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| <p class="apache">Apache HTTP Server Version 2.0</p> |
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| <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.0</a> > <a href="./">How-To / Tutorials</a></div><div id="page-content"><div id="preamble"><h1>Apache Tutorial: Dynamic Content with CGI</h1></div> |
| <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#intro">Introduction</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#configuring">Configuring Apache to permit CGI</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#writing">Writing a CGI program</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#troubleshoot">But it's still not working!</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#behindscenes">What's going on behind the scenes?</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#libraries">CGI modules/libraries</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#moreinfo">For more information</a></li> |
| </ul></div> |
| <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="intro" id="intro">Introduction</a></h2> |
| |
| |
| <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="../mod/mod_alias.html">mod_alias</a></code></li><li><code class="module"><a href="../mod/mod_cgi.html">mod_cgi</a></code></li></ul></td><td><ul><li><code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code></li><li><code class="directive"><a href="../mod/core.html#options">Options</a></code></li><li><code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code></li></ul></td></tr></table> |
| |
| <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> |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="configuring" id="configuring">Configuring Apache to permit CGI</a></h2> |
| |
| |
| <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> |
| |
| <h3><a name="scriptalias" id="scriptalias">ScriptAlias</a></h3> |
| |
| |
| <p>The |
| <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code> |
| |
| 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 <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code> |
| directive looks like:</p> |
| |
| <div class="example"><p><code> |
| ScriptAlias /cgi-bin/ /usr/local/apache/cgi-bin/ |
| </code></p></div> |
| |
| <p>The example shown is from your default <code>httpd.conf</code> |
| configuration file, if you installed Apache in the default |
| location. The <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code> |
| directive is much like the <code class="directive"><a href="../mod/mod_alias.html#alias">Alias</a></code> directive, which defines a URL prefix that |
| is to mapped to a particular directory. <code class="directive">Alias</code> |
| and <code class="directive">ScriptAlias</code> are usually used for |
| directories that are outside of the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> directory. The difference between |
| <code class="directive">Alias</code> and <code class="directive">ScriptAlias</code> |
| is that <code class="directive">ScriptAlias</code> 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/apache/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/apache/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> |
| |
| |
| <h3><a name="nonscriptalias" id="nonscriptalias">CGI outside of ScriptAlias directories</a></h3> |
| |
| |
| <p>CGI programs are often restricted to <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>'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 |
| <code class="directive"><a href="../mod/mod_userdir.html#userdir">UserDir</a></code> 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> |
| |
| |
| <h3><a name="options" id="options">Explicitly using Options to permit CGI execution</a></h3> |
| |
| |
| <p>You could explicitly use the <code class="directive"><a href="../mod/core.html#options">Options</a></code> directive, inside your main server configuration |
| file, to specify that CGI execution was permitted in a particular |
| directory:</p> |
| |
| <div class="example"><p><code> |
| <Directory /usr/local/apache/htdocs/somedir><br /> |
| <span class="indent"> |
| Options +ExecCGI<br /> |
| </span> |
| </Directory> |
| </code></p></div> |
| |
| <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 <code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code> directive tells the server to treat all |
| files with the <code>cgi</code> or <code>pl</code> extension as CGI |
| programs:</p> |
| |
| <div class="example"><p><code> |
| AddHandler cgi-script cgi pl |
| </code></p></div> |
| |
| |
| <h3><a name="htaccess" id="htaccess">.htaccess files</a></h3> |
| |
| |
| <p>A <a href="htaccess.html"><code>.htaccess</code> file</a> is a way |
| to set configuration directives on a per-directory basis. When Apache |
| serves a resource, it looks in the directory from which it is serving |
| a file for a file called <code>.htaccess</code>, and, if it |
| finds it, it will apply directives found therein. |
| |
| <code>.htaccess</code> files can be permitted with the |
| <code class="directive"><a href="../mod/core.html#allowoverride">AllowOverride</a></code> directive, |
| which specifies what types of directives can |
| appear in these files, or if they are not allowed at all. To |
| permit the directive we will need for this purpose, the |
| following configuration will be needed in your main server |
| configuration:</p> |
| |
| <div class="example"><p><code> |
| AllowOverride Options |
| </code></p></div> |
| |
| <p>In the <code>.htaccess</code> file, you'll need the |
| following directive:</p> |
| |
| <div class="example"><p><code> |
| Options +ExecCGI |
| </code></p></div> |
| |
| <p>which tells Apache that execution of CGI programs is |
| permitted in this directory.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="writing" id="writing">Writing a CGI program</a></h2> |
| |
| |
| <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 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:</p> |
| |
| <div class="example"><p><code> |
| Content-type: text/html |
| </code></p></div> |
| |
| <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> |
| |
| <h3><a name="firstcgi" id="firstcgi">Your first CGI program</a></h3> |
| |
| |
| <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> |
| |
| <div class="example"><p><code> |
| #!/usr/bin/perl<br /> |
| print "Content-type: text/html\n\n";<br /> |
| print "Hello, World."; |
| </code></p></div> |
| |
| <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> |
| |
| <div class="example"><p><code> |
| http://www.example.com/cgi-bin/first.pl |
| </code></p></div> |
| |
| <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> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="troubleshoot" id="troubleshoot">But it's still not working!</a></h2> |
| |
| |
| <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.</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="#configuringapachetopermitcgi">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> |
| |
| <h3><a name="permissions" id="permissions">File permissions</a></h3> |
| |
| |
| <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> |
| |
| <div class="example"><p><code> |
| chmod a+x first.pl |
| </code></p></div> |
| |
| <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> |
| |
| <p>The exception to this is when the server is configured to |
| use <a href="../suexec.html">suexec</a>. This 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 an "Internal Server |
| Error". In this case, you will need to check the suexec log |
| file to see what specific security check is failing.</p> |
| |
| |
| <h3><a name="pathinformation" id="pathinformation">Path information</a></h3> |
| |
| |
| <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 path, 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 does not have that 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.</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> |
| |
| <div class="example"><p><code> |
| #!/usr/bin/perl |
| </code></p></div> |
| |
| <p>Make sure that this is in fact the path to the |
| interpreter.</p> |
| |
| |
| <h3><a name="syntaxerrors" id="syntaxerrors">Syntax errors</a></h3> |
| |
| |
| <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. Always attempt to run your program |
| from the command line before you test if via a browser. This |
| will eliminate most of your problems.</p> |
| |
| |
| <h3><a name="errorlogs" id="errorlogs">Error logs</a></h3> |
| |
| |
| <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> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="behindscenes" id="behindscenes">What's going on behind the scenes?</a></h2> |
| |
| |
| <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> |
| |
| <h3><a name="env" id="env">Environment variables</a></h3> |
| |
| |
| <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 a 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://hoohoo.ncsa.uiuc.edu/cgi/env.html">http://hoohoo.ncsa.uiuc.edu/cgi/env.html</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> |
| |
| <div class="example"><p><code> |
| #!/usr/bin/perl<br /> |
| print "Content-type: text/html\n\n";<br /> |
| foreach $key (keys %ENV) {<br /> |
| <span class="indent"> |
| print "$key --> $ENV{$key}<br>";<br /> |
| </span> |
| } |
| </code></p></div> |
| |
| |
| <h3><a name="stdin" id="stdin">STDIN and STDOUT</a></h3> |
| |
| |
| <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 |
| (&). 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> |
| |
| <div class="example"><p><code> |
| name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey |
| </code></p></div> |
| |
| <p>You'll sometimes also see this type of string appended to |
| the 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> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="libraries" id="libraries">CGI modules/libraries</a></h2> |
| |
| |
| <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> |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="moreinfo" id="moreinfo">For more information</a></h2> |
| |
| |
| <p>There are a large number of CGI resources on the web. You |
| can discuss CGI problems with other users on the Usenet group |
| <a href="news:comp.infosystems.www.authoring.cgi">comp.infosystems.www.authoring.cgi</a>. And the -servers mailing |
| list from the HTML Writers Guild is a great source of answers |
| to your questions. You can find out more at |
| <a href="http://www.hwg.org/lists/hwg-servers/">http://www.hwg.org/lists/hwg-servers/</a>.</p> |
| |
| <p>And, of course, you should probably read the CGI |
| specification, which has all the details on the operation of |
| CGI programs. You can find the original version at the |
| <a href="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">NCSA</a> and there is an updated draft at the |
| <a href="http://web.golux.com/coar/cgi/">Common Gateway |
| Interface RFC project</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> |
| </div></div> |
| <div id="footer"> |
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