Installing and Debugging Your Own CGIs

Your Virtual Private Server is unique because it provides you with all of the flexibility, power, and control of a dedicated server. Because of this, you are free to customize and configure your Virtual Private Server for your specific circumstance and needs. This includes the ability to install your own custom developed CGI scripts or those that you have downloaded from a third party source.

• CGI Security Issues

This document is divided into several sections. Each section includes a discussion about a specific topic. Each topic, though important, may or may not have applicability to your specific situation.

The Virtual Environment

Your Virtual Private Server services operate in an environment completely separate of the root system (and any other Virtual Private Server hosted on the same host machine). This means that your script does not have access to any files residing on the root file system, only those files that are located in your home directory hierarchy.

Path Specification

Because your CGI scripts operate in a Virtual Environment, the pathnames that you specify in your CGI scripts should be declared with respect to your home directory. For example, your script may access a file to read from or write to. Instead of specifying a pathname that begins with /usr/home/LOGIN-NAME/usr/local/..., you would simply use /usr/local/... instead.

Setting Permissions

After you have uploaded your script or have created it on-line, make sure you give the script permission to execute. In a UNIX environment, each file has a specific mode or set of permissions which determine who can read or write to the file as well as who can execute the file (if anyone). Setting the execute bit on a file is easy to do. You can either use iManager or you can Telnet or SSH to your Virtual Private Server and type this command:

% chmod +x FILENAME

FILENAME is the name of your script. If a script does not have execute permissions, your web server will report a 403 Forbidden server error when it attempts to execute the script.

Common Problems with Perl Scripts

1. Failure to upload the Perl script in ASCII mode. - Perl scripts, unlike compiled executables, are plain text files. Plain text files should be transferred from your local computer to your Virtual Private Server using ASCII mode (not BINARY mode). Failure to transfer your Perl scripts to your Virtual Private Server in ASCII mode may result in 500 Server Errors.

2. Improper path specification of Perl interpreter. - The first line of a Perl script indicates the path name of the Perl interpreter. In the Virtual Private Server environment, the correct specification of your Perl 5 interpreter is /usr/bin/perl. If you downloaded a Perl script from a third party source, the Perl interpreter is most often defined based on the author's host environment which may be different from the Virtual Private Server environment (/usr/bin/perl is fairly common however).

Troubleshooting 500 Server Errors

If you encounter a 500 Server Error when you execute your scripts, the best way to diagnose the actual source of the problem is to examine your web server's error log. Your error log is typically stored in your ~/www/logs directory under the name error_log.

To review the server error generated in real time, perform the following steps, after connecting to your Virtual Private Server via Telnet or SSH:

1. Type this command:

   % tail -f ~/www/logs/error_log

   The tail command displays the last part of error_log file and will print anything appended to the error_log file to your console window. This in effect give you a real-time view of what is being written to your error log file.

2. Using your browser, attempt to execute your CGI script again. When you do this, the actual error message will be displayed in your Telnet session.

Common CGI Errors and Solutions

• HTTPd/CGI: exec of [CGI PATH INFO] failed, errno is 2

  Analysis and Solution: - The first line of your CGI script failed to specify the correct location of the interpreter. If your script is written in Perl, please see the Common Problems with Perl Scripts section above for the proper first line definition of the Perl interpreter. If your Perl interpreter definition is correct, it is very likely that you uploaded the script in BINARY mode from your PC to your Virtual Private Server. If you originally uploaded the script in BINARY mode, re-upload the script in ASCII mode to correct the problem.

• HTTPd: malformed header from script [CGI PATH INFO]

  Analysis and Solution: - Your script is not printing out a proper header response. When a CGI is executed, it communicates back to the web server a message which is divided into two parts: the header and the body. The header typically tells the web server the content type of the data that will be sent as the body of the response. The header and body are separated by a single blank line. An example of a CGI response is shown below:

  Content-type: text/html
  
  <html>
  <head><title>Title</title></head>
  <body bgcolor="white">
  Hello world!
  </body>
  </html>

  The malformed header from script error message indicates that your script is not properly returning the header portion of the response. You may have misspelled Content-type, not supplied a valid type (such as text/html), or failed to print out a blank line to separate the header from body of the response.