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