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With Apache 1.3.x, it is a bit simpler as the request does not go through ap_make_content_type(). Modules can set custom error responses but not be able to set the charset, so they have to code the charset in the html. Thus, it is useful to preserve 1.3.x behavior exactly. PR: 26467 git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@170354 13f79535-47bb-0310-9956-ffa450edef68
440 lines
18 KiB
XML
440 lines
18 KiB
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<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
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<?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
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<!-- $LastChangedRevision$ -->
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<!--
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Copyright 2002-2005 The Apache Software Foundation or its licensors, as
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applicable.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<manualpage metafile="env.xml.meta">
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<title>Environment Variables in Apache</title>
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<summary>
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<p>The Apache HTTP Server provides a mechanism for storing
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information in named variables that are called <em>environment
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variables</em>. This information can be used to control various
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operations such as logging or access control. The variables are
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also used as a mechanism to communicate with external programs
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such as CGI scripts. This document discusses different ways to
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manipulate and use these variables.</p>
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<p>Although these variables are referred to as <em>environment
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variables</em>, they are not the same as the environment
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variables controlled by the underlying operating system.
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Instead, these variables are stored and manipulated in an
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internal Apache structure. They only become actual operating
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system environment variables when they are provided to CGI
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scripts and Server Side Include scripts. If you wish to
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manipulate the operating system environment under which the
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server itself runs, you must use the standard environment
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manipulation mechanisms provided by your operating system
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shell.</p>
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</summary>
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<section id="setting">
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<title>Setting Environment Variables</title>
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<related>
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<modulelist>
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<module>mod_env</module>
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<module>mod_rewrite</module>
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<module>mod_setenvif</module>
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<module>mod_unique_id</module>
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</modulelist>
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<directivelist>
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<directive module="mod_setenvif">BrowserMatch</directive>
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<directive module="mod_setenvif">BrowserMatchNoCase</directive>
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<directive module="mod_env">PassEnv</directive>
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<directive module="mod_rewrite">RewriteRule</directive>
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<directive module="mod_env">SetEnv</directive>
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<directive module="mod_setenvif">SetEnvIf</directive>
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<directive module="mod_setenvif">SetEnvIfNoCase</directive>
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<directive module="mod_env">UnsetEnv</directive>
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</directivelist>
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</related>
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<section id="basic-manipulation">
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<title>Basic Environment Manipulation</title>
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<p>The most basic way to set an environment variable in Apache
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is using the unconditional <directive module="mod_env"
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>SetEnv</directive> directive. Variables may also be passed from
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the environment of the shell which started the server using the
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<directive module="mod_env">PassEnv</directive> directive.</p>
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</section>
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<section id="conditional">
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<title>Conditional Per-Request Settings</title>
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<p>For additional flexibility, the directives provided by
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mod_setenvif allow environment variables to be set on a
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per-request basis, conditional on characteristics of particular
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requests. For example, a variable could be set only when a
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specific browser (User-Agent) is making a request, or only when
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a specific Referer [sic] header is found. Even more flexibility
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is available through the mod_rewrite's <directive
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module="mod_rewrite">RewriteRule</directive> which uses the
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<code>[E=...]</code> option to set environment variables.</p>
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</section>
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<section id="unique-identifiers">
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<title>Unique Identifiers</title>
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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
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guaranteed to be unique across "all" requests under very
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specific conditions.</p>
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</section>
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<section id="standard-cgi">
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<title>Standard CGI Variables</title>
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<p>In addition to all environment variables set within the
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Apache configuration and passed from the shell, CGI scripts and
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SSI pages are provided with a set of environment variables
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containing meta-information about the request as required by
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the <a href="http://cgi-spec.golux.com/">CGI
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specification</a>.</p>
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</section>
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<section id="caveats">
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<title>Some Caveats</title>
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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 <program>suexec</program> is used to launch
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CGI 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
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list of <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
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variables may contain only letters, numbers, and the
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underscore character. In addition, the first character may
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not be a number. Characters which do not match this
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restriction will be replaced by an underscore when passed to
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CGI scripts and SSI pages.</li>
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</ul>
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</section>
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</section>
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<section id="using">
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<title>Using Environment Variables</title>
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<related>
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<modulelist>
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<module>mod_authz_host</module>
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<module>mod_cgi</module>
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<module>mod_ext_filter</module>
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<module>mod_headers</module>
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<module>mod_include</module>
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<module>mod_log_config</module>
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<module>mod_rewrite</module>
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</modulelist>
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<directivelist>
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<directive module="mod_authz_host">Allow</directive>
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<directive module="mod_log_config">CustomLog</directive>
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<directive module="mod_authz_host">Deny</directive>
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<directive module="mod_ext_filter">ExtFilterDefine</directive>
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<directive module="mod_headers">Header</directive>
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<directive module="mod_log_config">LogFormat</directive>
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<directive module="mod_rewrite">RewriteCond</directive>
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<directive module="mod_rewrite">RewriteRule</directive>
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</directivelist>
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</related>
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<section id="cgi-scripts">
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<title>CGI Scripts</title>
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<p>One of the primary uses of environment variables is to
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communicate information to CGI scripts. As discussed above, the
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environment passed to CGI scripts includes standard
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meta-information about the request in addition to any variables
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set within the Apache configuration. For more details, see the
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<a href="howto/cgi.html">CGI tutorial</a>.</p>
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</section>
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<section id="ssi-pages">
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<title>SSI Pages</title>
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<p>Server-parsed (SSI) documents processed by mod_include's
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<code>INCLUDES</code> filter can print environment variables
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using the <code>echo</code> element, and can use environment
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variables in flow control elements to makes parts of a page
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conditional on characteristics of a request. Apache also
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provides SSI pages with the standard CGI environment variables
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as discussed above. For more details, see the <a
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href="howto/ssi.html">SSI tutorial</a>.</p>
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</section>
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<section id="access-control">
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<title>Access Control</title>
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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>
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and <code>deny from env=</code> directives. In combination with
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<directive module="mod_setenvif">SetEnvIf</directive>, this
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allows for flexible control of access to the server based on
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characteristics of the client. For example, you can use these
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directives to deny access to a particular browser (User-Agent).
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</p>
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</section>
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<section id="logging">
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<title>Conditional Logging</title>
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<p>Environment variables can be logged in the access log using
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the <directive module="mod_log_config">LogFormat</directive>
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option <code>%e</code>. In addition, the decision on whether
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or not to log requests can be made based on the status of
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environment variables using the conditional form of the
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<directive module="mod_log_config">CustomLog</directive>
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directive. In combination with <directive module="mod_setenvif"
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>SetEnvIf</directive> this allows for flexible control of which
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requests are logged. For example, you can choose not to log
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requests for filenames ending in <code>gif</code>, or you can
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choose to only log requests from clients which are outside your
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subnet.</p>
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</section>
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<section id="response-headers">
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<title>Conditional Response Headers</title>
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<p>The <directive module="mod_headers">Header</directive>
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directive can use the presence or
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absence of an environment variable to determine whether or not
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a certain HTTP header will be placed in the response to the
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client. This allows, for example, a certain response header to
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be sent only if a corresponding header is received in the
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request from the client.</p>
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</section>
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<section id="external-filter">
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<title>External Filter Activation</title>
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<p>External filters configured by <module>mod_ext_filter</module>
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using the <directive
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module="mod_ext_filter">ExtFilterDefine</directive> directive can
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by activated conditional on an environment variable using the
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<code>disableenv=</code> and <code>enableenv=</code> options.</p>
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</section>
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<section id="url-rewriting">
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<title>URL Rewriting</title>
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<p>The <code>%{ENV:...}</code> form of <em>TestString</em> in
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the <directive module="mod_rewrite">RewriteCond</directive>
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allows mod_rewrite's rewrite
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engine to make decisions conditional on environment variables.
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Note that the variables accessible in mod_rewrite without the
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<code>ENV:</code> prefix are not actually environment
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variables. Rather, they are variables special to mod_rewrite
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which cannot be accessed from other modules.</p>
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</section>
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</section>
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<section id="special">
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<title>Special Purpose Environment Variables</title>
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<p>Interoperability problems have led to the introduction of
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mechanisms to modify the way Apache behaves when talking to
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particular clients. To make these mechanisms as flexible as
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possible, they are invoked by defining environment variables,
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typically with <directive module="mod_setenvif"
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>BrowserMatch</directive>, though <directive module="mod_env"
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>SetEnv</directive> and <directive module="mod_env"
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>PassEnv</directive> could also be used, for example.</p>
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<section id="downgrade">
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<title>downgrade-1.0</title>
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<p>This forces the request to be treated as a HTTP/1.0 request
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even if it was in a later dialect.</p>
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</section>
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<section id="force-gzip">
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<title>force-gzip</title>
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<p>If you have the <code>DEFLATE</code> filter activated, this
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environment variable will ignore the accept-encoding setting of
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your browser and will send compressed output unconditionally.</p>
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</section>
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<section id="force-no-vary">
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<title>force-no-vary</title>
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<p>This causes any <code>Vary</code> fields to be removed from
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the response header before it is sent back to the client. Some
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clients don't interpret this field correctly; setting this
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variable can work around this problem. Setting this variable
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also implies <strong>force-response-1.0</strong>.</p>
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</section>
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<section id="force-response">
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<title>force-response-1.0</title>
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<p>This forces an HTTP/1.0 response to clients making an HTTP/1.0
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request. It was originally
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implemented as a result of a problem with AOL's proxies. Some
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HTTP/1.0 clients may not behave correctly when given an HTTP/1.1
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response, and this can be used to interoperate with them.</p>
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</section>
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<section id="gzip-only-text-html">
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<title>gzip-only-text/html</title>
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<p>When set to a value of "1", this variable disables the DEFLATE
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output filter provided by <module>mod_deflate</module> for
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content-types other than <code>text/html</code>. If you'd rather
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use statically compressed files; <module>mod_negotiation</module>
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evaluates the variable as well (not only for gzip, but for all
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encodings that differ from "identity").</p>
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</section>
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<section id="no-gzip"><title>no-gzip</title>
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<p>When set, the <code>DEFLATE</code> filter of
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<module>mod_deflate</module> will be turned off and
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<module>mod_negotiation</module> will refuse to deliver encoded
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resources.</p>
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</section>
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<section id="nokeepalive">
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<title>nokeepalive</title>
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<p>This disables <directive module="core">KeepAlive</directive>
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when set.</p>
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</section>
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<section id="prefer-language"><title>prefer-language</title>
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<p>This influences <module>mod_negotiation</module>'s behaviour. If
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it contains a language tag (such as <code>en</code>, <code>ja</code>
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or <code>x-klingon</code>), <module>mod_negotiation</module> tries
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to deliver a variant with that language. If there's no such variant,
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the normal <a href="content-negotiation.html">negotiation</a> process
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applies.</p>
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</section>
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<section id="redirect-carefully">
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<title>redirect-carefully</title>
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<p>This forces the server to be more careful when sending a redirect
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to the client. This is typically used when a client has a known
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problem handling redirects. This was originally implemented as a
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result of a problem with Microsoft's WebFolders software which has
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a problem handling redirects on directory resources via DAV
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methods.</p>
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</section>
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<section id="suppress-error-charset">
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<title>suppress-error-charset</title>
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<p><em>Available in versions 2.2 and later</em></p>
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<p>When Apache issues a redirect in response to a client request,
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the response includes some actual text to be displayed in case
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the client can't (or doesn't) automatically follow the redirection.
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Apache ordinarily labels this text according to the character set
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which it uses, which is ISO-8859-1.</p>
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<p> However, if the redirection is to a page that uses a different
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character set, some broken browser versions will try to use the
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character set from the redirection text rather than the actual page.
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This can result in Greek, for instance, being incorrectly rendered.</p>
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<p>Setting this environment variable causes Apache to omit the character
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set for the redirection text, and these broken browsers will then correctly
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use that of the destination page.</p>
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</section>
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</section>
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<section id="examples">
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<title>Examples</title>
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<section id="misbehaving">
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<title>Changing protocol behavior with misbehaving clients</title>
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<p>We recommend that the following lines be included in
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httpd.conf to deal with known client problems.</p>
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<example><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</pre></example>
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</section>
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<section id="no-img-log">
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<title>Do not log requests for images in the access log</title>
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<p>This example keeps requests for images from appearing in the
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access log. It can be easily modified to prevent logging of
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particular directories, or to prevent logging of requests
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coming from particular hosts.</p>
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<example><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 common env=!image-request</pre></example>
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</section>
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<section id="image-theft">
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<title>Prevent "Image Theft"</title>
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<p>This example shows how to keep people not on your server
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from using images on your server as inline-images on their
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pages. This is not a recommended configuration, but it can work
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in limited circumstances. We assume that all your images are in
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a directory called /web/images.</p>
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<example><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></pre></example>
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<p>For more information about this technique, see the
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ApacheToday tutorial " <a
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href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">
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Keeping Your Images from Adorning Other Sites</a>".</p>
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</section>
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</section>
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</manualpage>
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