Apache HTTP Server Version 2.3
Apache HTTP Server Version 2.3
| Related Modules | Related Directives |
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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.
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.
httpd.conf you need to make sure the
LoadModule
directive has not been commented out. A correctly configured directive
may look like this:
LoadModule cgi_module modules/mod_cgi.so
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/ /usr/local/apache2/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 to 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 /usr/local/apache2/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
/usr/local/apache2/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 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.
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 /usr/local/apache2/htdocs/somedir>
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
The .htaccess tutorial
shows how to activate CGI programs if you do not have
access to httpd.conf.
To allow CGI program execution for any file ending in
.cgi in users' directories, you can use the
following configuration.
<Directory /home/*/public_html>
Options +ExecCGI
AddHandler cgi-script .cgi
</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 cgi-script
</Directory>
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.
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.
#!/usr/bin/perl
print "Content-type: text/html\n\n";
print "Hello, World.";
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.
There are four basic things that you may see in your browser when you try to access your CGI program from the web:
Content-Type set in your CGI program.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:
chmod a+x 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.
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.
If your CGI program depends on non-standard environment variables, you will need to assure that those variables are passed by Apache.
When you miss HTTP headers from the environment, make sure they are formatted according to RFC 2616, section 4.2: Header names must start with a letter, followed only by letters, numbers or hyphen. Any header violating this rule will be dropped silently.
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:
cd /usr/local/apache2/cgi-bin
./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.
The error logs are your friend. Anything that goes wrong generates message in the error log. You should always look there first. If the place where you are hosting your web site does not permit you access to the error log, you should probably host your site somewhere else. Learn to read the error logs, and you'll find that almost all of your problems are quickly identified, and quickly solved.
The suexec support program
allows CGI programs to be run under different user permissions,
depending on which virtual host or user home directory they are
located in. Suexec has very strict permission checking, and any
failure in that checking will result in your CGI programs
failing with Premature end of script headers.
To check if you are using suexec, run apachectl
-V and check for the location of SUEXEC_BIN.
If Apache finds an suexec binary there on startup,
suexec will be activated.
Unless you fully understand suexec, you should not be using it.
To disable suexec, simply remove (or rename) the suexec
binary pointed to by SUEXEC_BIN and then restart the
server. If, after reading about suexec,
you still wish to use it, then run suexec -V to find
the location of the suexec log file, and use that log file to
find what policy you are violating.
As you become more advanced in CGI programming, it will become useful to understand more about what's happening behind the scenes. Specifically, how the browser and server communicate with one another. Because although it's all very well to write a program that prints "Hello, World.", it's not particularly useful.
Environment variables are values that float around you as
you use your computer. They are useful things like your path
(where the computer searches for the actual file
implementing a command when you type it), your username, your
terminal type, and so on. For a full list of your normal,
every day environment variables, type
env at a command prompt.
During the CGI transaction, the server and the browser also set environment variables, so that they can communicate with one another. These are things like the browser type (Netscape, IE, Lynx), the server type (Apache, IIS, WebSite), the name of the CGI program that is being run, and so on.
These variables are available to the CGI programmer, and are half of the story of the client-server communication. The complete list of required variables is at Common Gateway Interface RFC.
This simple Perl CGI program will display all of the
environment variables that are being passed around. Two
similar programs are included in the
cgi-bin
directory of the Apache distribution. Note that some
variables are required, while others are optional, so you may
see some variables listed that were not in the official list.
In addition, Apache provides many different ways for you to
add your own environment variables
to the basic ones provided by default.
#!/usr/bin/perl
print "Content-type: text/html\n\n";
foreach $key (keys %ENV) {
print "$key --> $ENV{$key}<br>";
}
Other communication between the server and the client
happens over standard input (STDIN) and standard
output (STDOUT). In normal everyday context,
STDIN means the keyboard, or a file that a
program is given to act on, and STDOUT
usually means the console or screen.
When you POST a web form to a CGI program,
the data in that form is bundled up into a special format
and gets delivered to your CGI program over STDIN.
The program then can process that data as though it was
coming in from the keyboard, or from a file
The "special format" is very simple. A field name and its value are joined together with an equals (=) sign, and pairs of values are joined together with an ampersand (&). Inconvenient characters like spaces, ampersands, and equals signs, are converted into their hex equivalent so that they don't gum up the works. The whole data string might look something like:
name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
You'll sometimes also see this type of string appended to
a URL. When that is done, the server puts that string
into the environment variable called
QUERY_STRING. That's called a GET
request. Your HTML form specifies whether a GET
or a POST is used to deliver the data, by setting the
METHOD attribute in the FORM tag.
Your program is then responsible for splitting that string up into useful information. Fortunately, there are libraries and modules available to help you process this data, as well as handle other of the aspects of your CGI program.
When you write CGI programs, you should consider using a code library, or module, to do most of the grunt work for you. This leads to fewer errors, and faster development.
If you're writing CGI programs in Perl, modules are
available on CPAN. The most
popular module for this purpose is CGI.pm. You might
also consider CGI::Lite, which implements a minimal
set of functionality, which is all you need in most programs.
If you're writing CGI programs in C, there are a variety of
options. One of these is the CGIC library, from
http://www.boutell.com/cgic/.
The current CGI specification is available in the Common Gateway Interface RFC.
When you post a question about a CGI problem that you're having, whether to a mailing list, or to a newsgroup, make sure you provide enough information about what happened, what you expected to happen, and how what actually happened was different, what server you're running, what language your CGI program was in, and, if possible, the offending code. This will make finding your problem much simpler.
Note that questions about CGI problems should never be posted to the Apache bug database unless you are sure you have found a problem in the Apache source code.