation.
      • Line 108-112: else
        • If the above conditions are not met, then print passed the current test.

     

  7. Save and Quit
    • Instructions:
      1. Press the <Esc> key
      2. :q!
      3. Press the <Enter> key

 

Section 5. Proof of Lab
  1. Project
    • Instructions:
      1. Your project is to DEBUG and FIX all the BUGS.
      2. perl -c scan_system_resources.pl
        • Look at compiler errors.
        • Fix compiler errors.
        • Keep compiling until the syntax reports to be OK.
  2. Proof of Lab
    • Instructions
      1. chmod 700 scan_system_resources.pl
      2. perl -c scan_system_resources.pl
      3. ./scan_system_resources.pl -v
      4. date
      5. echo "Your Name"
        • Put in your actual name in place of "Your Name"
        • e.g., echo "John Gray"
    • Proof Of Lab Instructions:
      1. Press the <Ctrl><Alt> keys simultaneously
      2. Press the <PrtScn> key
      3. Paste into a word document
      4. Upload to Moodle
     


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