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

 <p>Many operating systems provide a facility for storage and
 transmission of information called environment variables.  Apache uses
 environment variables in many ways to control operations and to
 communicate with other programs like CGI scripts.  This document
 explains some of the ways to use environment variables in Apache.</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 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="misc/FAQ.html#cgi-spec">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 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_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_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>server-parsed</code> handler 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></a> 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>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 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>


 <hr>

 <h2><a 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 &quot;Image Theft&quot;</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 &quot;<a
 href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">Keeping
 Your Images from Adorning Other Sites</a>&quot;.</p>

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	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
	<HTML>
	<HEAD>
	<TITLE>Environment Variables in Apache</TITLE>
	</HEAD>

	<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
	<BODY
	BGCOLOR="#FFFFFF"
	TEXT="#000000"
	LINK="#0000FF"
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	<!--#include virtual="header.html" -->
	<h1 align="center">Environment Variables in Apache</h1>

	<p>Many operating systems provide a facility for storage and
	transmission of information called environment variables. Apache uses
	environment variables in many ways to control operations and to
	communicate with other programs like CGI scripts. This document
	explains some of the ways to use environment variables in Apache.</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 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="misc/FAQ.html#cgi-spec">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 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_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_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>server-parsed</code> handler 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></a> 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>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 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>


	<hr>

	<h2><a 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>

	<!--#include virtual="footer.html" -->
	</BODY>
	</HTML>