docs/manual/env.html - httpd - Git at Google

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     <h1 align="center">Environment Variables in Apache</h1>

     <p>The Apache HTTP Server provides a mechanism for storing
     information in named variables that are called <em>environment
     variables</em>. This information can be used to control various
     operations such as logging or access control. The variables are
     also used as a mechanism to communicate with external programs
     such as CGI scripts. This document discusses different ways to
     manipulate and use these variables.</p>

     <p>Although these variables are referred to as <em>environment
     variables</em>, they are not the same as the environment
     variables controlled by the underlying operating system.
     Instead, these variables are stored and manipulated in an
     internal Apache structure. They only become actual operating
     system environment variables when they are provided to CGI
     scripts and Server Side Include scripts. If you wish to
     manipulate the operating system environment under which the
     server itself runs, you must use the standard environment
     manipulation mechanisms provided by your operating system
     shell.</p>

     <ul>
       <li><a href="#setting">Setting Environment Variables</a></li>

       <li><a href="#using">Using Environment Variables</a></li>

       <li><a href="#special">Special Purpose Environment
       Variables</a></li>

       <li><a href="#examples">Examples</a></li>
     </ul>
     <hr />

     <h2><a id="setting" name="setting">Setting Environment
     Variables</a></h2>

     <table border="1">
       <tr>
         <td valign="top"><strong>Related Modules</strong><br />
          <br />
          <a href="mod/mod_env.html">mod_env</a><br />
          <a href="mod/mod_rewrite.html">mod_rewrite</a><br />
          <a href="mod/mod_setenvif.html">mod_setenvif</a><br />
          <a href="mod/mod_unique_id.html">mod_unique_id</a><br />
          </td>

         <td valign="top"><strong>Related Directives</strong><br />
          <br />
          <a
         href="mod/mod_setenvif.html#BrowserMatch">BrowserMatch</a><br />
          <a
         href="mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><br />
          <a href="mod/mod_env.html#passenv">PassEnv</a><br />
          <a
         href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
          <a href="mod/mod_env.html#setenv">SetEnv</a><br />
          <a
         href="mod/mod_setenvif.html#SetEnvIf">SetEnvIf</a><br />
          <a
         href="mod/mod_setenvif.html#SetEnvIfNoCase">SetEnvIfNoCase</a><br />
          <a href="mod/mod_env.html#unsetenv">UnsetEnv</a><br />
          </td>
       </tr>
     </table>

     <h3>Basic Environment Manipulation</h3>

     <p>The most basic way to set an environment variable in Apache
     is using the unconditional <code>SetEnv</code> directive.
     Variables may also be passed from the environment of the shell
     which started the server using the <code>PassEnv</code>
     directive.</p>

     <h3>Conditional Per-Request Settings</h3>

     <p>For additional flexibility, the directives provided by
     mod_setenvif allow environment variables to be set on a
     per-request basis, conditional on characteristics of particular
     requests. For example, a variable could be set only when a
     specific browser (User-Agent) is making a request, or only when
     a specific Referer [sic] header is found. Even more flexibility
     is available through the mod_rewrite's <code>RewriteRule</code>
     which uses the <code>[E=...]</code> option to set environment
     variables.</p>

     <h3>Unique Identifiers</h3>

     <p>Finally, mod_unique_id sets the environment variable
     <code>UNIQUE_ID</code> for each request to a value which is
     guaranteed to be unique across "all" requests under very
     specific conditions.</p>

     <h3>Standard CGI Variables</h3>

     <p>In addition to all environment variables set within the
     Apache configuration and passed from the shell, CGI scripts and
     SSI pages are provided with a set of environment variables
     containing meta-information about the request as required by
     the <a href="http://cgi-spec.golux.com/">CGI
     specification</a>.</p>

     <h3>Some Caveats</h3>

     <ul>
       <li>It is not possible to override or change the standard CGI
       variables using the environment manipulation directives.</li>

       <li>When <a href="suexec.html">suexec</a> is used to launch
       CGI scripts, the environment will be cleaned down to a set of
       <em>safe</em> variables before CGI scripts are launched. The
       list of <em>safe</em> variables is defined at compile-time in
       <code>suexec.c</code>.</li>

       <li>For portability reasons, the names of environment
       variables may contain only letters, numbers, and the
       underscore character. In addition, the first character may
       not be a number. Characters which do not match this
       restriction will be replaced by an underscore when passed to
       CGI scripts and SSI pages.</li>
     </ul>
     <hr />

     <h2><a id="using" name="using">Using Environment
     Variables</a></h2>

     <table border="1">
       <tr>
         <td valign="top"><strong>Related Modules</strong><br />
          <br />
          <a href="mod/mod_access.html">mod_access</a><br />
          <a href="mod/mod_cgi.html">mod_cgi</a><br />
          <a href="mod/mod_headers.html">mod_headers</a><br />
          <a href="mod/mod_include.html">mod_include</a><br />
          <a href="mod/mod_log_config.html">mod_log_config</a><br />
          <a href="mod/mod_rewrite.html">mod_rewrite</a><br />
          </td>

         <td valign="top"><strong>Related Directives</strong><br />
          <br />
          <a href="mod/mod_access.html#allow">Allow</a><br />
          <a
         href="mod/mod_log_config.html#customlog">CustomLog</a><br />
          <a href="mod/mod_access.html#deny">Deny</a><br />
          <a href="mod/mod_headers.html#Header">Header</a><br />
          <a
         href="mod/mod_log_config.html#logformat">LogFormat</a><br />
          <a
         href="mod/mod_rewrite.html#RewriteCond">RewriteCond</a><br />
          <a
         href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
          </td>
       </tr>
     </table>

     <h3>CGI Scripts</h3>

     <p>One of the primary uses of environment variables is to
     communicate information to CGI scripts. As discussed above, the
     environment passed to CGI scripts includes standard
     meta-information about the request in addition to any variables
     set within the Apache configuration. For more details, see the
     <a href="howto/cgi.html">CGI tutorial</a>.</p>

     <h3>SSI Pages</h3>

     <p>Server-parsed (SSI) documents processed by mod_include's
     <code>INCLUDES</code> filter can print environment variables
     using the <code>echo</code> element, and can use environment
     variables in flow control elements to makes parts of a page
     conditional on characteristics of a request. Apache also
     provides SSI pages with the standard CGI environment variables
     as discussed above. For more details, see the <a
     href="howto/ssi.html">SSI tutorial</a>.</p>

     <h3>Access Control</h3>

     <p>Access to the server can be controlled based on the value of
     environment variables using the <code>allow from env=</code>
     and <code>deny from env=</code> directives. In combination with
     <code>SetEnvIf</code>, this allows for flexible control of
     access to the server based on characteristics of the client.
     For example, you can use these directives to deny access to a
     particular browser (User-Agent).</p>

     <h3>Conditional Logging</h3>

     <p>Environment variables can be logged in the access log using
     the <code>LogFormat</code> option <code>%e</code>. In addition,
     the decision on whether or not to log requests can be made
     based on the status of environment variables using the
     conditional form of the <code>CustomLog</code> directive. In
     combination with <code>SetEnvIf</code> this allows for flexible
     control of which requests are logged. For example, you can
     choose not to log requests for filenames ending in
     <code>gif</code>, or you can choose to only log requests from
     clients which are outside your subnet.</p>

     <h3>Conditional Response Headers</h3>

     <p>The <code>Header</code> directive can use the presence or
     absence of an environment variable to determine whether or not
     a certain HTTP header will be placed in the response to the
     client. This allows, for example, a certain response header to
     be sent only if a corresponding header is received in the
     request from the client.</p>

     <h3>URL Rewriting</h3>

     <p>The <code>%{ENV:...}</code> form of <em>TestString</em> in
     the <code>RewriteCond</code> allows mod_rewrite's rewrite
     engine to make decisions conditional on environment variables.
     Note that the variables accessible in mod_rewrite without the
     <code>ENV:</code> prefix are not actually environment
     variables. Rather, they are variables special to mod_rewrite
     which cannot be accessed from other modules.</p>
     <hr />

     <h2><a id="special" name="special">Special Purpose Environment
     Variables</a></h2>

     <p>Interoperability problems have led to the introduction of
     mechanisms to modify the way Apache behaves when talking to
     particular clients. To make these mechanisms as flexible as
     possible, they are invoked by defining environment variables,
     typically with <a
     href="mod/mod_setenvif.html#browsermatch">BrowserMatch</a>,
     though <a href="mod/mod_env.html#setenv">SetEnv</a> and <a
     href="mod/mod_env.html#passenv">PassEnv</a> could also be used,
     for example.</p>

     <h2>downgrade-1.0</h2>

     <p>This forces the request to be treated as a HTTP/1.0 request
     even if it was in a later dialect.</p>

     <h2>force-no-vary</h2>

     <p>This causes any <code>Vary</code> fields to be removed from
     the response header before it is sent back to the client. Some
     clients don't interpret this field correctly (see the <a
     href="misc/known_client_problems.html">known client
     problems</a> page); setting this variable can work around this
     problem. Setting this variable also implies
     <strong>force-response-1.0</strong>.</p>

     <h2>force-response-1.0</h2>

     <p>This forces an HTTP/1.0 response when set. It was originally
     implemented as a result of a problem with AOL's proxies. Some
     clients may not behave correctly when given an HTTP/1.1
     response, and this can be used to interoperate with them.</p>

     <h2>nokeepalive</h2>

     <p>This disables <a
     href="mod/core.html#keepalive">KeepAlive</a> when set.</p>

     <h2>redirect-carefully</h2>

     <p>This forces the server to be more careful when sending a redirect
     to the client.  This is typically used when a client has a known
     problem handling redirects.  This was originally implemented as a
     result of a problem with Microsoft's WebFolders software which has
     a problem handling redirects on directory resources via DAV
     methods.</p>

     <hr />
     <h2><a id="examples" name="examples">Examples</a></h2>

     <h3>Changing protocol behavior with misbehaving clients</h3>

     <p>We recommend that the following lines be included in
     httpd.conf to deal with known client problems.</p>
 <pre>
 #
 # The following directives modify normal HTTP response behavior.
 # The first directive disables keepalive for Netscape 2.x and browsers that
 # spoof it. There are known problems with these browser implementations.
 # The second directive is for Microsoft Internet Explorer 4.0b2
 # which has a broken HTTP/1.1 implementation and does not properly
 # support keepalive when it is used on 301 or 302 (redirect) responses.
 #
 BrowserMatch "Mozilla/2" nokeepalive
 BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0

 #
 # The following directive disables HTTP/1.1 responses to browsers which
 # are in violation of the HTTP/1.0 spec by not being able to grok a
 # basic 1.1 response.
 #
 BrowserMatch "RealPlayer 4\.0" force-response-1.0
 BrowserMatch "Java/1\.0" force-response-1.0
 BrowserMatch "JDK/1\.0" force-response-1.0
 </pre>

     <h3>Do not log requests for images in the access log</h3>

     <p>This example keeps requests for images from appearing in the
     access log. It can be easily modified to prevent logging of
     particular directories, or to prevent logging of requests
     coming from particular hosts.</p>
 <pre>
     SetEnvIf Request_URI \.gif image-request
     SetEnvIf Request_URI \.jpg image-request
     SetEnvIf Request_URI \.png image-request
     CustomLog logs/access_log env=!image-request
 </pre>

     <h3>Prevent "Image Theft"</h3>

     <p>This example shows how to keep people not on your server
     from using images on your server as inline-images on their
     pages. This is not a recommended configuration, but it can work
     in limited circumstances. We assume that all your images are in
     a directory called /web/images.</p>
 <pre>
     SetEnvIf Referer "^http://www.example.com/" local_referal
     # Allow browsers that do not send Referer info
     SetEnvIf Referer "^$" local_referal
     &lt;Directory /web/images&gt;
        Order Deny,Allow
        Deny from all
        Allow from env=local_referal
     &lt;/Directory&gt;
 </pre>

     <p>For more information about this technique, see the
     ApacheToday tutorial " <a
     href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">
     Keeping Your Images from Adorning Other Sites</a>".</p>
     <!--#include virtual="footer.html" -->
   </body>
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	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

	<html xmlns="http://www.w3.org/1999/xhtml">
	<head>
	<meta name="generator" content="HTML Tidy, see www.w3.org" />

	<title>Environment Variables in Apache</title>
	</head>
	<!-- Background white, links blue (unvisited), navy (visited), red (active) -->

	<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
	vlink="#000080" alink="#FF0000">
	<!--#include virtual="header.html" -->

	<h1 align="center">Environment Variables in Apache</h1>

	<p>The Apache HTTP Server provides a mechanism for storing
	information in named variables that are called <em>environment
	variables</em>. This information can be used to control various
	operations such as logging or access control. The variables are
	also used as a mechanism to communicate with external programs
	such as CGI scripts. This document discusses different ways to
	manipulate and use these variables.</p>

	<p>Although these variables are referred to as <em>environment
	variables</em>, they are not the same as the environment
	variables controlled by the underlying operating system.
	Instead, these variables are stored and manipulated in an
	internal Apache structure. They only become actual operating
	system environment variables when they are provided to CGI
	scripts and Server Side Include scripts. If you wish to
	manipulate the operating system environment under which the
	server itself runs, you must use the standard environment
	manipulation mechanisms provided by your operating system
	shell.</p>

	<ul>
	<li><a href="#setting">Setting Environment Variables</a></li>

	<li><a href="#using">Using Environment Variables</a></li>

	<li><a href="#special">Special Purpose Environment
	Variables</a></li>

	<li><a href="#examples">Examples</a></li>
	</ul>
	<hr />

	<h2><a id="setting" name="setting">Setting Environment
	Variables</a></h2>

	<table border="1">
	<tr>
	<td valign="top"><strong>Related Modules</strong><br />
	<br />
	<a href="mod/mod_env.html">mod_env</a><br />
	<a href="mod/mod_rewrite.html">mod_rewrite</a><br />
	<a href="mod/mod_setenvif.html">mod_setenvif</a><br />
	<a href="mod/mod_unique_id.html">mod_unique_id</a><br />
	</td>

	<td valign="top"><strong>Related Directives</strong><br />
	<br />
	<a
	href="mod/mod_setenvif.html#BrowserMatch">BrowserMatch</a><br />
	<a
	href="mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><br />
	<a href="mod/mod_env.html#passenv">PassEnv</a><br />
	<a
	href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
	<a href="mod/mod_env.html#setenv">SetEnv</a><br />
	<a
	href="mod/mod_setenvif.html#SetEnvIf">SetEnvIf</a><br />
	<a
	href="mod/mod_setenvif.html#SetEnvIfNoCase">SetEnvIfNoCase</a><br />
	<a href="mod/mod_env.html#unsetenv">UnsetEnv</a><br />
	</td>
	</tr>
	</table>

	<h3>Basic Environment Manipulation</h3>

	<p>The most basic way to set an environment variable in Apache
	is using the unconditional <code>SetEnv</code> directive.
	Variables may also be passed from the environment of the shell
	which started the server using the <code>PassEnv</code>
	directive.</p>

	<h3>Conditional Per-Request Settings</h3>

	<p>For additional flexibility, the directives provided by
	mod_setenvif allow environment variables to be set on a
	per-request basis, conditional on characteristics of particular
	requests. For example, a variable could be set only when a
	specific browser (User-Agent) is making a request, or only when
	a specific Referer [sic] header is found. Even more flexibility
	is available through the mod_rewrite's <code>RewriteRule</code>
	which uses the <code>[E=...]</code> option to set environment
	variables.</p>

	<h3>Unique Identifiers</h3>

	<p>Finally, mod_unique_id sets the environment variable
	<code>UNIQUE_ID</code> for each request to a value which is
	guaranteed to be unique across "all" requests under very
	specific conditions.</p>

	<h3>Standard CGI Variables</h3>

	<p>In addition to all environment variables set within the
	Apache configuration and passed from the shell, CGI scripts and
	SSI pages are provided with a set of environment variables
	containing meta-information about the request as required by
	the <a href="http://cgi-spec.golux.com/">CGI
	specification</a>.</p>

	<h3>Some Caveats</h3>

	<ul>
	<li>It is not possible to override or change the standard CGI
	variables using the environment manipulation directives.</li>

	<li>When <a href="suexec.html">suexec</a> is used to launch
	CGI scripts, the environment will be cleaned down to a set of
	<em>safe</em> variables before CGI scripts are launched. The
	list of <em>safe</em> variables is defined at compile-time in
	<code>suexec.c</code>.</li>

	<li>For portability reasons, the names of environment
	variables may contain only letters, numbers, and the
	underscore character. In addition, the first character may
	not be a number. Characters which do not match this
	restriction will be replaced by an underscore when passed to
	CGI scripts and SSI pages.</li>
	</ul>
	<hr />

	<h2><a id="using" name="using">Using Environment
	Variables</a></h2>

	<table border="1">
	<tr>
	<td valign="top"><strong>Related Modules</strong><br />
	<br />
	<a href="mod/mod_access.html">mod_access</a><br />
	<a href="mod/mod_cgi.html">mod_cgi</a><br />
	<a href="mod/mod_headers.html">mod_headers</a><br />
	<a href="mod/mod_include.html">mod_include</a><br />
	<a href="mod/mod_log_config.html">mod_log_config</a><br />
	<a href="mod/mod_rewrite.html">mod_rewrite</a><br />
	</td>

	<td valign="top"><strong>Related Directives</strong><br />
	<br />
	<a href="mod/mod_access.html#allow">Allow</a><br />
	<a
	href="mod/mod_log_config.html#customlog">CustomLog</a><br />
	<a href="mod/mod_access.html#deny">Deny</a><br />
	<a href="mod/mod_headers.html#Header">Header</a><br />
	<a
	href="mod/mod_log_config.html#logformat">LogFormat</a><br />
	<a
	href="mod/mod_rewrite.html#RewriteCond">RewriteCond</a><br />
	<a
	href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
	</td>
	</tr>
	</table>

	<h3>CGI Scripts</h3>

	<p>One of the primary uses of environment variables is to
	communicate information to CGI scripts. As discussed above, the
	environment passed to CGI scripts includes standard
	meta-information about the request in addition to any variables
	set within the Apache configuration. For more details, see the
	<a href="howto/cgi.html">CGI tutorial</a>.</p>

	<h3>SSI Pages</h3>

	<p>Server-parsed (SSI) documents processed by mod_include's
	<code>INCLUDES</code> filter can print environment variables
	using the <code>echo</code> element, and can use environment
	variables in flow control elements to makes parts of a page
	conditional on characteristics of a request. Apache also
	provides SSI pages with the standard CGI environment variables
	as discussed above. For more details, see the <a
	href="howto/ssi.html">SSI tutorial</a>.</p>

	<h3>Access Control</h3>

	<p>Access to the server can be controlled based on the value of
	environment variables using the <code>allow from env=</code>
	and <code>deny from env=</code> directives. In combination with
	<code>SetEnvIf</code>, this allows for flexible control of
	access to the server based on characteristics of the client.
	For example, you can use these directives to deny access to a
	particular browser (User-Agent).</p>

	<h3>Conditional Logging</h3>

	<p>Environment variables can be logged in the access log using
	the <code>LogFormat</code> option <code>%e</code>. In addition,
	the decision on whether or not to log requests can be made
	based on the status of environment variables using the
	conditional form of the <code>CustomLog</code> directive. In
	combination with <code>SetEnvIf</code> this allows for flexible
	control of which requests are logged. For example, you can
	choose not to log requests for filenames ending in
	<code>gif</code>, or you can choose to only log requests from
	clients which are outside your subnet.</p>

	<h3>Conditional Response Headers</h3>

	<p>The <code>Header</code> directive can use the presence or
	absence of an environment variable to determine whether or not
	a certain HTTP header will be placed in the response to the
	client. This allows, for example, a certain response header to
	be sent only if a corresponding header is received in the
	request from the client.</p>

	<h3>URL Rewriting</h3>

	<p>The <code>%{ENV:...}</code> form of <em>TestString</em> in
	the <code>RewriteCond</code> allows mod_rewrite's rewrite
	engine to make decisions conditional on environment variables.
	Note that the variables accessible in mod_rewrite without the
	<code>ENV:</code> prefix are not actually environment
	variables. Rather, they are variables special to mod_rewrite
	which cannot be accessed from other modules.</p>
	<hr />

	<h2><a id="special" name="special">Special Purpose Environment
	Variables</a></h2>

	<p>Interoperability problems have led to the introduction of
	mechanisms to modify the way Apache behaves when talking to
	particular clients. To make these mechanisms as flexible as
	possible, they are invoked by defining environment variables,
	typically with <a
	href="mod/mod_setenvif.html#browsermatch">BrowserMatch</a>,
	though <a href="mod/mod_env.html#setenv">SetEnv</a> and <a
	href="mod/mod_env.html#passenv">PassEnv</a> could also be used,
	for example.</p>

	<h2>downgrade-1.0</h2>

	<p>This forces the request to be treated as a HTTP/1.0 request
	even if it was in a later dialect.</p>

	<h2>force-no-vary</h2>

	<p>This causes any <code>Vary</code> fields to be removed from
	the response header before it is sent back to the client. Some
	clients don't interpret this field correctly (see the <a
	href="misc/known_client_problems.html">known client
	problems</a> page); setting this variable can work around this
	problem. Setting this variable also implies
	<strong>force-response-1.0</strong>.</p>

	<h2>force-response-1.0</h2>

	<p>This forces an HTTP/1.0 response when set. It was originally
	implemented as a result of a problem with AOL's proxies. Some
	clients may not behave correctly when given an HTTP/1.1
	response, and this can be used to interoperate with them.</p>

	<h2>nokeepalive</h2>

	<p>This disables <a
	href="mod/core.html#keepalive">KeepAlive</a> when set.</p>

	<h2>redirect-carefully</h2>

	<p>This forces the server to be more careful when sending a redirect
	to the client. This is typically used when a client has a known
	problem handling redirects. This was originally implemented as a
	result of a problem with Microsoft's WebFolders software which has
	a problem handling redirects on directory resources via DAV
	methods.</p>

	<hr />
	<h2><a id="examples" name="examples">Examples</a></h2>

	<h3>Changing protocol behavior with misbehaving clients</h3>

	<p>We recommend that the following lines be included in
	httpd.conf to deal with known client problems.</p>
	<pre>
	#
	# The following directives modify normal HTTP response behavior.
	# The first directive disables keepalive for Netscape 2.x and browsers that
	# spoof it. There are known problems with these browser implementations.
	# The second directive is for Microsoft Internet Explorer 4.0b2
	# which has a broken HTTP/1.1 implementation and does not properly
	# support keepalive when it is used on 301 or 302 (redirect) responses.
	#
	BrowserMatch "Mozilla/2" nokeepalive
	BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0

	#
	# The following directive disables HTTP/1.1 responses to browsers which
	# are in violation of the HTTP/1.0 spec by not being able to grok a
	# basic 1.1 response.
	#
	BrowserMatch "RealPlayer 4\.0" force-response-1.0
	BrowserMatch "Java/1\.0" force-response-1.0
	BrowserMatch "JDK/1\.0" force-response-1.0
	</pre>

	<h3>Do not log requests for images in the access log</h3>

	<p>This example keeps requests for images from appearing in the
	access log. It can be easily modified to prevent logging of
	particular directories, or to prevent logging of requests
	coming from particular hosts.</p>
	<pre>
	SetEnvIf Request_URI \.gif image-request
	SetEnvIf Request_URI \.jpg image-request
	SetEnvIf Request_URI \.png image-request
	CustomLog logs/access_log env=!image-request
	</pre>

	<h3>Prevent "Image Theft"</h3>

	<p>This example shows how to keep people not on your server
	from using images on your server as inline-images on their
	pages. This is not a recommended configuration, but it can work
	in limited circumstances. We assume that all your images are in
	a directory called /web/images.</p>
	<pre>
	SetEnvIf Referer "^http://www.example.com/" local_referal
	# Allow browsers that do not send Referer info
	SetEnvIf Referer "^$" local_referal
	<Directory /web/images>
	Order Deny,Allow
	Deny from all
	Allow from env=local_referal
	</Directory>
	</pre>

	<p>For more information about this technique, see the
	ApacheToday tutorial " <a
	href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">
	Keeping Your Images from Adorning Other Sites</a>".</p>
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