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d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h1 align="center">Environment Variables in Apache</h1>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<p>Many operating systems provide a facility for storage and
d73ae94e59c019f5cc3221ee0a0012d02091b40egctransmission of information called environment variables. Apache uses
d73ae94e59c019f5cc3221ee0a0012d02091b40egcenvironment variables in many ways to control operations and to
d73ae94e59c019f5cc3221ee0a0012d02091b40egccommunicate with other programs like CGI scripts. This document
d73ae94e59c019f5cc3221ee0a0012d02091b40egcexplains some of the ways to use environment variables in Apache.</p>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
489b7c4ab76ae8df137fbfcc2214f7baa52883a0Raymond Chen<ul>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<li><a href="#setting">Setting Environment Variables</a></li>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<li><a href="#using">Using Environment Variables</a></li>
e2c88f0c2610f16de7b639746b40dea5f3e2256eGarrett D'Amore<li><a href="#special">Special Purpose Environment Variables</a></li>
e2c88f0c2610f16de7b639746b40dea5f3e2256eGarrett D'Amore<li><a href="#examples">Examples</a></li>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc</ul>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<hr>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h2><a name="setting">Setting Environment Variables</a></h2>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<table border="1">
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<tr><td valign="top">
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<strong>Related Modules</strong><br><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_env.html">mod_env</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_rewrite.html">mod_rewrite</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_setenvif.html">mod_setenvif</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_unique_id.html">mod_unique_id</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc</td><td valign="top">
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<strong>Related Directives</strong><br><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_setenvif.html#BrowserMatch">BrowserMatch</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_env.html#passenv">PassEnv</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_rewrite.html#RewriteRule">RewriteRule</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_env.html#setenv">SetEnv</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_setenvif.html#SetEnvIf">SetEnvIf</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_setenvif.html#SetEnvIfNoCase">SetEnvIfNoCase</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_env.html#unsetenv">UnsetEnv</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc</td></tr></table>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h3>Basic Environment Manipulation</h3>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<p>The most basic way to set an environment variable in Apache is
d73ae94e59c019f5cc3221ee0a0012d02091b40egcusing the unconditional <code>SetEnv</code> directive. Variables
d73ae94e59c019f5cc3221ee0a0012d02091b40egcmay also be passed from the environment of the shell which started
d73ae94e59c019f5cc3221ee0a0012d02091b40egcthe server using the <code>PassEnv</code> directive.</p>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h3>Conditional Per-Request Settings</h3>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<p>For additional flexibility, the directives provided by mod_setenvif
d73ae94e59c019f5cc3221ee0a0012d02091b40egcallow environment variables to be set on a per-request basis,
d73ae94e59c019f5cc3221ee0a0012d02091b40egcconditional on characteristics of particular requests. For example, a
d73ae94e59c019f5cc3221ee0a0012d02091b40egcvariable could be set only when a specific browser (User-Agent) is
d73ae94e59c019f5cc3221ee0a0012d02091b40egcmaking a request, or only when a specific Referer [sic] header is
d73ae94e59c019f5cc3221ee0a0012d02091b40egcfound. Even more flexibility is available through the mod_rewrite's
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<code>RewriteRule</code> which uses the <code>[E=...]</code> option to
d73ae94e59c019f5cc3221ee0a0012d02091b40egcset environment variables.</p>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h3>Unique Identifiers</h3>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<p>Finally, mod_unique_id sets the environment variable
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<code>UNIQUE_ID</code> for each request to a value which is guaranteed
d73ae94e59c019f5cc3221ee0a0012d02091b40egcto be unique across "all" requests under very specific conditions.</p>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h3>Standard CGI Variables</h3>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<p>In addition to all environment variables set within the Apache
d73ae94e59c019f5cc3221ee0a0012d02091b40egcconfiguration and passed from the shell, CGI scripts and SSI pages are
d73ae94e59c019f5cc3221ee0a0012d02091b40egcprovided with a set of environment variables containing
d73ae94e59c019f5cc3221ee0a0012d02091b40egcmeta-information about the request as required by the <a
d73ae94e59c019f5cc3221ee0a0012d02091b40egchref="misc/FAQ.html#cgi-spec">CGI specification</a>.</p>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h3>Some Caveats</h3>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<ul>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<li>It is not possible to override or change the standard CGI
d73ae94e59c019f5cc3221ee0a0012d02091b40egcvariables using the environment manipulation directives.</li>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<li>When <a href="suexec.html">suexec</a> is used to launch CGI
d73ae94e59c019f5cc3221ee0a0012d02091b40egcscripts, the environment will be cleaned down to a set of
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<em>safe</em> variables before CGI scripts are launched. The list of
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<em>safe</em> variables is defined at compile-time in
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<code>suexec.c</code>.</li>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<li>For portability reasons, the names of environment variables
d73ae94e59c019f5cc3221ee0a0012d02091b40egcmay contain only letters, numbers, and the underscore character.
d73ae94e59c019f5cc3221ee0a0012d02091b40egcIn addition, the first character may not be a number. Characters
d73ae94e59c019f5cc3221ee0a0012d02091b40egcwhich do not match this restriction will be replaced by an
d73ae94e59c019f5cc3221ee0a0012d02091b40egcunderscore when passed to CGI scripts and SSI pages.</li>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
e2c88f0c2610f16de7b639746b40dea5f3e2256eGarrett D'Amore</ul>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<hr>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h2><a name="using">Using Environment Variables</a></h2>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
bf56214c0556fa6864189c826d39dbe156bb22a0stevel<table border=1><tr><td valign="top">
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<strong>Related Modules</strong><br><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_access.html">mod_access</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_cgi.html">mod_cgi</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_include.html">mod_include</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_log_config.html">mod_log_config</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_rewrite.html">mod_rewrite</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
bf56214c0556fa6864189c826d39dbe156bb22a0stevel</td><td valign="top">
bf56214c0556fa6864189c826d39dbe156bb22a0stevel<strong>Related Directives</strong><br><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_access.html#allow">Allow</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_log_config.html#customlog">CustomLog</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_access.html#deny">Deny</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<a href="mod/mod_log_config.html#logformat">LogFormat</a><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_rewrite.html#RewriteCond">RewriteCond</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<A HREF="mod/mod_rewrite.html#RewriteRule">RewriteRule</A><br>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc</td></tr></table>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<h3>CGI Scripts</h3>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<p>One of the primary uses of environment variables is to communicate
d73ae94e59c019f5cc3221ee0a0012d02091b40egcinformation to CGI scripts. As discussed above, the environment
d73ae94e59c019f5cc3221ee0a0012d02091b40egcpassed to CGI scripts includes standard meta-information about the request
d73ae94e59c019f5cc3221ee0a0012d02091b40egcin addition to any variables set within the Apache configuration.
d73ae94e59c019f5cc3221ee0a0012d02091b40egcFor more details, see the <a href="howto/cgi.html">CGI tutorial</a>.
d73ae94e59c019f5cc3221ee0a0012d02091b40egc</p>
d73ae94e59c019f5cc3221ee0a0012d02091b40egc
d73ae94e59c019f5cc3221ee0a0012d02091b40egc<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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