docs/manual/env.xml

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<manualpage metafile="env.xml.meta">

  <title>Environment Variables in Apache</title>

  <summary>
    <p>There are two kinds of environment variables that affect
    the Apache HTTP Server.</p>

    <p>First, there are the environment variables controlled by
    the underlying operating system.  These are set before the
    server starts.  They can be used in expansions in configuration
    files, and can optionally be passed to CGI scripts and SSI
    using the PassEnv directive.</p>

    <p>Second, the Apache HTTP Server provides a mechanism for storing
    information in named variables that are also 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>
  </summary>

  <section id="setting">
    <title>Setting Environment Variables</title>
    <related>
      <modulelist>
        <module>mod_cache</module>
        <module>mod_env</module>
        <module>mod_rewrite</module>
        <module>mod_setenvif</module>
        <module>mod_unique_id</module>
      </modulelist>
      <directivelist>
        <directive module="mod_setenvif">BrowserMatch</directive>
        <directive module="mod_setenvif">BrowserMatchNoCase</directive>
        <directive module="mod_env">PassEnv</directive>
        <directive module="mod_rewrite">RewriteRule</directive>
        <directive module="mod_env">SetEnv</directive>
        <directive module="mod_setenvif">SetEnvIf</directive>
        <directive module="mod_setenvif">SetEnvIfNoCase</directive>
        <directive module="mod_env">UnsetEnv</directive>
      </directivelist>
    </related>

    <section id="basic-manipulation">
        <title>Basic Environment Manipulation</title>

        <p>The most basic way to set an environment variable in Apache
        is using the unconditional <directive module="mod_env"
        >SetEnv</directive> directive. Variables may also be passed from
        the environment of the shell which started the server using the
        <directive module="mod_env">PassEnv</directive> directive.</p>

    </section>
    <section id="conditional">
        <title>Conditional Per-Request Settings</title>

        <p>For additional flexibility, the directives provided by
        <module>mod_setenvif</module> 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 <module>mod_rewrite</module>'s <directive
        module="mod_rewrite">RewriteRule</directive> which uses the
        <code>[E=...]</code> option to set environment variables.</p>

    </section>
    <section id="unique-identifiers">
        <title>Unique Identifiers</title>

        <p>Finally, <module>mod_unique_id</module> 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>

    </section>
    <section id="standard-cgi">
        <title>Standard CGI Variables</title>

        <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://www.ietf.org/rfc/rfc3875">CGI
        specification</a>.</p>

    </section>
    <section id="caveats">
        <title>Some Caveats</title>

        <ul>
          <li>It is not possible to override or change the standard CGI
          variables using the environment manipulation directives.</li>

          <li>When <program>suexec</program> 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>

          <li>A special case are HTTP headers which are passed to CGI
          scripts and the like via environment variables (see below).
          They are converted to uppercase and only dashes are replaced with
          underscores; if the header contains any other (invalid) character,
          the whole header is silently dropped. See <a href="#fixheader">
          below</a> for a workaround.</li>

          <li>The <directive module="mod_env">SetEnv</directive> directive runs
          late during request processing meaning that directives such as
          <directive module="mod_setenvif">SetEnvIf</directive> and <directive
          module="mod_rewrite">RewriteCond</directive> will not see the
          variables set with it.</li>

          <li>When the server looks up a path via an internal
          <glossary ref="subrequest">subrequest</glossary> such as looking
          for a <directive module="mod_dir" >DirectoryIndex</directive>
          or generating a directory listing with <module>mod_autoindex</module>,
          per-request environment variables are <em>not</em> inherited in the
          subrequest. Additionally,
          <directive module="mod_setenvif">SetEnvIf</directive> directives
          are not separately evaluated in the subrequest due to the API phases
           <module>mod_setenvif</module> takes action in.</li>
        </ul>
    </section>
  </section>
  <section id="using">
    <title>Using Environment Variables</title>

    <related>
      <modulelist>
        <module>mod_authz_host</module>
        <module>mod_cgi</module>
        <module>mod_ext_filter</module>
        <module>mod_headers</module>
        <module>mod_include</module>
        <module>mod_log_config</module>
        <module>mod_rewrite</module>
      </modulelist>
      <directivelist>
        <directive module="mod_authz_core">Require</directive>
        <directive module="mod_log_config">CustomLog</directive>
        <directive module="mod_access_compat">Deny</directive>
        <directive module="mod_ext_filter">ExtFilterDefine</directive>
        <directive module="mod_headers">Header</directive>
        <directive module="mod_log_config">LogFormat</directive>
        <directive module="mod_rewrite">RewriteCond</directive>
        <directive module="mod_rewrite">RewriteRule</directive>
      </directivelist>
    </related>

    <section id="cgi-scripts">
        <title>CGI Scripts</title>

        <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>

    </section>
    <section id="ssi-pages">
        <title>SSI Pages</title>

        <p>Server-parsed (SSI) documents processed by
        <module>mod_include</module>'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>

    </section>
    <section id="access-control">
        <title>Access Control</title>

        <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
        <directive module="mod_setenvif">SetEnvIf</directive>, 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>

    </section>
    <section id="logging">
        <title>Conditional Logging</title>

        <p>Environment variables can be logged in the access log using
        the <directive module="mod_log_config">LogFormat</directive>
        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
        <directive module="mod_log_config">CustomLog</directive>
        directive. In combination with <directive module="mod_setenvif"
        >SetEnvIf</directive> 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>

    </section>
    <section id="response-headers">
        <title>Conditional Response Headers</title>

        <p>The <directive module="mod_headers">Header</directive>
        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>

    </section>

    <section id="external-filter">
        <title>External Filter Activation</title>

        <p>External filters configured by <module>mod_ext_filter</module>
        using the <directive
        module="mod_ext_filter">ExtFilterDefine</directive> directive can
        by activated conditional on an environment variable using the
        <code>disableenv=</code> and <code>enableenv=</code> options.</p>
    </section>

    <section id="url-rewriting">
        <title>URL Rewriting</title>

        <p>The <code>%{ENV:<em>variable</em>}</code> form of
        <em>TestString</em> in the <directive module="mod_rewrite"
        >RewriteCond</directive> allows <module>mod_rewrite</module>'s rewrite
        engine to make decisions conditional on environment variables.
        Note that the variables accessible in <module>mod_rewrite</module>
        without the <code>ENV:</code> prefix are not actually environment
        variables. Rather, they are variables special to
        <module>mod_rewrite</module> which cannot be accessed from other
        modules.</p>
    </section>
  </section>

  <section id="special">
    <title>Special Purpose Environment Variables</title>

        <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 <directive module="mod_setenvif"
        >BrowserMatch</directive>, though <directive module="mod_env"
        >SetEnv</directive> and  <directive module="mod_env"
        >PassEnv</directive> could also be used, for example.</p>

    <section id="downgrade">
        <title>downgrade-1.0</title>

        <p>This forces the request to be treated as a HTTP/1.0 request
        even if it was in a later dialect.</p>

    </section>
    <section id="force-gzip">
        <title>force-gzip</title>
          <p>If you have the <code>DEFLATE</code> filter activated, this
          environment variable will ignore the accept-encoding setting of
          your browser and will send compressed output unconditionally.</p>
    </section>
    <section id="force-no-vary">
        <title>force-no-vary</title>

        <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; setting this
        variable can work around this problem. Setting this variable
        also implies <strong>force-response-1.0</strong>.</p>

    </section>
    <section id="force-response">
        <title>force-response-1.0</title>

      <p>This forces an HTTP/1.0 response to clients making an HTTP/1.0
      request. It was originally
      implemented as a result of a problem with AOL's proxies. Some
      HTTP/1.0 clients may not behave correctly when given an HTTP/1.1
      response, and this can be used to interoperate with them.</p>

    </section>

    <section id="gzip-only-text-html">
        <title>gzip-only-text/html</title>

        <p>When set to a value of "1", this variable disables the
        <code>DEFLATE</code> output filter provided by
        <module>mod_deflate</module> for content-types other than
        <code>text/html</code>. If you'd rather
        use statically compressed files, <module>mod_negotiation</module>
        evaluates the variable as well (not only for gzip, but for all
        encodings that differ from "identity").</p>
    </section>

    <section id="no-gzip"><title>no-gzip</title>

        <p>When set, the <code>DEFLATE</code> filter of
        <module>mod_deflate</module> will be turned off and
        <module>mod_negotiation</module> will refuse to deliver encoded
        resources.</p>

    </section>

    <section id="no-cache"><title>no-cache</title>
        <p><em>Available in versions 2.2.12 and later</em></p>

        <p>When set, <module>mod_cache</module> will not save an otherwise
        cacheable response.  This environment variable does not influence
        whether a response already in the cache will be served for the current
        request.</p>

    </section>

    <section id="nokeepalive">
        <title>nokeepalive</title>

        <p>This disables <directive module="core">KeepAlive</directive>
        when set.</p>

    </section>

    <section id="prefer-language"><title>prefer-language</title>

        <p>This influences <module>mod_negotiation</module>'s behaviour. If
        it contains a language tag (such as <code>en</code>, <code>ja</code>
        or <code>x-klingon</code>), <module>mod_negotiation</module> tries
        to deliver a variant with that language. If there's no such variant,
        the normal <a href="content-negotiation.html">negotiation</a> process
        applies.</p>

    </section>

    <section id="redirect-carefully">
        <title>redirect-carefully</title>

        <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>

    </section>

   <section id="suppress-error-charset">
       <title>suppress-error-charset</title>

    <p><em>Available in versions after 2.0.54</em></p>

    <p>When Apache issues a redirect in response to a client request,
    the response includes some actual text to be displayed in case
    the client can't (or doesn't) automatically follow the redirection.
    Apache ordinarily labels this text according to the character set
    which it uses, which is ISO-8859-1.</p>

    <p> However, if the redirection is to a page that uses a different
    character set, some broken browser versions will try to use the
    character set from the redirection text rather than the actual page.
    This can result in Greek, for instance, being incorrectly rendered.</p>

    <p>Setting this environment variable causes Apache to omit the character
    set for the redirection text, and these broken browsers will then correctly
    use that of the destination page.</p>

    <note type="warning">
      <title>Security note</title>

      <p>Sending error pages without a specified character set may
      allow a cross-site-scripting attack for existing browsers (MSIE)
      which do not follow the HTTP/1.1 specification and attempt to
      "guess" the character set from the content.  Such browsers can
      be easily fooled into using the UTF-7 character set, and UTF-7
      content from input data (such as the request-URI) will not be
      escaped by the usual escaping mechanisms designed to prevent
      cross-site-scripting attacks.</p>
    </note>

   </section>

   <section id="proxy"><title>force-proxy-request-1.0, proxy-nokeepalive, proxy-sendchunked,
   proxy-sendcl, proxy-chain-auth, proxy-interim-response, proxy-initial-not-pooled</title>

   <p>These directives alter the protocol behavior of
   <module>mod_proxy</module>.  See the <module>mod_proxy</module> and <module>mod_proxy_http</module>
   documentation for more details.</p>
   </section>

  </section>

  <section id="examples">
    <title>Examples</title>

    <section id="fixheader">
      <title>Passing broken headers to CGI scripts</title>

      <p>Starting with version 2.4, Apache is more strict about how HTTP
      headers are converted to environment variables in <module>mod_cgi
      </module> and other modules:  Previously any invalid characters
      in header names were simply translated to underscores.  This allowed
      for some potential cross-site-scripting attacks via header injection
      (see <a href="http://events.ccc.de/congress/2007/Fahrplan/events/2212.en.html">
      Unusual Web Bugs</a>, slide 19/20).</p>

      <p>If you have to support a client which sends broken headers and
      which can't be fixed, a simple workaround involving <module>mod_setenvif
      </module> and <module>mod_headers</module> allows you to still accept
      these headers:</p>

<highlight language="config">
#
# The following works around a client sending a broken Accept_Encoding
# header.
#
SetEnvIfNoCase ^Accept.Encoding$ ^(.*)$ fix_accept_encoding=$1
RequestHeader set Accept-Encoding %{fix_accept_encoding}e env=fix_accept_encoding
</highlight>

    </section>

    <section id="misbehaving">
        <title>Changing protocol behavior with misbehaving clients</title>

        <p>Earlier versions recommended that the following lines be included in
        httpd.conf to deal with known client problems.  Since the affected clients
        are no longer seen in the wild, this configuration is likely no-longer
        necessary.</p>
<highlight language="config">
#
# 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 understand 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
</highlight>

    </section>
    <section id="no-img-log">
        <title>Do not log requests for images in the access log</title>

        <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>

        <highlight language="config">
SetEnvIf Request_URI \.gif image-request
SetEnvIf Request_URI \.jpg image-request
SetEnvIf Request_URI \.png image-request
CustomLog "logs/access_log" common env=!image-request
        </highlight>

    </section>
    <section id="image-theft">
        <title>Prevent "Image Theft"</title>

        <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 <code>/web/images</code>.</p>

        <highlight language="config">
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;
    Require env local_referal
&lt;/Directory&gt;
        </highlight>

        <p>For more information about this technique, see the
        "<a href="http://www.serverwatch.com/tutorials/article.php/1132731"
        >Keeping Your Images from Adorning Other Sites</a>"
        tutorial on ServerWatch.</p>
    </section>
  </section>
</manualpage>