apache/docs/manual/howto/cgi.xml

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<manualpage metafile="cgi.xml.meta">
  <parentdocument href="./">How-To / Tutorials</parentdocument>

  <title>Apache Tutorial: Dynamic Content with CGI</title>

  <section id="intro">
    <title>Introduction</title>

    <related>
      <modulelist>
        <module>mod_alias</module>
        <module>mod_cgi</module>
      </modulelist>

      <directivelist>
        <directive module="mod_mime">AddHandler</directive>
        <directive module="core">Options</directive>
        <directive module="mod_alias">ScriptAlias</directive>
      </directivelist>
    </related>

    <p>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.</p>
  </section>

  <section id="configuring">
    <title>Configuring Apache to permit CGI</title>

    <p>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.</p>

    <note type="warning">Note: If Apache has been built with shared module
    support you need to ensure that the module is loaded; in your
    <code>httpd.conf</code> you need to make sure the
    <directive module="mod_so">LoadModule</directive>
    directive has not been commented out.  A correctly configured directive
    may look like this:

    <example>
      LoadModule cgi_module modules/mod_cgi.so
    </example></note>

    <section id="scriptalias">
      <title>ScriptAlias</title>

      <p>The 
      <directive module="mod_alias">ScriptAlias</directive>

      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.</p>

      <p>The <directive module="mod_alias">ScriptAlias</directive>
      directive looks like:</p>

      <example>
        ScriptAlias /cgi-bin/ /usr/local/apache2/cgi-bin/
      </example>

      <p>The example shown is from your default <code>httpd.conf</code>
      configuration file, if you installed Apache in the default
      location. The <directive module="mod_alias">ScriptAlias</directive>
      directive is much like the <directive module="mod_alias"
      >Alias</directive> directive, which defines a URL prefix that
      is to mapped to a particular directory. <directive>Alias</directive>
      and <directive>ScriptAlias</directive> are usually used for
      directories that are outside of the <directive module="core"
      >DocumentRoot</directive> directory. The difference between
      <directive>Alias</directive> and <directive>ScriptAlias</directive>
      is that <directive>ScriptAlias</directive> 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 <code>/cgi-bin/</code> should be served from
      the directory  <code>/usr/local/apache2/cgi-bin/</code>, and should be
      treated as a CGI program.</p>

      <p>For example, if the URL
      <code>http://www.example.com/cgi-bin/test.pl</code>
      is requested, Apache will attempt to execute the file 
      <code>/usr/local/apache2/cgi-bin/test.pl</code>
      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.</p>
    </section>

    <section id="nonscriptalias">
      <title>CGI outside of ScriptAlias directories</title>

      <p>CGI programs are often restricted to <directive module="mod_alias"
      >ScriptAlias</directive>'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 
      <directive module="mod_userdir">UserDir</directive> directive.
      If they want to have their own CGI programs, but don't have access to
      the main <code>cgi-bin</code> directory, they will need to be able to
      run CGI programs elsewhere.</p>

      <p>There are two steps to allowing CGI execution in an arbitrary
      directory.  First, the <code>cgi-script</code> handler must be
      activated using the <directive
      module="mod_mime">AddHandler</directive> or <directive
      module="core">SetHandler</directive> directive.  Second,
      <code>ExecCGI</code> must be specified in the <directive
      module="core">Options</directive> directive.</p> 
    </section>

    <section id="options">
      <title>Explicitly using Options to permit CGI execution</title>

      <p>You could explicitly use the <directive module="core"
      >Options</directive> directive, inside your main server configuration
      file, to specify that CGI execution was permitted in a particular
      directory:</p>

      <example>
        &lt;Directory /usr/local/apache2/htdocs/somedir&gt;<br />
        <indent>
          Options +ExecCGI<br />
        </indent>
        &lt;/Directory&gt;
      </example>

      <p>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 <directive module="mod_mime"
      >AddHandler</directive> directive tells the server to treat all
      files with the <code>cgi</code> or <code>pl</code> extension as CGI
      programs:</p>

      <example>
        AddHandler cgi-script .cgi .pl
      </example>
    </section>

    <section id="htaccess">
      <title>.htaccess files</title>

      <p>The <a href="htaccess.html"><code>.htaccess</code> tutorial</a>
      shows how to activate CGI programs if you do not have
      access to <code>httpd.conf</code>.</p>
    </section>

    <section id="userdir">
      <title>User Directories</title>

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

      <example>
      &lt;Directory /home/*/public_html&gt;<br/>
      <indent>
        Options +ExecCGI<br/>
        AddHandler cgi-script .cgi<br/>
      </indent>
      &lt;/Directory&gt;
      </example>

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

      <example>
      &lt;Directory /home/*/public_html/cgi-bin&gt;<br/>
      <indent>
        Options ExecCGI<br/>
        SetHandler cgi-script<br/>
      </indent>
      &lt;/Directory&gt;
      </example>

    </section>

  </section>

  <section id="writing">
    <title>Writing a CGI program</title>

    <p>There are two main differences between ``regular''
    programming, and CGI programming.</p>

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

    <example>
      Content-type: text/html
    </example>

    <p>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.</p>

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

    <section id="firstcgi">
      <title>Your first CGI program</title>

      <p>The following is an example CGI program that prints one
      line to your browser. Type in the following, save it to a
      file called <code>first.pl</code>, and put it in your 
      <code>cgi-bin</code> directory.</p>

      <example>
        #!/usr/bin/perl<br />
        print "Content-type: text/html\n\n";<br />
        print "Hello, World.";
      </example>

      <p>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 <code>/usr/bin/perl</code>.
      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.</p>

      <p>If you open your favorite browser and tell it to get the
      address</p>

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

      <p>or wherever you put your file, you will see the one line 
      <code>Hello, World.</code> 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.</p>
    </section>
  </section>

  <section id="troubleshoot">
    <title>But it's still not working!</title>

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

    <dl>
      <dt>The output of your CGI program</dt>
      <dd>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 <code>Content-Type</code> set in your CGI program.</dd>

      <dt>The source code of your CGI program or a "POST Method Not
      Allowed" message</dt>
      <dd>That means that you have not properly configured Apache
      to process your CGI program. Reread the section on 
      <a href="#configuring">configuring
      Apache</a> and try to find what you missed.</dd>

      <dt>A message starting with "Forbidden"</dt>
      <dd>That means that there is a permissions problem. Check the
      <a href="#errorlogs">Apache error log</a> and the section below on
      <a href="#permissions">file permissions</a>.</dd>

      <dt>A message saying "Internal Server Error"</dt>
      <dd>If you check the 
      <a href="#errorlogs">Apache error log</a>, 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.</dd>
    </dl>

    <section id="permissions">
      <title>File permissions</title>

      <p>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 <code>nobody</code>, or
      <code>www</code> - 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 <code>nobody</code>
      is to give everyone execute permission on the file:</p>

      <example>
        chmod a+x first.pl
      </example>

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

    </section>

    <section id="pathinformation">
      <title>Path information and environment</title>

      <p>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 <code>PATH</code>,
      which tells the shell where it can look for files that you
      reference.</p>

      <p>When a program runs through the web server as a CGI program,
      it may not have the same <code>PATH</code>. Any programs that you
      invoke in your CGI program (like <code>sendmail</code>, 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.</p>

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

      <example>
        #!/usr/bin/perl
      </example>

      <p>Make sure that this is in fact the path to the
      interpreter.</p>
      <note type="warning">
      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.
      </note>
    </section>

    <section id="missingenv">
      <title>Missing environment variables</title>

      <p>If your CGI program depends on non-standard <a
      href="#env">environment variables</a>, you will need to
      assure that those variables are passed by Apache.</p>

      <p>When you miss HTTP headers from the environment, make
      sure they are formatted according to 
      <a href="http://tools.ietf.org/html/rfc2616">RFC 2616</a>,
      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.</p>

    </section>

    <section id="syntaxerrors">
      <title>Program errors</title>

      <p>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:</p>

      <example>
      cd /usr/local/apache2/cgi-bin<br/>
      ./first.pl
      </example>

      <p>(Do not call the <code>perl</code> interpreter.  The shell
      and Apache should find the interpreter using the <a
      href="#pathinformation">path information</a> on the first line of
      the script.)</p>

      <p>The first thing you see written by your program should be
      a set of HTTP headers, including the <code>Content-Type</code>,
      followed by a blank line.  If you see anything else, Apache will
      return the <code>Premature end of script headers</code> error if
      you try to run it through the server. See <a
      href="#writing">Writing a CGI program</a> above for more
      details.</p>
    </section>

    <section id="errorlogs">
      <title>Error logs</title>

      <p>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.</p>
    </section>

    <section id="suexec">
      <title>Suexec</title>

      <p>The <a href="../suexec.html">suexec</a> 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 <code>Premature end of script headers</code>.</p>

      <p>To check if you are using suexec, run <code>apachectl
      -V</code> and check for the location of <code>SUEXEC_BIN</code>.
      If Apache finds an <program>suexec</program> binary there on startup,
      suexec will be activated.</p>

      <p>Unless you fully understand suexec, you should not be using it.
      To disable suexec, simply remove (or rename) the <program>suexec</program>
      binary pointed to by <code>SUEXEC_BIN</code> and then restart the
      server.  If, after reading about <a href="../suexec.html">suexec</a>,
      you still wish to use it, then run <code>suexec -V</code> to find
      the location of the suexec log file, and use that log file to
      find what policy you are violating.</p>
    </section>
  </section>

  <section id="behindscenes">
    <title>What's going on behind the scenes?</title>

    <p>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.</p>

    <section id="env">
      <title>Environment variables</title>

      <p>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 
      <code>env</code> at a command prompt.</p>

      <p>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.</p>

      <p>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 
      <a href="http://www.ietf.org/rfc/rfc3875">Common Gateway
      Interface RFC</a>.</p>

      <p>This simple Perl CGI program will display all of the
      environment variables that are being passed around. Two
      similar programs are included in the 
      <code>cgi-bin</code>

      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 
      <a href="../env.html">add your own environment variables</a>
      to the basic ones provided by default.</p>

      <example>
        #!/usr/bin/perl<br />
        print "Content-type: text/html\n\n";<br />
        foreach $key (keys %ENV) {<br />
        <indent>
          print "$key --&gt; $ENV{$key}&lt;br&gt;";<br />
        </indent>
        }
      </example>
    </section>

    <section id="stdin">
      <title>STDIN and STDOUT</title>

      <p>Other communication between the server and the client
      happens over standard input (<code>STDIN</code>) and standard
      output (<code>STDOUT</code>). In normal everyday context, 
      <code>STDIN</code> means the keyboard, or a file that a 
      program is given to act on, and <code>STDOUT</code>
      usually means the console or screen.</p> 

      <p>When you <code>POST</code> 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 <code>STDIN</code>.
      The program then can process that data as though it was
      coming in from the keyboard, or from a file</p>

      <p>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
      (&amp;). 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:</p>

      <example>
        name=Rich%20Bowen&amp;city=Lexington&amp;state=KY&amp;sidekick=Squirrel%20Monkey
      </example>

      <p>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 
      <code>QUERY_STRING</code>. That's called a <code>GET</code>
      request. Your HTML form specifies whether a <code>GET</code>
      or a <code>POST</code> is used to deliver the data, by setting the 
      <code>METHOD</code> attribute in the <code>FORM</code> tag.</p>

      <p>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.</p>
    </section>
  </section>

  <section id="libraries">
    <title>CGI modules/libraries</title>

    <p>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.</p>

    <p>If you're writing CGI programs in Perl, modules are
    available on <a href="http://www.cpan.org/">CPAN</a>. The most
    popular module for this purpose is <code>CGI.pm</code>. You might
    also consider <code>CGI::Lite</code>, which implements a minimal
    set of functionality, which is all you need in most programs.</p>

    <p>If you're writing CGI programs in C, there are a variety of
    options. One of these is the <code>CGIC</code> library, from 
    <a href="http://www.boutell.com/cgic/"
    >http://www.boutell.com/cgic/</a>.</p>
  </section>

  <section id="moreinfo">
    <title>For more information</title>

    <p>The current CGI specification is available in the 
    <a href="http://www.ietf.org/rfc/rfc3875">Common Gateway
    Interface RFC</a>.</p>

    <p>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.</p>

    <p>Note that questions about CGI problems should <strong>never</strong>
    be posted to the Apache bug database unless you are sure you
    have found a problem in the Apache source code.</p>
  </section>
</manualpage>