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the system administrator, or if a shadowed password file does not exist, you might want to suggest that one be created for improved security. You can perform a file test by creating an expression using the e operator, which tests for existence. Thus, an expression like this,
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(-e /etc/passwd)
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when evaluated, returns true if the file /etc/passwd exists, and returns false if the file does not exist. Other file operators used in PERL include the following:
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Tests whether the file contains binary data Tests whether the file is a directory entry Tests whether the file contains text data Tests whether the file is writeable
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To test for the existence of both the password file and the shadowed password file, you can create a program like this:
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#!/usr/bin/perl $passwdfile="/etc/passwd"; $shadowfile="/etc/shadow"; if (-e $passwdfile) { print "Found standard Solaris password file\n"; } else { print "No standard Solaris password file found\n"; } if (-e $shadowfile) { print "Found shadow password file - good security move!\n"; } else { print "No shadow password file found!\n"; }
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When executing the file on a Solaris 9 system, you should see output like this:
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$ ./checkpasswords.pl Found standard Solaris password file Found shadow password file - good security move!
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Te x t P r o c e s s i n g a n d E d i t i n g
This kind of check could be added as a cron job for the root user, meaning that it could be executed on a regular basis as part of a security check. If any errors were detected, instead of writing a message to standard output, a mail message could be sent to the system administrator. Of course, password files are not the only kinds of files that might be included as part of a security check. Imagine the situation where a Trojan horse or virus has deleted one of the major shells or changed its permissions to render it inoperable. Thus, it is not adequate to just check for the existence of a file. You may also need to check other characteristics, such as being executable ( x), being readable ( r), and having a file size greater than zero ( s). Imagine that you want to check the status of the default Bourne Again Shell (/bin/bash): You can define a valid shell state as existing, being readable, being executable, and having a file size greater than zero, where logical AND is represented by the operator &&. If the shell does not have these attributes, you can generate a warning message. A simple program to achieve this could look like this:
#!/usr/bin/perl $shell="/bin/bash"; if (-e $shell && -x $shell && -r $shell && -s $shell) { print "Valid shell found\n"; } else { print "No valid shell found\n"; }
When executed, the program prints the following message:
$ ./checkbash.pl Valid shell found
Other logical operators commonly used in PERL include the following:
|| ! | ^
Logical OR Logical NOT Bitwise OR Bitwise XOR
Of course, there is more than one shell to be found on Solaris systems, and users are free to choose any one of them for their default login. You can modify the shell-checking program to verify the attributes of each of these shells by using an array that contains the name of each shell, rather than just creating a single scalar variable (e.g., $shell in the previous example). If you create an array called @shell that stores the names of all
Part II:
System Essentials
shells on the system, you can just iterate through the list using the foreach command, as shown in this program:
#!/usr/bin/perl @shells=("/bin/sh", "/bin/csh", "/bin/sh", "/bin/tcsh", "/bin/zsh"); foreach $i (@shells) { if (-e $i && -x $i && -r $i && -s $i) { print "Valid shell: ".$i."\n"; } else { print "Invalid shell: ".$i."\n"; } }
When you execute the program on a Solaris system, you might see output like this:
Valid shell: /bin/sh Valid shell: /bin/csh Valid shell: /bin/sh Valid shell: /bin/tcsh Invalid shell: /bin/zsh
Oops you can see that the first four shells check out okay, but a problem occurs with the /bin/zsh shell. This means that a system administrator should check whether there is a problem. Again, this could be achieved by creating a cron job that runs once per day, and e-mails the administrator if a problem is detected. However, it may be much more useful to actually run this application through a Web browser, which is possible by using CGI. Few modifications are necessary to convert a PERL program to use the CGI: you simply need to print out a content-type header and then continue to print output as usual. For example, the earlier program could be restated in CGI terms as follows:
#!/usr/bin/perl print "Content-type: text/html\n\n"; @shells=("/bin/sh", "/bin/csh", "/bin/sh", "/bin/tcsh", "/bin/zsh"); foreach $i (@shells) { if (-e $i && -x $i && -r $i && -s $i) { print "<b>Valid shell:</b> ".$i."<br>\n"; } else
6:
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