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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<title>Environment Variables in Apache</title>
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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
<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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