CGI (Common Gateway Interface)


Introduction


The CGI (Common Gateway Interface) defines a way for a web server to interact with external content-generating programs, which are often referred to as CGI programs or CGI scripts. It is the simplest, and most common, way to put dynamic content on your web site. This document will be an introduction to setting up CGI on your Apache web server, and getting started writing CGI programs.


Configuring Apache to permit CGI


In order to get your CGI programs to work properly, you'll need to have Apache configured to permit CGI execution. There are several ways to do this.


ScriptAlias


The ScriptAlias directive tells Apache that a particular directory is set aside for CGI programs. Apache will assume that every file in this directory is a CGI program, and will attempt to execute it, when that particular resource is requested by a client.

The ScriptAlias directive looks like:

ScriptAlias /cgi-bin/ /var/www/cgi-bin/

The example shown is from your default httpd.conf configuration file, if you installed Apache in the default location. The ScriptAlias directive is much like the Alias directive, which defines a URL prefix that is too mapped to a particular directory. Alias and ScriptAlias are usually used for directories that are outside of the DocumentRoot directory. The difference between Alias and ScriptAlias is that ScriptAlias has the added meaning that everything under that URL prefix will be considered a CGI program. So, the example above tells Apache that any request for a resource beginning with /cgi-bin/ should be served from the directory ‘/var/www/cgi-bin/’, and should be treated as a CGI program.

For example, if the URL http://www.example.com/cgi-bin/test.pl is requested, Apache will attempt to execute the file ‘/var/www/cgi-bin/test.pl’ and return the output. Of course, the file will have to exist, and be executable, and return output in a particular way, or Apache will return an error message.


CGI outside of ScriptAlias directories


CGI programs are often restricted to ScriptAlias'ed directories for security reasons. In this way, administrators can tightly control who is allowed to use CGI programs. However, if the proper security precautions are taken, there is no reason why CGI programs cannot be run from arbitrary directories. For example, you may wish to let users have web content in their home directories with the UserDir directive. If they want to have their own CGI programs, but don't have access to the main cgi-bin directory, they will need to be able to run CGI programs elsewhere.

There are two steps to allowing CGI execution in an arbitrary directory. First, the cgi-script handler must be activated using the AddHandler or SetHandler directive. Second, ExecCGI must be specified in the Options directive.

Explicitly using Options to permit CGI execution


You could explicitly use the Options directive, inside your main server configuration file, to specify that CGI execution was permitted in a particular directory:

<Directory /var/www/cgi-bin/>
Options +ExecCGI
</Directory>

The above directive tells Apache to permit the execution of CGI files. You will also need to tell the server what files are CGI files. The following AddHandler directive tells the server to treat all files with the cgi or pl extension as CGI programs:

AddHandler cgi-script .cgi .pl


User Directories


To allow CGI program execution for any file ending in .cgi in users' directories, you can use the following configuration.

<Directory /var/www/cgi-bin/>
Options ExecCGI
AddHandler test .pl .cgi
AllowOverride None
Order allow,deny
Allow from all
</Directory>

If you wish designate a cgi-bin subdirectory of a user's directory where everything will be treated as a CGI program, you can use the following.

<Directory /home/*/public_html/cgi-bin>
Options ExecCGI
SetHandler test .pl cgi-script
AllowOverride None
Order allow,deny
Allow from all
</Directory>


Writing a CGI program


There are two main differences between “regular” programming, and CGI programming.

First, all output from your CGI program must be preceded by a MIME-type header. This is HTTP header that tells the client what sort of content it is receiving. Most of the time, this will look like:

Content-type: text/html

Secondly, your output needs to be in HTML, or some other format that a browser will be able to display. Most of the time, this will be HTML, but occasionally you might write a CGI program that outputs a gif image, or other non-HTML content.

Apart from those two things, writing a CGI program will look a lot like any other program that you might write.

Your first CGI program


The following is an example CGI program that prints one line to your browser. Type in the following, save it to a file called first.pl, and put it in your cgi-bin directory.
[root@server1 ~]# vim /var/www/cgi-bin/first.pl
#!/usr/bin/perl
print "Content-type: text/html \n\n";
print "Hello, World..!!";
                             :wq

Even if you are not familiar with Perl, you should be able to see what is happening here. The first line tells Apache (or whatever shell you happen to be running under) that this program can be executed by feeding the file to the interpreter found at the location /usr/bin/perl. The second line prints the content-type declaration we talked about, followed by two carriage-return newline pairs. This puts a blank line after the header, to indicate the end of the HTTP headers, and the beginning of the body. The third line prints the string "Hello, World.". And that's the end of it.

If you open your favorite browser and tell it to get the address.
http://www.example.com/cgi-bin/first.pl

or wherever you put your file, you will see the one line Hello, World. Appear in your browser window. It's not very exciting, but once you get that working, you'll have a good chance of getting just about anything working.

But it's still not working!

There are four basic things that you may see in your browser when you try to access your CGI program from the web:

The output of your CGI program

    Great! That means everything worked fine. If the output is correct, but the browser is not processing it correctly, make sure you have the correct Content-Type set in your CGI program.
The source code of your CGI program or a "POST Method Not Allowed" message
    That means that you have not properly configured Apache to process your CGI program. Reread the section on configuring Apache and try to find what you missed.
A message starting with "Forbidden"
    That means that there is a permissions problem. Check the Apache error log and the section below on file permissions.
A message saying "Internal Server Error"
    If you check the Apache error log, you will probably find that it says "Premature end of script headers", possibly along with an error message generated by your CGI program. In this case, you will want to check each of the below sections to see what might be preventing your CGI program from emitting the proper HTTP headers.

File permissions


Remember that the server does not run as you. That is, when the server starts up, it is running with the permissions of an unprivileged user - usually nobody, or www - and so it will need extra permissions to execute files that are owned by you. Usually, the way to give a file sufficient permissions to be executed by nobody is to give everyone execute permission on the file:

[root@server1 ~]# chmod a+x /var/www/cgi-bin/first.pl

Also, if your program reads from, or writes to, any other files, those files will need to have the correct permissions to permit this.

Path information and environment


When you run a program from your command line, you have certain information that is passed to the shell without you thinking about it. For example, you have a PATH, which tells the shell where it can look for files that you reference.

When a program runs through the web server as a CGI program, it may not have the same PATH. Any programs that you invoke in your CGI program (like sendmail, for example) will need to be specified by a full path, so that the shell can find them when it attempts to execute your CGI program.

A common manifestation of this is the path to the script interpreter (often perl) indicated in the first line of your CGI program, which will look something like:

#!/usr/bin/perl

Make sure that this is in fact the path to the interpreter.

In addition, if your CGI program depends on other environment variables, you will need to assure that those variables are passed by Apache.
Program errors

Most of the time when a CGI program fails, it's because of a problem with the program itself, This is particularly true once you get the hang of this CGI stuff, and no longer make the above two mistakes. The first thing to do is to make sure that your program runs from the command line before testing it via the web server. For example, try:

[root@server1 ~]# cd /var/www/cgi-bin/
[root@server1 ~]# ./first.pl

(Do not call the perl interpreter. The shell and Apache should find the interpreter using the path information on the first line of the script.)

The first thing you see written by your program should be a set of HTTP headers, including the Content-Type, followed by a blank line. If you see anything else, Apache will return the premature end of script headers error if you try to run it through the server. See Writing a CGI program above for more details.


Open your favorite browser and tell it to get the address.

http://www.example.com/cgi-bin/first.pl

          Hello, World..!!


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