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Apache Tutorial: Dynamic Content with CGI - Apache HTTP Server Version 2.4








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Apache > HTTP Server > Documentation > Version 2.4 > How-To / TutorialsApache Tutorial: Dynamic Content with CGI

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 Introduction
 Configuring Apache to permit CGI
 Writing a CGI program
 But it's still not working!
 What's going on behind the scenes?
 CGI modules/libraries
 For more information
See alsoComments


Introduction
    

    Related ModulesRelated Directivesmod_aliasmod_cgimod_cgidAddHandlerOptionsScriptAlias

    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 a simple way to put dynamic content on
    your web site, using whatever programming language you're most
    familiar with. 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.

    Note: If Apache has been built with shared module
    support you need to ensure that the module is loaded; in your
    httpd.conf you need to make sure the
    LoadModule
    directive has not been commented out.  A correctly configured directive
    may look like this:

    LoadModule cgid_module modules/mod_cgid.so



     On Windows, or using a non-threaded MPM like prefork,  A correctly 
     configured directive may look like this:

    LoadModule cgi_module modules/mod_cgi.so



    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/" "/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 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 "/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

    

    .htaccess files
      

      The .htaccess tutorial
      shows how to activate CGI programs if you do not have
      access to httpd.conf.
    

    User Directories
      

      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>


    

  

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.

      #!/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.
    
  

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:

      
        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.

    

    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.
      
      When editing CGI scripts on Windows, end-of-line characters may be
      appended to the interpreter path. Ensure that files are then
      transferred to the server in ASCII mode. Failure to do so may
      result in "Command not found" warnings from the OS, due to the
      unrecognized end-of-line character being interpreted as a part of
      the interpreter filename.
      
    

    Missing environment variables
      

      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.

    

    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:

      
      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.
    

    Error logs
      

      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.
    

    Suexec
      

      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.
    
  

What's going on behind the scenes?
    

    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
      

      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
use strict;
use warnings;

print "Content-type: text/html\n\n";
foreach my $key (keys %ENV) {
    print "$key --> $ENV{$key}<br>";
}

    

    STDIN and STDOUT
      

      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.
    
  

CGI modules/libraries
    

    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/.
  

For more information
    

    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.
  

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