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environment variable docs. Here is what I have done: 1. Removed some obsolete CERN references and done a tiny bit of cleanup of mod_env.html. 2. Included references from mod_env.html and mod_setenvif.html to env.html. 3. Prettied-up env.html a little bit by including sub-headings. 4. Added some "caveats" to env.html: - No overriding basic CGI variables - suexec restrictions - no non-alphanumeric characters in env variable names git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87621 13f79535-47bb-0310-9956-ffa450edef68
306 lines
11 KiB
Plaintext
306 lines
11 KiB
Plaintext
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<TITLE>Environment Variables in Apache</TITLE>
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TEXT="#000000"
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<!--#include virtual="header.html" -->
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<h1 align="center">Environment Variables in Apache</h1>
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<p>Many operating systems provide a facility for storage and
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transmission of information called environment variables. Apache uses
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environment variables in many ways to control operations and to
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communicate with other programs like CGI scripts. This document
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explains some of the ways to use environment variables in Apache.</p>
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<ul>
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<li><a href="#setting">Setting Environment Variables</a></li>
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<li><a href="#using">Using Environment Variables</a></li>
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<li><a href="#special">Special Purpose Environment Variables</a></li>
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<li><a href="#examples">Examples</a></li>
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</ul>
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<hr>
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<h2><a name="setting">Setting Environment Variables</a></h2>
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<table border="1">
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<tr><td valign="top">
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<strong>Related Modules</strong><br><br>
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<a href="mod/mod_env.html">mod_env</a><br>
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<a href="mod/mod_rewrite.html">mod_rewrite</a><br>
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<a href="mod/mod_setenvif.html">mod_setenvif</a><br>
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<a href="mod/mod_unique_id.html">mod_unique_id</a><br>
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</td><td valign="top">
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<strong>Related Directives</strong><br><br>
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<A HREF="mod/mod_setenvif.html#BrowserMatch">BrowserMatch</A><br>
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<A HREF="mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</A><br>
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<A HREF="mod/mod_env.html#passenv">PassEnv</A><br>
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<A HREF="mod/mod_rewrite.html#RewriteRule">RewriteRule</A><br>
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<A HREF="mod/mod_env.html#setenv">SetEnv</A><br>
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<A HREF="mod/mod_setenvif.html#SetEnvIf">SetEnvIf</A><br>
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<A HREF="mod/mod_setenvif.html#SetEnvIfNoCase">SetEnvIfNoCase</A><br>
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<A HREF="mod/mod_env.html#unsetenv">UnsetEnv</A><br>
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</td></tr></table>
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<h3>Basic Environment Manipulation</h3>
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<p>The most basic way to set an environment variable in Apache is
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using the unconditional <code>SetEnv</code> directive. Variables
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may also be passed from the environment of the shell which started
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the server using the <code>PassEnv</code> directive.</p>
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<h3>Conditional Per-Request Settings</h3>
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<p>For additional flexibility, the directives provided by mod_setenvif
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allow environment variables to be set on a per-request basis,
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conditional on characteristics of particular requests. For example, a
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variable could be set only when a specific browser (User-Agent) is
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making a request, or only when a specific Referer [sic] header is
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found. Even more flexibility is available through the mod_rewrite's
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<code>RewriteRule</code> which uses the <code>[E=...]</code> option to
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set environment variables.</p>
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<h3>Unique Identifiers</h3>
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<p>Finally, mod_unique_id sets the environment variable
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<code>UNIQUE_ID</code> for each request to a value which is guaranteed
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to be unique across "all" requests under very specific conditions.</p>
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<h3>Standard CGI Variables</h3>
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<p>In addition to all environment variables set within the Apache
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configuration and passed from the shell, CGI scripts and SSI pages are
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provided with a set of environment variables containing
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meta-information about the request as required by the <a
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href="misc/FAQ.html#cgi-spec">CGI specification</a>.</p>
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<h3>Some Caveats</h3>
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<ul>
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<li>It is not possible to override or change the standard CGI
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variables using the environment manipulation directives.</li>
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<li>When <a href="suexec.html">suexec</a> is used to launch CGI
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scripts, the environment will be cleaned down to a set of
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<em>safe</em> variables before CGI scripts are launched. The list of
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<em>safe</em> variables is defined at compile-time in
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<code>suexec.c</code>.</li>
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<li>For portability reasons, the names of environment variables
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may contain only letters, numbers, and the underscore character.
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In addition, the first character may not be a number. Characters
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which do not match this restriction will be replaced by an
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underscore when passed to CGI scripts and SSI pages.</li>
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</ul>
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<hr>
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<h2><a name="using">Using Environment Variables</a></h2>
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<table border=1><tr><td valign="top">
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<strong>Related Modules</strong><br><br>
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<a href="mod/mod_access.html">mod_access</a><br>
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<a href="mod/mod_cgi.html">mod_cgi</a><br>
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<a href="mod/mod_include.html">mod_include</a><br>
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<a href="mod/mod_log_config.html">mod_log_config</a><br>
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<a href="mod/mod_rewrite.html">mod_rewrite</a><br>
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</td><td valign="top">
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<strong>Related Directives</strong><br><br>
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<A HREF="mod/mod_access.html#allow">Allow</A><br>
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<a href="mod/mod_log_config.html#customlog">CustomLog</a><br>
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<A HREF="mod/mod_access.html#deny">Deny</A><br>
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<a href="mod/mod_log_config.html#logformat">LogFormat</a><br>
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<A HREF="mod/mod_rewrite.html#RewriteCond">RewriteCond</A><br>
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<A HREF="mod/mod_rewrite.html#RewriteRule">RewriteRule</A><br>
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</td></tr></table>
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<h3>CGI Scripts</h3>
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<p>One of the primary uses of environment variables is to communicate
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information to CGI scripts. As discussed above, the environment
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passed to CGI scripts includes standard meta-information about the request
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in addition to any variables set within the Apache configuration.
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For more details, see the <a href="howto/cgi.html">CGI tutorial</a>.
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</p>
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<h3>SSI Pages</h3>
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<p>Server-parsed (SSI) documents processed by mod_include's
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<code>server-parsed</code> handler can print environment variables
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using the <code>echo</code> element, and can use environment variables
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in flow control elements to makes parts of a page conditional on
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characteristics of a request. Apache also provides SSI pages with the
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standard CGI environment variables as discussed above. For more
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details, see the <a href="howto/ssi.html">SSI tutorial</a>.
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</p>
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<h3>Access Control</h3>
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<p>Access to the server can be controlled based on the value of
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environment variables using the <code>allow from env=</code> and
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<code>deny from env=</code></a> directives. In combination with
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<code>SetEnvIf</code>, this allows for flexible control of access to
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the server based on characteristics of the client. For example, you
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can use these directives to deny access to a particular browser
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(User-Agent).
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</p>
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<h3>Conditional Logging</h3>
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<p>Environment variables can be logged in the access log using the
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<code>LogFormat</code> option <code>%e</code>. In addition, the
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decision on whether or not to log requests can be made based on the
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status of environment variables using the conditional form of the
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<code>CustomLog</code> directive. In combination with
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<code>SetEnvIf</code> this allows for flexible control of which
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requests are logged. For example, you can choose not to log requests
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for filenames ending in <code>gif</code>, or you can choose to only
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log requests from clients which are outside your subnet.
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</p>
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<h3>URL Rewriting</h3>
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<p>The <code>%{ENV:...}</code> form of <em>TestString</em> in the
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<code>RewriteCond</code> allows mod_rewrite's rewrite engine to make
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decisions conditional on environment variables. Note that the
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variables accessible in mod_rewrite without the <code>ENV:</code>
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prefix are not actually environment variables. Rather, they
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are variables special to mod_rewrite which cannot be accessed from
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other modules.</p>
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<hr>
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<H2><a name="special">Special Purpose Environment Variables</a></H2>
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<P>
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Interoperability problems have led to the introduction of
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mechanisms to modify the way Apache behaves when talking to particular
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clients. To make these mechanisms as flexible as possible, they
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are invoked by defining environment variables, typically with
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<A HREF="mod/mod_setenvif.html#browsermatch">BrowserMatch</A>, though
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<A HREF="mod/mod_env.html#setenv">SetEnv</A> and
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<A HREF="mod/mod_env.html#passenv">PassEnv</A> could also be used, for
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example.
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</P>
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<H2>downgrade-1.0</H2>
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<P>
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This forces the request to be treated as a HTTP/1.0 request even if it
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was in a later dialect.
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</P>
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<H2>force-no-vary</H2>
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<P>
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This causes any <CODE>Vary</CODE> fields to be removed from the response
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header before it is sent back to the client. Some clients don't
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interpret this field correctly (see the
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<A HREF="misc/known_client_problems.html">known client problems</A>
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page); setting this variable can work around this problem. Setting
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this variable also implies <STRONG>force-response-1.0</STRONG>.
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</P>
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<H2>force-response-1.0</H2>
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<P>
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This forces an HTTP/1.0 response when set. It was originally implemented as a
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result of a problem with AOL's proxies. Some clients may not behave correctly
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when given an HTTP/1.1 response, and this can be used to interoperate with
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them.
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</P>
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<H2>nokeepalive</H2>
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<P>
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This disables <A HREF="mod/core.html#keepalive">KeepAlive</A> when set.
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</P>
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<hr>
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<h2><a name="examples">Examples</a></h2>
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<h3>Changing protocol behavior with misbehaving clients</h3>
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<p>We recommend that the following lines be included in httpd.conf
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to deal with known client problems.</p>
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<pre>
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#
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# The following directives modify normal HTTP response behavior.
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# The first directive disables keepalive for Netscape 2.x and browsers that
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# spoof it. There are known problems with these browser implementations.
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# The second directive is for Microsoft Internet Explorer 4.0b2
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# which has a broken HTTP/1.1 implementation and does not properly
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# support keepalive when it is used on 301 or 302 (redirect) responses.
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#
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BrowserMatch "Mozilla/2" nokeepalive
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BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0
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#
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# The following directive disables HTTP/1.1 responses to browsers which
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# are in violation of the HTTP/1.0 spec by not being able to grok a
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# basic 1.1 response.
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#
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BrowserMatch "RealPlayer 4\.0" force-response-1.0
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BrowserMatch "Java/1\.0" force-response-1.0
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BrowserMatch "JDK/1\.0" force-response-1.0
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</pre>
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<h3>Do not log requests for images in the access log</h3>
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<p>This example keeps requests for images from appearing
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in the access log. It can be easily modified to prevent logging
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of particular directories, or to prevent logging of requests
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coming from particular hosts.</p>
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<pre>
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SetEnvIf Request_URI \.gif image-request
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SetEnvIf Request_URI \.jpg image-request
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SetEnvIf Request_URI \.png image-request
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CustomLog logs/access_log env=!image-request
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</pre>
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<h3>Prevent "Image Theft"</h3>
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<p>This example shows how to keep people not on your server from using
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images on your server as inline-images on their pages. This is not
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a recommended configuration, but it can work in limited
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circumstances. We assume that all your images are in a directory
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called /web/images.</p>
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<pre>
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SetEnvIf Referer "^http://www.example.com/" local_referal
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# Allow browsers that do not send Referer info
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SetEnvIf Referer "^$" local_referal
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<Directory /web/images>
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Order Deny,Allow
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Deny from all
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Allow from env=local_referal
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</Directory>
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</pre>
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<p>For more information about this technique, see the ApacheToday
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tutorial "<a
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href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">Keeping
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Your Images from Adorning Other Sites</a>".</p>
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<!--#include virtual="footer.html" -->
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</BODY>
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</HTML>
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