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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>

<h3>Related Modules</h3>

<ul>
<li><a href="mod/mod_env.html">mod_env</a></li>
<li><a href="mod/mod_rewrite.html">mod_rewrite</a></li>
<li><a href="mod/mod_setenvif.html">mod_setenvif</a></li>
<li><a href="mod/mod_unique_id.html">mod_unique_id</a></li>
</ul>

<h3>Related Directives</h3>

<ul>
<li><A HREF="mod/mod_setenvif.html#BrowserMatch">BrowserMatch</A></li>
<li><A HREF="mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</A></li>
<li><A HREF="mod/mod_env.html#passenv">PassEnv</A></li>
<li><A HREF="mod/mod_rewrite.html#RewriteRule">RewriteRule</A></li>
<li><A HREF="mod/mod_env.html#setenv">SetEnv</A></li>
<li><A HREF="mod/mod_setenvif.html#SetEnvIf">SetEnvIf</A></li>
<li><A HREF="mod/mod_setenvif.html#SetEnvIfNoCase">SetEnvIfNoCase</A></li>
<li><A HREF="mod/mod_env.html#unsetenv">UnsetEnv</A></li>
</ul>

<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 when Apache is started
using the <code>PassEnv</code> directive.</p>

<p>The directives provided by mod_setenvif allow environment variables
to be set on a per-request basis based 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 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>

<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>

<hr>

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

<h3>Related Modules</h3>

<ul>
<li><a href="mod/mod_access.html">mod_access</a></li>
<li><a href="mod/mod_cgi.html">mod_cgi</a></li>
<li><a href="mod/mod_include.html">mod_include</a></li>
<li><a href="mod/mod_log_config.html">mod_log_config</a></li>
<li><a href="mod/mod_rewrite.html">mod_rewrite</a></li>
</ul>

<h3>Related Directives</h3>

<ul>
<LI><A HREF="mod/mod_access.html#allowfromenv">Allow from env=</A></li>
<li><a href="mod/mod_log_config.html#customlog-conditional">CustomLog
(conditional)</a></li>
<LI><A HREF="mod/mod_access.html#denyfromenv">Deny from env=</A></li>
<li><a href="mod/mod_log_config.html#logformat">LogFormat</a></li>
<LI><A HREF="mod/mod_rewrite.html#RewriteCond">RewriteCond</A></li>
<LI><A HREF="mod/mod_rewrite.html#RewriteRule">RewriteRule</A></li>
</ul>


<p>One of the primary uses of environment variables is to communicate
information to CGI scripts.  In addition to all environment variables
set within Apache, CGI scripts are provided with a set of
meta-information about the request as provided for in the <a
href="misc/FAQ.html#cgi-spec">CGI specification</a>.  If you are using
<a href="suexec.html">Suexec</a> to execute CGI scripts under
different userids, note that the environment will be cleaned down to a
set of <em>safe</em> environment variables before the CGI script is
executed.  The set of safe environment variables is defined at
compile time in <code>suexec.c</code>.
</p>

<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.
</p>

<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>

<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>

<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.

<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_browser.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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