CS 480 M01 Lecture Notes 4 - Aug 31, 2022

Last Time

Ch 3. Access Control and Rootly Powers Continued

• Password
• Extensions to Standard Access Control Model
• Modern Access Control
• Other Special Pseudo Users

Perl Warm-Up

Perl related chapter from the previous edition of our textbook is available on canvas.


• Perl Example:

  #!/usr/bin/perl
  
  # an exampple of a basic perl script in practice
  # not commented on purpose here (very bad idea normally)
  
  use strict;
  use warnings;
  
  $msgsf="/var/log/secure";            # scalar
  $ignore="128.123.";
  open(in,"grep 'Accepted password' $msgsf | grep -v $ignore |") || die "cannot open $msgsf $!\n";
  while(<in>){
          $ln++;
  	#Jan 12 03:04:05 host sshd[12345]: Accepted password for username from 111.33.44.55 port 54321 ssh2
  	#Aug 30 14:38:52 host sshd[223686]: Accepted keyboard-interactive/pam for username from 10.88.14.3 port 56418 ssh2
  	#0   1   2       3      4            5        6                         7   8    9    10
          if(/^(\w{3})\s+(\d+)/) {
                  $newdate = $1 . "_" . $2;
          } else {
                  print STDERR "wrong date line $ln ...\n";
          }
          if($newdate ne $date) {
                  $date = $newdate;
                  print $date,"\n";
          }
  	$daily{$date} +=1;             # %daily - hash / associative array  (of scalars $daily{'Aug_30'}, $daily{'Aug_31'}, ...)
          @flds=split(/\s+/);            # @flds - array (of scalars $flds[0], $flds[1], ...) 
          $userslogins{$flds[8]} +=1;
  	print "\t$flds[2] $flds[8] on $flds[3] from $flds[10]\n";
  }
  close(in);
  
  print "\nDAILY:\n";
  foreach $d (sort keys %daily){
          print "$d $daily{$d}\n";
  }
  print "\nPER USER:\n";
  foreach $u (sort keys %userslogins){
  	$un++;
          print "$un $u $userslogins{$u}\n";
  }
  
  

  or better ?

  #!/usr/bin/perl
  
  $msgsf="/var/log/secure";
  open(in,$msgsf) || die "cannot open $msgsf $!\n";
  while(<in>){
          $ln++;
          next unless /Accept/;
          @flds=split(/\s+/);
          ...
  }
  

• perl as a filter:

  head -5 /etc/passwd | perl -pe 's#/bin/bash$#/bin/sh#'
  head -5 /etc/passwd | perl -aF: -ne 'print $F[0],"\n";'
  

• References:

  while (<>) {
          $array_ref = [ split /:/ ];                       # return a reference to an array containing the results of the split
          $passwd_by_uid{$array_ref->[2]} = $array_ref;   # the UID field, the third member of the array referenced by $array_ref
  }
  

Next Ch 4. Process Control