path: root/docs/manual/mod/mod_proxy.xml

<?xml version="1.0"?>
<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<!-- $LastChangedRevision$ -->

<!--
 Licensed to the Apache Software Foundation (ASF) under one or more
 contributor license agreements.  See the NOTICE file distributed with
 this work for additional information regarding copyright ownership.
 The ASF licenses this file to You under the Apache License, Version 2.0
 (the "License"); you may not use this file except in compliance with
 the License.  You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
-->

<modulesynopsis metafile="mod_proxy.xml.meta">

<name>mod_proxy</name>
<description>HTTP/1.1 proxy/gateway server</description>
<status>Extension</status>
<sourcefile>mod_proxy.c</sourcefile>
<identifier>proxy_module</identifier>

<summary>
    <note type="warning"><title>Warning</title>
      <p>Do not enable proxying with <directive module="mod_proxy"
      >ProxyRequests</directive> until you have <a href="#access"
      >secured your server</a>. Open proxy servers are dangerous both to your
      network and to the Internet at large.</p>
    </note>

    <p>This module implements a proxy/gateway for Apache. It implements
    proxying capability for <code>AJP13</code> (Apache JServe Protocol
    version 1.3), <code>FTP</code>, <code>CONNECT</code> (for SSL),
    <code>HTTP/0.9</code>, <code>HTTP/1.0</code>, and <code>HTTP/1.1</code>.
    The module can be configured to connect to other proxy modules for these
    and other protocols.</p>

    <p>Apache's proxy features are divided into several modules in
    addition to <module>mod_proxy</module>:
    <module>mod_proxy_http</module>, <module>mod_proxy_ftp</module>,
    <module>mod_proxy_ajp</module>, <module>mod_proxy_balancer</module>,
    and <module>mod_proxy_connect</module>.  Thus, if you want to use
    one or more of the particular proxy functions, load
    <module>mod_proxy</module> <em>and</em> the appropriate module(s)
    into the server (either statically at compile-time or dynamically
    via the <directive module="mod_so">LoadModule</directive>
    directive).</p>

    <p>In addition, extended features are provided by other modules.
    Caching is provided by <module>mod_cache</module> and related
    modules.  The ability to contact remote servers using the SSL/TLS
    protocol is provided by the <code>SSLProxy*</code> directives of
    <module>mod_ssl</module>.  These additional modules will need
    to be loaded and configured to take advantage of these features.</p>
</summary>
<seealso><module>mod_cache</module></seealso>
<seealso><module>mod_proxy_http</module></seealso>
<seealso><module>mod_proxy_ftp</module></seealso>
<seealso><module>mod_proxy_connect</module></seealso>
<seealso><module>mod_proxy_balancer</module></seealso>
<seealso><module>mod_ssl</module></seealso>

    <section id="forwardreverse"><title>Forward Proxies and Reverse 
       Proxies/Gateways</title>
      <p>Apache can be configured in both a <dfn>forward</dfn> and
      <dfn>reverse</dfn> proxy (also known as <dfn>gateway</dfn>) mode.</p>

      <p>An ordinary <dfn>forward proxy</dfn> is an intermediate
      server that sits between the client and the <em>origin
      server</em>.  In order to get content from the origin server,
      the client sends a request to the proxy naming the origin server
      as the target and the proxy then requests the content from the
      origin server and returns it to the client.  The client must be
      specially configured to use the forward proxy to access other
      sites.</p>

      <p>A typical usage of a forward proxy is to provide Internet
      access to internal clients that are otherwise restricted by a
      firewall.  The forward proxy can also use caching (as provided
      by <module>mod_cache</module>) to reduce network usage.</p>

      <p>The forward proxy is activated using the <directive
      module="mod_proxy">ProxyRequests</directive> directive.  Because
      forward proxies allow clients to access arbitrary sites through
      your server and to hide their true origin, it is essential that
      you <a href="#access">secure your server</a> so that only
      authorized clients can access the proxy before activating a
      forward proxy.</p>

      <p>A <dfn>reverse proxy</dfn> (or <dfn>gateway</dfn>), by
      contrast, appears to the client just like an ordinary web
      server.  No special configuration on the client is necessary.
      The client makes ordinary requests for content in the name-space
      of the reverse proxy.  The reverse proxy then decides where to
      send those requests, and returns the content as if it was itself
      the origin.</p>

      <p>A typical usage of a reverse proxy is to provide Internet
      users access to a server that is behind a firewall.  Reverse
      proxies can also be used to balance load among several back-end
      servers, or to provide caching for a slower back-end server.
      In addition, reverse proxies can be used simply to bring
      several servers into the same URL space.</p>

      <p>A reverse proxy is activated using the <directive
      module="mod_proxy">ProxyPass</directive> directive or the
      <code>[P]</code> flag to the <directive
      module="mod_rewrite">RewriteRule</directive> directive.  It is
      <strong>not</strong> necessary to turn <directive
      module="mod_proxy">ProxyRequests</directive> on in order to
      configure a reverse proxy.</p>
    </section> <!-- /forwardreverse -->

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

    <p>The examples below are only a very basic idea to help you
    get started.  Please read the documentation on the individual
    directives.</p>

    <p>In addition, if you wish to have caching enabled, consult
    the documentation from <module>mod_cache</module>.</p>

    <example><title>Forward Proxy</title>
    ProxyRequests On<br />
    ProxyVia On<br />
    <br />
    &lt;Proxy *&gt;<br />
    <indent>
      Order deny,allow<br />
      Deny from all<br />
      Allow from internal.example.com<br />
    </indent>
    &lt;/Proxy&gt;
    </example>

    <example><title>Reverse Proxy</title>
    ProxyRequests Off<br />
    <br />
    &lt;Proxy *&gt;<br />
    <indent>
      Order deny,allow<br />
      Allow from all<br />
    </indent>
    &lt;/Proxy&gt;<br />
    <br />
    ProxyPass /foo http://foo.example.com/bar<br />
    ProxyPassReverse /foo http://foo.example.com/bar
    </example>
    </section> <!-- /examples -->


    <section id="access"><title>Controlling access to your proxy</title>
      <p>You can control who can access your proxy via the <directive
      module="mod_proxy" type="section">Proxy</directive> control block as in
      the following example:</p>

      <example>
        &lt;Proxy *&gt;<br />
        <indent>
          Order Deny,Allow<br />
          Deny from all<br />
          Allow from 192.168.0<br />
        </indent>
        &lt;/Proxy&gt;
      </example>

      <p>For more information on access control directives, see
      <module>mod_authz_host</module>.</p>

      <p>Strictly limiting access is essential if you are using a
      forward proxy (using the <directive
      module="mod_proxy">ProxyRequests</directive> directive).
      Otherwise, your server can be used by any client to access
      arbitrary hosts while hiding his or her true identity.  This is
      dangerous both for your network and for the Internet at large.
      When using a reverse proxy (using the <directive
      module="mod_proxy">ProxyPass</directive> directive with
      <code>ProxyRequests Off</code>), access control is less
      critical because clients can only contact the hosts that you
      have specifically configured.</p>

      <p><strong>See Also</strong> the <a href="mod_proxy_http.html#env"
      >Proxy-Chain-Auth</a> environment variable.</p>

    </section> <!-- /access -->

    <section id="startup"><title>Slow Startup</title>
      <p>If you're using the <directive module="mod_proxy"
      >ProxyBlock</directive> directive, hostnames' IP addresses are looked up
      and cached during startup for later match test. This may take a few
      seconds (or more) depending on the speed with which the hostname lookups
      occur.</p>
    </section> <!-- /startup -->

    <section id="intranet"><title>Intranet Proxy</title>
      <p>An Apache proxy server situated in an intranet needs to forward
      external requests through the company's firewall (for this, configure
      the <directive module="mod_proxy">ProxyRemote</directive> directive
      to forward the respective <var>scheme</var> to the firewall proxy).
      However, when it has to
      access resources within the intranet, it can bypass the firewall when
      accessing hosts. The <directive module="mod_proxy">NoProxy</directive>
      directive is useful for specifying which hosts belong to the intranet and
      should be accessed directly.</p>

      <p>Users within an intranet tend to omit the local domain name from their
      WWW requests, thus requesting "http://somehost/" instead of
      <code>http://somehost.example.com/</code>. Some commercial proxy servers
      let them get away with this and simply serve the request, implying a
      configured local domain. When the <directive module="mod_proxy"
      >ProxyDomain</directive> directive is used and the server is <a
      href="#proxyrequests">configured for proxy service</a>, Apache can return
      a redirect response and send the client to the correct, fully qualified,
      server address. This is the preferred method since the user's bookmark
      files will then contain fully qualified hosts.</p>
    </section> <!-- /intranet -->

    <section id="envsettings"><title>Protocol Adjustments</title>
      <p>For circumstances where <module>mod_proxy</module> is sending
      requests to an origin server that doesn't properly implement
      keepalives or HTTP/1.1, there are two <a
      href="../env.html">environment variables</a> that can force the
      request to use HTTP/1.0 with no keepalive. These are set via the
      <directive module="mod_env">SetEnv</directive> directive.</p>

      <p>These are the <code>force-proxy-request-1.0</code> and
      <code>proxy-nokeepalive</code> notes.</p>

      <example>
        &lt;Location /buggyappserver/&gt;<br />
        <indent>
          ProxyPass http://buggyappserver:7001/foo/<br />
          SetEnv force-proxy-request-1.0 1<br />
          SetEnv proxy-nokeepalive 1<br />
        </indent>
        &lt;/Location&gt;
      </example>

    </section> <!-- /envsettings -->

    <section id="request-bodies"><title>Request Bodies</title>

    <p>Some request methods such as POST include a request body.
    The HTTP protocol requires that requests which include a body
    either use chunked transfer encoding or send a
    <code>Content-Length</code> request header.  When passing these
    requests on to the origin server, <module>mod_proxy_http</module>
    will always attempt to send the <code>Content-Length</code>.  But
    if the body is large and the original request used chunked
    encoding, then chunked encoding may also be used in the upstream
    request.  You can control this selection using <a
    href="../env.html">environment variables</a>.  Setting
    <code>proxy-sendcl</code> ensures maximum compatibility with
    upstream servers by always sending the
    <code>Content-Length</code>, while setting
    <code>proxy-sendchunked</code> minimizes resource usage by using
    chunked encoding.</p>

    </section> <!-- /request-bodies -->

    <section id="x-headers"><title>Reverse Proxy Request Headers</title>

    <p>When acting in a reverse-proxy mode (using the <directive
    module="mod_proxy">ProxyPass</directive> directive, for example),
    <module>mod_proxy_http</module> adds several request headers in
    order to pass information to the origin server. These headers
    are:</p>

    <dl>
      <dt><code>X-Forwarded-For</code></dt>
      <dd>The IP address of the client.</dd>
      <dt><code>X-Forwarded-Host</code></dt>
      <dd>The original host requested by the client in the <code>Host</code> 
       HTTP request header.</dd>
      <dt><code>X-Forwarded-Server</code></dt>
      <dd>The hostname of the proxy server.</dd>
    </dl>

    <p>Be careful when using these headers on the origin server, since
    they will contain more than one (comma-separated) value if the
    original request already contained one of these headers. For
    example, you can use <code>%{X-Forwarded-For}i</code> in the log
    format string of the origin server to log the original clients IP
    address, but you may get more than one address if the request
    passes through several proxies.</p>

    <p>See also the <directive
    module="mod_proxy">ProxyPreserveHost</directive> and <directive
    module="mod_proxy">ProxyVia</directive> directives, which control
    other request headers.</p>

   </section> <!--/x-headers -->


<directivesynopsis type="section">
<name>Proxy</name>
<description>Container for directives applied to proxied resources</description>
<syntax>&lt;Proxy <var>wildcard-url</var>&gt; ...&lt;/Proxy&gt;</syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>Directives placed in <directive type="section">Proxy</directive>
    sections apply only to matching proxied content.  Shell-style wildcards are
    allowed.</p>

    <p>For example, the following will allow only hosts in
    <code>yournetwork.example.com</code> to access content via your proxy
    server:</p>

    <example>
      &lt;Proxy *&gt;<br />
      <indent>
        Order Deny,Allow<br />
        Deny from all<br />
        Allow from yournetwork.example.com<br />
      </indent>
      &lt;/Proxy&gt;
    </example>

    <p>The following example will process all files in the <code>foo</code>
    directory of <code>example.com</code> through the <code>INCLUDES</code>
    filter when they are sent through the proxy server:</p>

    <example>
      &lt;Proxy http://example.com/foo/*&gt;<br />
      <indent>
        SetOutputFilter INCLUDES<br />
      </indent>
      &lt;/Proxy&gt;
    </example>

</usage>
<seealso><directive type="section" module="mod_proxy">ProxyMatch</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>ProxyBadHeader</name>
<description>Determines how to handle bad header lines in a
response</description>
<syntax>ProxyBadHeader IsError|Ignore|StartBody</syntax>
<default>ProxyBadHeader IsError</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<compatibility>Available in Apache 2.0.44 and later</compatibility>

<usage>
    <p>The <directive>ProxyBadHeader</directive> directive determines the
    behaviour of <module>mod_proxy</module> if it receives syntactically invalid
    header lines (<em>i.e.</em> containing no colon). The following arguments
    are possible:</p>

    <dl>
    <dt><code>IsError</code></dt>
    <dd>Abort the request and end up with a 502 (Bad Gateway) response. This is
    the default behaviour.</dd>

    <dt><code>Ignore</code></dt>
    <dd>Treat bad header lines as if they weren't sent.</dd>

    <dt><code>StartBody</code></dt>
    <dd>When receiving the first bad header line, finish reading the headers and
    treat the remainder as body. This helps to work around buggy backend servers
    which forget to insert an empty line between the headers and the body.</dd>
    </dl>
</usage>
</directivesynopsis>

<directivesynopsis type="section">
<name>ProxyMatch</name>
<description>Container for directives applied to regular-expression-matched 
proxied resources</description>
<syntax>&lt;ProxyMatch <var>regex</var>&gt; ...&lt;/ProxyMatch&gt;</syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive type="section">ProxyMatch</directive> directive is
    identical to the <directive module="mod_proxy"
    type="section">Proxy</directive> directive, except it matches URLs
    using <glossary ref="regex">regular expressions</glossary>.</p>
</usage>
<seealso><directive type="section" module="mod_proxy">Proxy</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>ProxyPreserveHost</name>
<description>Use incoming Host HTTP request header for proxy
request</description>
<syntax>ProxyPreserveHost On|Off</syntax>
<default>ProxyPreserveHost Off</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context>
</contextlist>
<compatibility>Available in Apache 2.0.31 and later.</compatibility>

<usage>
    <p>When enabled, this option will pass the Host: line from the incoming
    request to the proxied host, instead of the hostname specified in the
    <directive>ProxyPass</directive> line.</p>

    <p>This option should normally be turned <code>Off</code>. It is mostly 
    useful in special configurations like proxied mass name-based virtual
    hosting, where the original Host header needs to be evaluated by the
    backend server.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyRequests</name>
<description>Enables forward (standard) proxy requests</description>
<syntax>ProxyRequests On|Off</syntax>
<default>ProxyRequests Off</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>This allows or prevents Apache from functioning as a forward proxy
    server. (Setting ProxyRequests to <code>Off</code> does not disable use of
    the <directive module="mod_proxy">ProxyPass</directive> directive.)</p>

    <p>In a typical reverse proxy or gateway configuration, this
    option should be set to
    <code>Off</code>.</p>

    <p>In order to get the functionality of proxying HTTP or FTP sites, you
    need also <module>mod_proxy_http</module> or <module>mod_proxy_ftp</module>
    (or both) present in the server.</p>

    <note type="warning"><title>Warning</title>
      <p>Do not enable proxying with <directive
      module="mod_proxy">ProxyRequests</directive> until you have <a
      href="#access">secured your server</a>.  Open proxy servers are dangerous
      both to your network and to the Internet at large.</p>
    </note>
</usage>
<seealso><a href="#forwardreverse">Forward and Reverse Proxies/Gateways</a></seealso>
</directivesynopsis>

<directivesynopsis>
<name>ProxyRemote</name>
<description>Remote proxy used to handle certain requests</description>
<syntax>ProxyRemote <var>match</var> <var>remote-server</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>This defines remote proxies to this proxy. <var>match</var> is either the
    name of a URL-scheme that the remote server supports, or a partial URL
    for which the remote server should be used, or <code>*</code> to indicate
    the server should be contacted for all requests. <var>remote-server</var> is
    a partial URL for the remote server. Syntax:</p>

    <example>
      <dfn>remote-server</dfn> =
          <var>scheme</var>://<var>hostname</var>[:<var>port</var>]
    </example>

    <p><var>scheme</var> is effectively the protocol that should be used to
    communicate with the remote server; only <code>http</code> is supported by
    this module.</p>

    <example><title>Example</title>
      ProxyRemote http://goodguys.example.com/ http://mirrorguys.example.com:8000<br />
      ProxyRemote * http://cleverproxy.localdomain<br />
      ProxyRemote ftp http://ftpproxy.mydomain:8080
    </example>

    <p>In the last example, the proxy will forward FTP requests, encapsulated
    as yet another HTTP proxy request, to another proxy which can handle
    them.</p>

    <p>This option also supports reverse proxy configuration - a backend
    webserver can be embedded within a virtualhost URL space even if that
    server is hidden by another forward proxy.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyRemoteMatch</name>
<description>Remote proxy used to handle requests matched by regular
expressions</description>
<syntax>ProxyRemoteMatch <var>regex</var> <var>remote-server</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>ProxyRemoteMatch</directive> is identical to the
    <directive module="mod_proxy">ProxyRemote</directive> directive, except the
    first argument is a <glossary ref="regex">regular expression</glossary>
    match against the requested URL.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>BalancerMember</name>
<description>Add a member to a load balancing group</description>
<syntax>BalancerMember [<var>balancerurl</var>] <var>url</var> [<var
  >key=value [key=value ...]]</var></syntax>
<contextlist><context>directory</context>
</contextlist>
<compatibility>BalancerMember is only available in Apache 2.2
  and later.</compatibility>
<usage>
    <p>This directive adds a member to a load balancing group. It could be used
    within a <code>&lt;Proxy <var>balancer://</var>...&gt;</code> container
    directive, and can take any of the key value pair parameters available to
    <directive module="mod_proxy">ProxyPass</directive> directives.</p>
    <p>One additional parameter is available only to <directive
    module="mod_proxy">BalancerMember</directive> directives:
    <var>loadfactor</var>. This is the member load factor - a number between 1 
    (default) and 100, which defines the weighted load to be applied to the 
    member in question.</p>
    <p>The balancerurl is only needed when not in <code>&lt;Proxy <var>balancer://</var>...&gt;</code>
    container directive. It corresponds to the url of a balancer defined in
    <directive module="mod_proxy">ProxyPass</directive> directive.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxySet</name>
<description>Set various Proxy balancer or member parameters</description>
<syntax>ProxySet <var>url</var> <var>key=value [key=value ...]</var></syntax>
<contextlist><context>directory</context>
</contextlist>
<compatibility>ProxySet is only available in Apache 2.2
  and later.</compatibility>
<usage>
    <p>This directive is used as an alternate method of setting any of the
    parameters available to Proxy balancers and workers normally done via the
    <directive module="mod_proxy">ProxyPass</directive> directive. If used
    within a <code>&lt;Proxy <var>balancer url|worker url</var>&gt;</code>
    container directive, the <var>url</var> argument is not required. As a side
    effect the respective balancer or worker gets created. This can be useful
    when doing reverse proxying via a
    <directive module="mod_rewrite">RewriteRule</directive> instead of a
    <directive module="mod_proxy">ProxyPass</directive> directive.</p>

    <example>
      &lt;Proxy balancer://hotcluster&gt;<br />
      <indent>
        BalancerMember http://www2.example.com:8009 loadfactor=1<br />
        BalancerMember http://www3.example.com:8009 loadfactor=2<br />
        ProxySet lbmethod=bytraffic<br />
      </indent>
      &lt;/Proxy&gt;
    </example>

    <example>
      &lt;Proxy http://backend&gt;<br />
      <indent>
        ProxySet keepalive=On<br />
      </indent>
      &lt;/Proxy&gt;
    </example>

    <example>
        ProxySet balancer://foo lbmethod=bytraffic timeout=15
    </example>

    <example>
        ProxySet ajp://backend:7001 timeout=15
    </example>

   <note type="warning"><title>Warning</title>
      <p>Keep in mind that the same parameter key can have a different meaning
      depending whether it is applied to a balancer or a worker as shown by the two
      examples above regarding timeout.</p>
   </note>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyPass</name>
<description>Maps remote servers into the local server URL-space</description>
<syntax>ProxyPass [<var>path</var>] !|<var>url</var> [<var>key=value</var>
  <var>[key=value</var> ...]] [nocanon] [interpolate]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context>
</contextlist>

<usage>
    <p>This directive allows remote servers to be mapped into the
    space of the local server; the local server does not act as a
    proxy in the conventional sense, but appears to be a mirror of the
    remote server. The local server is often called a <dfn>reverse
    proxy</dfn> or <dfn>gateway</dfn>. The <var>path</var> is the name of
    a local virtual path; <var>url</var> is a partial URL for the
    remote server and cannot include a query string.</p>

    <note type="warning">The <directive
    module="mod_proxy">ProxyRequests</directive> directive should
    usually be set <strong>off</strong> when using
    <directive>ProxyPass</directive>.</note>

    <p>Suppose the local server has address <code>http://example.com/</code>;
    then</p>

    <example>
      ProxyPass /mirror/foo/ http://backend.example.com/
    </example>

    <p>will cause a local request for
    <code>http://example.com/mirror/foo/bar</code> to be internally converted
    into a proxy request to <code>http://backend.example.com/bar</code>.</p>

    <note type="warning">
    <p>If the first argument ends with a trailing <strong>/</strong>, the second
       argument should also end with a trailing <strong>/</strong> and vice
       versa. Otherwise the resulting requests to the backend may miss some
       needed slashes and do not deliver the expected results.
    </p>
    </note>

    <p>The <code>!</code> directive is useful in situations where you don't want
    to reverse-proxy a subdirectory, <em>e.g.</em></p>

    <example>
      ProxyPass /mirror/foo/i !<br />
      ProxyPass /mirror/foo http://backend.example.com
    </example>

    <p>will proxy all requests to <code>/mirror/foo</code> to
    <code>backend.example.com</code> <em>except</em> requests made to
    <code>/mirror/foo/i</code>.</p>

    <note><title>Note</title>
      <p>Order is important: exclusions must come <em>before</em> the
      general <directive>ProxyPass</directive> directive.</p>
    </note>

    <p>As of Apache 2.1, the ability to use pooled connections to a backend
    server is available. Using the <code>key=value</code> parameters it is
    possible to tune this connection pooling. The default for a <code>Hard
    Maximum</code> for the number of connections is the number of threads per
    process in the active MPM. In the Prefork MPM, this is always 1, while with
    the Worker MPM it is controlled by the
    <directive>ThreadsPerChild</directive>.</p>

    <p>Setting <code>min</code> will determine how many connections will always 
    be open to the backend server. Upto the Soft Maximum or <code>smax</code> 
    number of connections will be created on demand. Any connections above 
    <code>smax</code> are subject to a time to live or <code>ttl</code>.  Apache
    will never create more than the Hard Maximum or <code>max</code> connections
    to the backend server.</p>

    <example>
        ProxyPass /example http://backend.example.com smax=5 max=20 ttl=120 retry=300
    </example>

    <table>
    <tr><th>Parameter</th>
        <th>Default</th>
        <th>Description</th></tr>
    <tr><td>min</td>
        <td>0</td>
        <td>Minimum number of connections that will always
            be open to the backend server.</td></tr>
    <tr><td>max</td>
        <td>1...n</td>
        <td>Hard Maximum number of connections that will be
    allowed to the backend server. The default for a Hard Maximum
    for the number of connections is the number of threads per process in the 
    active MPM. In the Prefork MPM, this is always 1, while with the Worker MPM
    it is controlled by the <directive>ThreadsPerChild</directive>.
    Apache will never create more than the Hard Maximum connections
    to the backend server.</td></tr>
    <tr><td>smax</td>
        <td>max</td>
        <td>Upto the Soft Maximum
    number of connections will be created on demand. Any connections above 
    <code>smax</code> are subject to a time to live or <code>ttl</code>.
    </td></tr>
    <tr><td>acquire</td>
        <td>-</td>
        <td>If set this will be the maximum time to wait for a free
    connection in the connection pool, in milliseconds. If there are no free
    connections in the pool the Apache will return <code>SERVER_BUSY</code>
    status to the client.
    </td></tr>
    <tr><td>connectiontimeout</td>
        <td>timeout</td>
        <td>Connect timeout in seconds.
        The number of seconds Apache waits for the creation of a connection to
        the backend to complete. By adding a postfix of ms the timeout can be
        also set in milliseconds.
    </td></tr>
    <tr><td>disablereuse</td>
        <td>Off</td>
        <td>This parameter should be used when you want to force mod_proxy
    to immediately close a connection to the backend after being used, and
    thus, disable its persistent connection and pool for that backend.
    This helps in various situations where a firewall between Apache and
    the backend server (regardless of protocol) tends to silently
    drop connections or when backends themselves may be under round-
    robin DNS. To disable connection pooling reuse,
    set this property value to <code>On</code>. 
    </td></tr>
    <tr><td>flushpackets</td>
        <td>off</td>
        <td>Determines whether the proxy module will auto-flush the output
        brigade after each "chunk" of data. 'off' means that it will flush
        only when needed, 'on' means after each chunk is sent and
        'auto' means poll/wait for a period of time and flush if
        no input has been received for 'flushwait' milliseconds.
        Currently this is in effect only for AJP.
    </td></tr>
    <tr><td>flushwait</td>
        <td>10</td>
        <td>The time to wait for additional input, in milliseconds, before
        flushing the output brigade if 'flushpackets' is 'auto'.
    </td></tr>
    <tr><td>iobuffersize</td>
        <td>8192</td>
        <td>Adjusts the size of the internal scratchpad IO buffer. This allows you
        to override the <directive>ProxyIOBufferSize</directive> for a specific worker.
        This must be at least 512 or set to 0 for the system default of 8192.
    </td></tr>
    <tr><td>keepalive</td>
        <td>Off</td>
        <td>This parameter should be used when you have a firewall between your
    Apache and the backend server, who tend to drop inactive connections.
    This flag will tell the Operating System to send <code>KEEP_ALIVE</code>
    messages on inactive connections (interval depends on global OS settings,
    generally 120ms), and thus prevent the firewall to drop the connection.
    To enable keepalive set this property value to <code>On</code>. 
    </td></tr>
    <tr><td>lbset</td>
        <td>0</td>
        <td>Sets the load balancer cluster set that the worker is a member
         of. The load balancer will try all members of a lower numbered
         lbset before trying higher numbered ones.
    </td></tr>
    <tr><td>ping</td>
        <td>0</td>
        <td>Ping property tells webserver to send a <code>CPING</code>
        request on ajp13 connection before forwarding a request.
        The parameter is the delay in seconds to wait for the
        <code>CPONG</code> reply.
        This features has been added to avoid problem with hung and
        busy Tomcat's and require ajp13 ping/pong support which has
        been implemented on Tomcat 3.3.2+, 4.1.28+ and 5.0.13+.
        This will increase the network traffic during the normal operation
        which could be an issue, but it will lower the
        traffic in case some of the cluster nodes are down or busy.
        Currently this has an effect only for AJP.
        By adding a postfix of ms the delay can be also set in
        milliseconds.
    </td></tr>
    <tr><td>receivebuffersize</td>
        <td>0</td>
        <td>Adjusts the size of the explicit (TCP/IP) network buffer size for
        proxied connections. This allows you to override the
        <directive>ProxyReceiveBufferSize</directive> for a specific worker.
        This must be at least 512 or set to 0 for the system default.
    </td></tr>
    <tr><td>redirect</td>
        <td>-</td>
        <td>Redirection Route of the worker. This value is usually
        set dynamically to enable safe removal of the node from
        the cluster. If set all requests without session id will be
        redirected to the BalancerMember that has route parametar
        equal as this value.
    </td></tr>
    <tr><td>retry</td>
        <td>60</td>
        <td>Connection pool worker retry timeout in seconds.
    If the connection pool worker to the backend server is in the error state,
    Apache will not forward any requests to that server until the timeout
    expires. This enables to shut down the backend server for maintenance,
    and bring it back online later. A value of 0 means always retry workers
    in an error state with no timeout.
    </td></tr>
    <tr><td>route</td>
        <td>-</td>
        <td>Route of the worker when used inside load balancer.
        The route is a value appended to session id.
    </td></tr>
    <tr><td>status</td>
        <td>-</td>
        <td>Single letter value defining the initial status of
        this worker: 'D' is disabled, 'S' is stopped, 'I' is ignore-errors,
        'H' is hot-standby and 'E' is in an error state. Status 
        can be set (which is the default) by prepending with '+' or 
        cleared by prepending with '-'.
        Thus, a setting of 'S-E' sets this worker to Stopped and
        clears the in-error flag.
    </td></tr>
    <tr><td>timeout</td>
        <td><directive module="mod_proxy">ProxyTimeout</directive></td>
        <td>Connection timeout in seconds.
        The number of seconds Apache waits for data sent by / to the backend.
    </td></tr>
    <tr><td>ttl</td>
        <td>-</td>
        <td>Time To Live for the inactive connections above the
        <code>smax</code> connections in seconds. Apache will close all
        connections that has not been used inside that time period.
    </td></tr>

    </table>

    <p>If the Proxy directive scheme starts with the
    <code>balancer://</code> (eg: <code>balancer://cluster/</code>, 
    any path information is ignored)  then a virtual worker that does not really
    communicate with the backend server will be created. Instead it is responsible
    for the management of several "real" workers. In that case the special set of
    parameters can be add to this virtual worker. See <module>mod_proxy_balancer</module>
    for more information about how the balancer works.
    </p>
    <table>
    <tr><th>Parameter</th>
        <th>Default</th>
        <th>Description</th></tr>
    <tr><td>lbmethod</td>
        <td>byrequests</td>
        <td>Balancer load-balance method. Select the load-balancing scheduler
        method to use. Either <code>byrequests</code>, to perform weighted
        request counting, <code>bytraffic</code>, to perform weighted
        traffic byte count balancing, or <code>bybusyness</code>, to perform 
        pending request balancing. Default is <code>byrequests</code>.
    </td></tr>
    <tr><td>maxattempts</td>
        <td>1</td>
        <td>Maximum number of failover attempts before giving up. 
    </td></tr>
    <tr><td>nofailover</td>
        <td>Off</td>
        <td>If set to <code>On</code> the session will break if the worker is in
        error state or disabled. Set this value to On if backend servers do not
        support session replication.
    </td></tr>
    <tr><td>stickysession</td>
        <td>-</td>
        <td>Balancer sticky session name. The value is usually set to something
        like <code>JSESSIONID</code> or <code>PHPSESSIONID</code>,
        and it depends on the backend application server that support sessions.
        If the backend application server uses different name for cookies
        and url encoded id (like servlet containers) use | to to separate them.
        The first part is for the cookie the second for the path.
    </td></tr>
    <tr><td>scolonpathdelim</td>
        <td>Off</td>
        <td>If set to <code>On</code> the semi-colon character ';' will be
        used as an additional sticky session path deliminator/separator. This
        is mainly used to emulate mod_jk's behavior when dealing with paths such
        as <code>JSESSIONID=6736bcf34;foo=aabfa</code>
    </td></tr>
    <tr><td>timeout</td>
        <td>0</td>
        <td>Balancer timeout in seconds. If set this will be the maximum time
        to wait for a free worker. Default is not to wait. 
    </td></tr>
    
    </table>
    <p>A sample balancer setup</p>
    <example>
      ProxyPass /special-area http://special.example.com smax=5 max=10<br />
      ProxyPass / balancer://mycluster/ stickysession=JSESSIONID|jsessionid nofailover=On<br />
      &lt;Proxy balancer://mycluster&gt;<br />
      <indent>
        BalancerMember http://1.2.3.4:8009<br />
        BalancerMember http://1.2.3.5:8009 smax=10<br />
        # Less powerful server, don't send as many requests there<br />
        BalancerMember http://1.2.3.6:8009 smax=1 loadfactor=20<br />
      </indent>
      &lt;/Proxy&gt;
    </example>

    <p>Setting up a hot-standby, that will only be used if no other
     members are available</p>
    <example>
      ProxyPass / balancer://hotcluster/ <br />
      &lt;Proxy balancer://hotcluster&gt;<br />
      <indent>
        BalancerMember http://1.2.3.4:8009 loadfactor=1<br />
        BalancerMember http://1.2.3.5:8009 loadfactor=2<br />
        # The below is the hot standby<br />
        BalancerMember http://1.2.3.6:8009 status=+H<br />
        ProxySet lbmethod=bytraffic
      </indent>
      &lt;/Proxy&gt;
    </example>

    <p>Normally, mod_proxy will canonicalise ProxyPassed URLs.
    But this may be incompatible with some backends, particularly those
    that make use of <var>PATH_INFO</var>.  The optional <var>nocanon</var>
    keyword suppresses this, and passes the URL path "raw" to the
    backend.  Note that may affect the security of your backend, as it
    removes the normal limited protection against URL-based attacks
    provided by the proxy.</p>

    <p>The optional <var>interpolate</var> keyword (available in
    httpd 2.2.9 and later), in combination with
    <directive>ProxyPassInterpolateEnv</directive> causes the ProxyPass
    to interpolate environment variables, using the syntax
    <var>${VARNAME}</var>.  Note that many of the standard CGI-derived
    environment variables will not exist when this interpolation happens,
    so you may still have to resort to <module>mod_rewrite</module>
    for complex rules.</p>

    <p>When used inside a <directive type="section" module="core"
    >Location</directive> section, the first argument is omitted and the local
    directory is obtained from the <directive type="section" module="core"
    >Location</directive>.</p>

    <p>If you require a more flexible reverse-proxy configuration, see the
    <directive module="mod_rewrite">RewriteRule</directive> directive with the
    <code>[P]</code> flag.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyPassMatch</name>
<description>Maps remote servers into the local server URL-space using regular expressions</description>
<syntax>ProxyPassMatch [<var>regex</var>] !|<var>url</var> [<var>key=value</var>
	<var>[key=value</var> ...]]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context>
</contextlist>

<usage>
    <p>This directive is equivalent to <directive module="mod_proxy">ProxyPass</directive>,
       but makes use of regular expressions, instead of simple prefix matching. The
       supplied regular expression is matched against the <var>url</var>, and if it
       matches, the server will substitute any parenthesized matches into the given
       string and use it as a new <var>url</var>.</p>

    <p>Suppose the local server has address <code>http://example.com/</code>;
    then</p>

    <example>
      ProxyPassMatch ^(/.*\.gif)$ http://backend.example.com$1
    </example>

    <p>will cause a local request for
    <code>http://example.com/foo/bar.gif</code> to be internally converted
    into a proxy request to <code>http://backend.example.com/foo/bar.gif</code>.</p>
    <note><title>Note</title>
      <p>The URL argument must be parsable as a URL <em>before</em> regexp
      substitutions (as well as after).  This limits the matches you can use.
      For instance, if we had used</p>
      <example>
        ProxyPassMatch ^(/.*\.gif)$ http://backend.example.com:8000$1
      </example>
      <p>in our previous example, it would fail with a syntax error
      at server startup.  This is a bug (PR 46665 in the ASF bugzilla),
      and the workaround is to reformulate the match:</p>
      <example>
        ProxyPassMatch ^/(.*\.gif)$ http://backend.example.com:8000/$1
      </example>
    </note>
    <p>The <code>!</code> directive is useful in situations where you don't want
    to reverse-proxy a subdirectory.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyPassReverse</name>
<description>Adjusts the URL in HTTP response headers sent from a reverse
proxied server</description>
<syntax>ProxyPassReverse [<var>path</var>] <var>url</var>
[<var>interpolate</var>]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context>
</contextlist>

<usage>
    <p>This directive lets Apache adjust the URL in the <code>Location</code>,
    <code>Content-Location</code> and <code>URI</code> headers on HTTP
    redirect responses. This is essential when Apache is used as a
    reverse proxy (or gateway) to avoid by-passing the reverse proxy
    because of HTTP redirects on the backend servers which stay behind
    the reverse proxy.</p>

    <p>Only the HTTP response headers specifically mentioned above
    will be rewritten.  Apache will not rewrite other response
    headers, nor will it rewrite URL references inside HTML pages.
    This means that if the proxied content contains absolute URL
    references, they will by-pass the proxy.  A third-party module
    that will look inside the HTML and rewrite URL references is Nick
    Kew's <a href="http://apache.webthing.com/mod_proxy_html/"
    >mod_proxy_html</a>.</p>

    <p><var>path</var> is the name of a local virtual path. <var>url</var> is a
    partial URL for the remote server - the same way they are used for the
    <directive module="mod_proxy">ProxyPass</directive> directive.</p>

    <p>For example, suppose the local server has address
    <code>http://example.com/</code>; then</p>

    <example>
      ProxyPass         /mirror/foo/ http://backend.example.com/<br />
      ProxyPassReverse  /mirror/foo/ http://backend.example.com/<br />
      ProxyPassReverseCookieDomain  backend.example.com  public.example.com<br />
      ProxyPassReverseCookiePath  /  /mirror/foo/
    </example>

    <p>will not only cause a local request for the
    <code>http://example.com/mirror/foo/bar</code> to be internally converted
    into a proxy request to <code>http://backend.example.com/bar</code>
    (the functionality <code>ProxyPass</code> provides here). It also takes care
    of redirects the server <code>backend.example.com</code> sends: when
    <code>http://backend.example.com/bar</code> is redirected by him to
    <code>http://backend.example.com/quux</code> Apache adjusts this to
    <code>http://example.com/mirror/foo/quux</code> before forwarding the HTTP
    redirect response to the client. Note that the hostname used for
    constructing the URL is chosen in respect to the setting of the <directive
    module="core">UseCanonicalName</directive> directive.</p>

    <p>Note that this <directive>ProxyPassReverse</directive> directive can
    also be used in conjunction with the proxy pass-through feature
    (<code>RewriteRule ...  [P]</code>) from <module>mod_rewrite</module>
    because it doesn't depend on a corresponding <directive module="mod_proxy"
    >ProxyPass</directive> directive.</p>

    <p>The optional <var>interpolate</var> keyword (available in
    httpd 2.2.9 and later), used together with
    <directive>ProxyPassInterpolateEnv</directive>, enables interpolation
    of environment variables specified using the format <var>${VARNAME}</var>.
    </p>

    <p>When used inside a <directive type="section" module="core"
    >Location</directive> section, the first argument is omitted and the local
    directory is obtained from the <directive type="section" module="core"
    >Location</directive>.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyPassReverseCookieDomain</name>
<description>Adjusts the Domain string in Set-Cookie headers from a reverse-
proxied server</description>
<syntax>ProxyPassReverseCookieDomain <var>internal-domain</var>
<var>public-domain</var> [<var>interpolate</var>]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context>
</contextlist>
<usage>
<p>Usage is basically similar to
<directive module="mod_proxy">ProxyPassReverse</directive>, but instead of
rewriting headers that are a URL, this rewrites the <code>domain</code>
string in <code>Set-Cookie</code> headers.</p>
</usage>
</directivesynopsis>
<directivesynopsis>
<name>ProxyPassReverseCookiePath</name>
<description>Adjusts the Path string in Set-Cookie headers from a reverse-
proxied server</description>
<syntax>ProxyPassReverseCookiePath <var>internal-path</var>
<var>public-path</var> [<var>interpolate</var>]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context>
</contextlist>
<usage>
<p>Usage is basically similar to
<directive module="mod_proxy">ProxyPassReverse</directive>, but instead of
rewriting headers that are a URL, this rewrites the <code>path</code>
string in <code>Set-Cookie</code> headers.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyBlock</name>
<description>Words, hosts, or domains that are banned from being
proxied</description>
<syntax>ProxyBlock *|<var>word</var>|<var>host</var>|<var>domain</var>
[<var>word</var>|<var>host</var>|<var>domain</var>] ...</syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>ProxyBlock</directive> directive specifies a list of
    words, hosts and/or domains, separated by spaces.  HTTP, HTTPS, and
    FTP document requests to sites whose names contain matched words,
    hosts or domains are <em>blocked</em> by the proxy server. The proxy
    module will also attempt to determine IP addresses of list items which
    may be hostnames during startup, and cache them for match test as
    well. That may slow down the startup time of the server.</p>

    <example><title>Example</title>
      ProxyBlock joes-garage.com some-host.co.uk rocky.wotsamattau.edu
    </example>

    <p><code>rocky.wotsamattau.edu</code> would also be matched if referenced by
    IP address.</p>

    <p>Note that <code>wotsamattau</code> would also be sufficient to match
    <code>wotsamattau.edu</code>.</p>

    <p>Note also that</p>

    <example>
      ProxyBlock *
    </example>

    <p>blocks connections to all sites.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyReceiveBufferSize</name>
<description>Network buffer size for proxied HTTP and FTP
connections</description>
<syntax>ProxyReceiveBufferSize <var>bytes</var></syntax>
<default>ProxyReceiveBufferSize 0</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>ProxyReceiveBufferSize</directive> directive specifies an
    explicit (TCP/IP) network buffer size for proxied HTTP and FTP connections,
    for increased throughput. It has to be greater than <code>512</code> or set
    to <code>0</code> to indicate that the system's default buffer size should
    be used.</p>

    <example><title>Example</title>
      ProxyReceiveBufferSize 2048
    </example>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyIOBufferSize</name>
<description>Determine size of internal data throughput buffer</description>
<syntax>ProxyIOBufferSize <var>bytes</var></syntax>
<default>ProxyIOBufferSize 8192</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>ProxyIOBufferSize</directive> directive adjusts the size
    of the internal buffer, which is used as a scratchpad for the data between
    input and output. The size must be at least <code>512</code>.</p>

    <p>In almost every case there's no reason to change that value.</p>
    <p>If used with AJP this directive sets the maximum AJP packet size in
    bytes. If you change it from the default, you must also change the 
    <code>packetSize</code> attribute of your AJP connector on the
    Tomcat side! The attribute <code>packetSize</code> is only available
    in Tomcat <code>5.5.20+</code> and <code>6.0.2+</code></p>
    <p>Normally it is not necessary to change the maximum packet size.
    Problems with the default value have been reported when sending
    certificates or certificate chains.</p>
    
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyMaxForwards</name>
<description>Maximium number of proxies that a request can be forwarded
through</description>
<syntax>ProxyMaxForwards <var>number</var></syntax>
<default>ProxyMaxForwards -1</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<compatibility>Available in Apache 2.0 and later;
	default behaviour changed in 2.2.7/2.3</compatibility>

<usage>
    <p>The <directive>ProxyMaxForwards</directive> directive specifies the
    maximum number of proxies through which a request may pass, if there's no
    <code>Max-Forwards</code> header supplied with the request. This may
    be set to prevent infinite proxy loops, or a DoS attack.</p>

    <example><title>Example</title>
      ProxyMaxForwards 15
    </example>

    <p>Note that setting <directive>ProxyMaxForwards</directive> is a
    violation of the HTTP/1.1 protocol (RFC2616), which forbids a Proxy
    setting <code>Max-Forwards</code> if the Client didn't set it.
    Earlier Apache versions would always set it.  A negative
    <directive>ProxyMaxForwards</directive> value, including the
    default -1, gives you protocol-compliant behaviour, but may
    leave you open to loops.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>NoProxy</name>
<description>Hosts, domains, or networks that will be connected to
directly</description>
<syntax>NoProxy <var>host</var> [<var>host</var>] ...</syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>This directive is only useful for Apache proxy servers within
    intranets.  The <directive>NoProxy</directive> directive specifies a
    list of subnets, IP addresses, hosts and/or domains, separated by
    spaces. A request to a host which matches one or more of these is
    always served directly, without forwarding to the configured
    <directive module="mod_proxy">ProxyRemote</directive> proxy server(s).</p>

    <example><title>Example</title>
      ProxyRemote  *  http://firewall.example.com:81<br />
      NoProxy         .example.com 192.168.112.0/21
    </example>

    <p>The <var>host</var> arguments to the <directive>NoProxy</directive>
    directive are one of the following type list:</p>

    <dl>
    <!-- ===================== Domain ======================= -->
    <dt><var><a name="domain" id="domain">Domain</a></var></dt>
    <dd>
    <p>A <dfn>Domain</dfn> is a partially qualified DNS domain name, preceded
    by a period. It represents a list of hosts which logically belong to the
    same DNS domain or zone (<em>i.e.</em>, the suffixes of the hostnames are
    all ending in <var>Domain</var>).</p>

    <example><title>Examples</title>
      .com .apache.org.
    </example>

    <p>To distinguish <var>Domain</var>s from <var><a href="#hostname"
    >Hostname</a></var>s (both syntactically and semantically; a DNS domain can
    have a DNS A record, too!), <var>Domain</var>s are always written with a
    leading period.</p>
    
    <note><title>Note</title>
      <p>Domain name comparisons are done without regard to the case, and
      <var>Domain</var>s are always assumed to be anchored in the root of the
      DNS tree, therefore two domains <code>.ExAmple.com</code> and
      <code>.example.com.</code> (note the trailing period) are considered
      equal. Since a domain comparison does not involve a DNS lookup, it is much
      more efficient than subnet comparison.</p>
    </note></dd>

    <!-- ===================== SubNet ======================= -->
    <dt><var><a name="subnet" id="subnet">SubNet</a></var></dt>
    <dd>
    <p>A <dfn>SubNet</dfn> is a partially qualified internet address in
    numeric (dotted quad) form, optionally followed by a slash and the netmask,
    specified as the number of significant bits in the <var>SubNet</var>. It is
    used to represent a subnet of hosts which can be reached over a common
    network interface. In the absence of the explicit net mask it is assumed
    that omitted (or zero valued) trailing digits specify the mask. (In this
    case, the netmask can only be multiples of 8 bits wide.) Examples:</p>

    <dl>
    <dt><code>192.168</code> or <code>192.168.0.0</code></dt>
    <dd>the subnet 192.168.0.0 with an implied netmask of 16 valid bits
    (sometimes used in the netmask form <code>255.255.0.0</code>)</dd>
    <dt><code>192.168.112.0/21</code></dt>
    <dd>the subnet <code>192.168.112.0/21</code> with a netmask of 21
    valid bits (also used in the form <code>255.255.248.0</code>)</dd>
    </dl>

    <p>As a degenerate case, a <em>SubNet</em> with 32 valid bits is the
    equivalent to an <var><a href="#ipadr">IPAddr</a></var>, while a <var>SubNet</var> with zero
    valid bits (<em>e.g.</em>, 0.0.0.0/0) is the same as the constant
    <var>_Default_</var>, matching any IP address.</p></dd>

    <!-- ===================== IPAddr ======================= -->
    <dt><var><a name="ipaddr" id="ipaddr">IPAddr</a></var></dt>
    <dd>
    <p>A <dfn>IPAddr</dfn> represents a fully qualified internet address in
    numeric (dotted quad) form. Usually, this address represents a host, but
    there need not necessarily be a DNS domain name connected with the
    address.</p>
    <example><title>Example</title>
      192.168.123.7
    </example>
    
    <note><title>Note</title>
      <p>An <var>IPAddr</var> does not need to be resolved by the DNS system, so
      it can result in more effective apache performance.</p>
    </note></dd>

    <!-- ===================== Hostname ======================= -->
    <dt><var><a name="hostname" id="hostname">Hostname</a></var></dt>
    <dd>
    <p>A <dfn>Hostname</dfn> is a fully qualified DNS domain name which can
    be resolved to one or more <var><a href="#ipaddr">IPAddrs</a></var> via the
    DNS domain name service. It represents a logical host (in contrast to
    <var><a href="#domain">Domain</a></var>s, see above) and must be resolvable
    to at least one <var><a href="#ipaddr">IPAddr</a></var> (or often to a list
    of hosts with different <var><a href="#ipaddr">IPAddr</a></var>s).</p>

    <example><title>Examples</title>
      prep.ai.example.com<br />
      www.apache.org
    </example>

    <note><title>Note</title>
      <p>In many situations, it is more effective to specify an <var><a
      href="#ipaddr">IPAddr</a></var> in place of a <var>Hostname</var> since a
      DNS lookup can be avoided. Name resolution in Apache can take a remarkable
      deal of time when the connection to the name server uses a slow PPP
      link.</p>
      <p><var>Hostname</var> comparisons are done without regard to the case,
      and <var>Hostname</var>s are always assumed to be anchored in the root
      of the DNS tree, therefore two hosts <code>WWW.ExAmple.com</code>
      and <code>www.example.com.</code> (note the trailing period) are
      considered equal.</p>
     </note></dd>
    </dl>
</usage>
<seealso><a href="../dns-caveats.html">DNS Issues</a></seealso>
</directivesynopsis>

<directivesynopsis>
<name>ProxyTimeout</name>
<description>Network timeout for proxied requests</description>
<syntax>ProxyTimeout <var>seconds</var></syntax>
<default>Value of <directive module="core">Timeout</directive></default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<compatibility>Available in Apache 2.0.31 and later</compatibility>

<usage>
    <p>This directive allows a user to specifiy a timeout on proxy requests.
    This is useful when you have a slow/buggy appserver which hangs, and you
    would rather just return a timeout and fail gracefully instead of waiting
    however long it takes the server to return.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyDomain</name>
<description>Default domain name for proxied requests</description>
<syntax>ProxyDomain <var>Domain</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>This directive is only useful for Apache proxy servers within
    intranets. The <directive>ProxyDomain</directive> directive specifies
    the default domain which the apache proxy server will belong to. If a
    request to a host without a domain name is encountered, a redirection
    response to the same host with the configured <var>Domain</var> appended
    will be generated.</p>

    <example><title>Example</title>
      ProxyRemote  *  http://firewall.example.com:81<br />
      NoProxy         .example.com 192.168.112.0/21<br />
      ProxyDomain     .example.com
    </example>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyVia</name>
<description>Information provided in the <code>Via</code> HTTP response
header for proxied requests</description>
<syntax>ProxyVia On|Off|Full|Block</syntax>
<default>ProxyVia Off</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>This directive controls the use of the <code>Via:</code> HTTP
    header by the proxy. Its intended use is to control the flow of
    proxy requests along a chain of proxy servers.  See <a
    href="http://www.ietf.org/rfc/rfc2616.txt">RFC 2616</a> (HTTP/1.1), section
    14.45 for an explanation of <code>Via:</code> header lines.</p>

    <ul>
    <li>If set to <code>Off</code>, which is the default, no special processing
    is performed. If a request or reply contains a <code>Via:</code> header,
    it is passed through unchanged.</li>

    <li>If set to <code>On</code>, each request and reply will get a
    <code>Via:</code> header line added for the current host.</li>

    <li>If set to <code>Full</code>, each generated <code>Via:</code> header
    line will additionally have the Apache server version shown as a
    <code>Via:</code> comment field.</li>

    <li>If set to <code>Block</code>, every proxy request will have all its
    <code>Via:</code> header lines removed. No new <code>Via:</code> header will
    be generated.</li>
    </ul>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyErrorOverride</name>
<description>Override error pages for proxied content</description>
<syntax>ProxyErrorOverride On|Off</syntax>
<default>ProxyErrorOverride Off</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<compatibility>Available in version 2.0 and later</compatibility>

<usage>
    <p>This directive is useful for reverse-proxy setups, where you want to 
    have a common look and feel on the error pages seen by the end user. 
    This also allows for included files (via
    <module>mod_include</module>'s SSI) to get
    the error code and act accordingly (default behavior would display
    the error page of the proxied server, turning this on shows the SSI
    Error message).</p>

    <p>This directive does not affect the processing of informational (1xx),
    normal success (2xx), or redirect (3xx) responses.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyPassInterpolateEnv</name>
<description>Enable Environment Variable interpolation in Reverse Proxy configurations</description>
<syntax>ProxyPassInterpolateEnv On|Off</syntax>
<default>ProxyPassInterpolateEnv Off</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
</contextlist>
<compatibility>Available in httpd 2.2.9 and later</compatibility>

<usage>
    <p>This directive, together with the <var>interpolate</var> argument to
    <directive>ProxyPass</directive>, <directive>ProxyPassReverse</directive>,
    <directive>ProxyPassReverseCookieDomain</directive> and
    <directive>ProxyPassReverseCookiePath</directive>
    enables reverse proxies to be dynamically
    configured using environment variables, which may be set by
    another module such as <module>mod_rewrite</module>.
    It affects the <directive>ProxyPass</directive>,
    <directive>ProxyPassReverse</directive>,
    <directive>ProxyPassReverseCookieDomain</directive>, and
    <directive>ProxyPassReverseCookiePath</directive> directives,
    and causes them to substitute the value of an environment
    variable <code>varname</code> for the string <code>${varname}</code>
    in configuration directives.</p>
    <p>Keep this turned off (for server performance) unless you need it!</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyStatus</name>
<description>Show Proxy LoadBalancer status in mod_status</description>
<syntax>ProxyStatus Off|On|Full</syntax>
<default>ProxyStatus Off</default>
<contextlist><context>server config</context>
<context>virtual host</context>
</contextlist>
<compatibility>Available in version 2.2 and later</compatibility>

<usage>
    <p>This directive determines whether or not proxy
    loadbalancer status data is displayed via the <module>mod_status</module>
    server-status page.</p>
    <note><title>Note</title>
      <p><strong>Full</strong> is synonymous with <strong>On</strong></p>
    </note>

</usage>
</directivesynopsis>

</modulesynopsis>