Documentation rebuild

[apache] / docs / manual / sections.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "./style/manualpage.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.
-->

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

<title>Configuration Sections</title>

<summary> <p>Directives in the <a
href="configuring.html">configuration files</a> may apply to the
entire server, or they may be restricted to apply only to particular
directories, files, hosts, or URLs.  This document describes how to
use configuration section containers or <code>.htaccess</code> files
to change the scope of other configuration directives.</p>
</summary>

<section id="types"><title>Types of Configuration Section Containers</title>

<related>
<modulelist>
<module>core</module>
<module>mod_version</module>
<module>mod_proxy</module>
</modulelist>
<directivelist>
<directive type="section" module="core">Directory</directive>
<directive type="section" module="core">DirectoryMatch</directive>
<directive type="section" module="core">Files</directive>
<directive type="section" module="core">FilesMatch</directive>
<directive type="section" module="core">If</directive>
<directive type="section" module="core">IfDefine</directive>
<directive type="section" module="core">IfModule</directive>
<directive type="section" module="mod_version">IfVersion</directive>
<directive type="section" module="core">Location</directive>
<directive type="section" module="core">LocationMatch</directive>
<directive type="section" module="md">MangedDomain</directive>
<directive type="section" module="mod_proxy">Proxy</directive>
<directive type="section" module="mod_proxy">ProxyMatch</directive>
<directive type="section" module="ssl">SSLPolicy</directive>
<directive type="section" module="core">VirtualHost</directive>
</directivelist>
</related>

<p>There are two basic types of containers.  Most containers are
evaluated for each request.  The enclosed directives are applied only
for those requests that match the containers.  The <directive
type="section" module="core">IfDefine</directive>, <directive
type="section" module="core">IfModule</directive>, and
<directive type="section" module="mod_version">IfVersion</directive>
containers, on the other hand, are evaluated only at server startup
and restart.  If their conditions are true at startup, then the
enclosed directives will apply to all requests.  If the conditions are
not true, the enclosed directives will be ignored.</p>

<p>The <directive type="section" module="core">IfDefine</directive> directive
encloses directives that will only be applied if an appropriate
parameter is defined on the <program>httpd</program> command line.  For example,
with the following configuration, all requests will be redirected
to another site only if the server is started using
<code>httpd -DClosedForNow</code>:</p>

<highlight language="config">
&lt;IfDefine ClosedForNow&gt;
    Redirect "/" "http://otherserver.example.com/"
&lt;/IfDefine&gt;
</highlight>

<p>The <directive type="section" module="core">IfModule</directive>
directive is very similar, except it encloses directives that will
only be applied if a particular module is available in the server.
The module must either be statically compiled in the server, or it
must be dynamically compiled and its <directive
module="mod_so">LoadModule</directive> line must be earlier in the
configuration file.  This directive should only be used if you need
your configuration file to work whether or not certain modules are
installed.  It should not be used to enclose directives that you want
to work all the time, because it can suppress useful error messages
about missing modules.</p>

<p>In the following example, the <directive
module="mod_mime_magic">MimeMagicFile</directive> directive will be
applied only if <module>mod_mime_magic</module> is available.</p>

<highlight language="config">
&lt;IfModule mod_mime_magic.c&gt;
    MimeMagicFile conf/magic
&lt;/IfModule&gt;
</highlight>

<p>The <directive type="section" module="mod_version">IfVersion</directive>
directive is very similar to <directive type="section"
module="core">IfDefine</directive> and <directive type="section"
module="core">IfModule</directive>, except it encloses directives that will
only be applied if a particular version of the server is executing.  This
module is designed for the use in test suites and large networks which have to
deal with different httpd versions and different configurations.</p>

<highlight language="config">
&lt;IfVersion >= 2.4&gt;
    # this happens only in versions greater or
    # equal 2.4.0.
&lt;/IfVersion&gt;
</highlight>

<p><directive type="section" module="core">IfDefine</directive>,
<directive type="section" module="core">IfModule</directive>, and the
<directive type="section" module="mod_version">IfVersion</directive>
can apply negative conditions by preceding their test with "!".
Also, these sections can be nested to achieve more complex
restrictions.</p>
</section>

<section id="file-and-web"><title>Filesystem, Webspace, and Boolean Expressions</title>

<p>The most commonly used configuration section containers are the
ones that change the configuration of particular places in the
filesystem or webspace.  First, it is important to understand the
difference between the two.  The filesystem is the view of your disks
as seen by your operating system.  For example, in a default install,
Apache httpd resides at <code>/usr/local/apache2</code> in the Unix
filesystem or <code>"c:/Program Files/Apache Group/Apache2"</code> in
the Windows filesystem.  (Note that forward slashes should always be
used as the path separator in Apache httpd configuration files, even for Windows.)  In contrast,
the webspace is the view of your site as delivered by the web server
and seen by the client.  So the path <code>/dir/</code> in the
webspace corresponds to the path
<code>/usr/local/apache2/htdocs/dir/</code> in the filesystem of a
default Apache httpd install on Unix.  The webspace need not map directly to
the filesystem, since webpages may be generated dynamically
from databases or other locations.</p>

<section id="filesystem"><title>Filesystem Containers</title>

<p>The <directive type="section" module="core">Directory</directive>
and <directive type="section" module="core">Files</directive>
directives, along with their <glossary ref="regex">regex</glossary>
counterparts, apply directives to
parts of the filesystem.  Directives enclosed in a <directive
type="section" module="core">Directory</directive> section apply to
the named filesystem directory and all subdirectories of that
directory (as well as the files in those directories).
The same effect can be obtained using <a
href="howto/htaccess.html">.htaccess files</a>.  For example, in the
following configuration, directory indexes will be enabled for the
<code>/var/web/dir1</code> directory and all subdirectories.</p>

<highlight language="config">
&lt;Directory "/var/web/dir1"&gt;
    Options +Indexes
&lt;/Directory&gt;
</highlight>

<p>Directives enclosed in a <directive type="section"
module="core">Files</directive> section apply to any file with
the specified name, regardless of what directory it lies in.
So for example, the following configuration directives will,
when placed in the main section of the configuration file,
deny access to any file named <code>private.html</code> regardless
of where it is found.</p>

<highlight language="config">
&lt;Files "private.html"&gt;
    Require all denied
&lt;/Files&gt;
</highlight>

<p>To address files found in a particular part of the filesystem, the
<directive type="section" module="core">Files</directive> and
<directive type="section" module="core">Directory</directive> sections
can be combined.  For example, the following configuration will deny
access to <code>/var/web/dir1/private.html</code>,
<code>/var/web/dir1/subdir2/private.html</code>,
<code>/var/web/dir1/subdir3/private.html</code>, and any other instance
of <code>private.html</code> found under the <code>/var/web/dir1/</code>
directory.</p>

<highlight language="config">
&lt;Directory "/var/web/dir1"&gt;
    &lt;Files "private.html"&gt;
        Require all denied
    &lt;/Files&gt;
&lt;/Directory&gt;
</highlight>
</section>

<section id="webspace"><title>Webspace Containers</title>

<p>The <directive type="section" module="core">Location</directive>
directive and its <glossary ref="regex">regex</glossary> counterpart, on
the other hand, change the
configuration for content in the webspace.  For example, the following
configuration prevents access to any URL-path that begins in /private.
In particular, it will apply to requests for
<code>http://yoursite.example.com/private</code>,
<code>http://yoursite.example.com/private123</code>, and
<code>http://yoursite.example.com/private/dir/file.html</code> as well
as any other requests starting with the <code>/private</code> string.</p>

<highlight language="config">
&lt;LocationMatch "^/private"&gt;
    Require all denied
&lt;/LocationMatch&gt;
</highlight>

<p>The <directive type="section" module="core">Location</directive>
directive need not have anything to do with the filesystem.
For example, the following example shows how to map a particular
URL to an internal Apache HTTP Server handler provided by <module>mod_status</module>.
No file called <code>server-status</code> needs to exist in the
filesystem.</p>

<highlight language="config">
&lt;Location "/server-status"&gt;
    SetHandler server-status
&lt;/Location&gt;
</highlight>
</section>

<section id="overlapping-webspace"><title>Overlapping Webspace</title>
<p>In order to have two overlapping URLs one has to consider the order in which
certain sections or directives are evaluated. For
<directive type="section" module="core">Location</directive> this would be:</p>
<highlight language="config">
&lt;Location "/foo"&gt;
&lt;/Location&gt;
&lt;Location "/foo/bar"&gt;
&lt;/Location>
</highlight>
<p><directive type="section" module="mod_alias">Alias</directive>es on the other hand,
are mapped vice-versa:</p>
<highlight language="config">
Alias "/foo/bar" "/srv/www/uncommon/bar"
Alias "/foo"     "/srv/www/common/foo"
</highlight>
<p>The same is true for the <directive module="mod_proxy">ProxyPass</directive>
directives:</p>
<highlight language="config">
ProxyPass "/special-area" "http://special.example.com" smax=5 max=10
ProxyPass "/" "balancer://mycluster/" stickysession=JSESSIONID|jsessionid nofailover=On
</highlight>
</section>

<section id="wildcards"><title>Wildcards and Regular Expressions</title>

<p>The <directive type="section" module="core">Directory</directive>,
<directive type="section" module="core">Files</directive>, and
<directive type="section" module="core">Location</directive>
directives can each use shell-style wildcard characters as in
<code>fnmatch</code> from the C standard library.  The character "*"
matches any sequence of characters, "?" matches any single character,
and "[<em>seq</em>]" matches any character in <em>seq</em>.  The "/"
character will not be matched by any wildcard; it must be specified
explicitly.</p>

<p>If even more flexible matching is required, each
container has a regular expression (regex) counterpart <directive
type="section" module="core">DirectoryMatch</directive>, <directive
type="section" module="core">FilesMatch</directive>, and <directive
type="section" module="core">LocationMatch</directive> that allow
perl-compatible
<glossary ref="regex">regular expressions</glossary>
to be used in choosing the matches.  But see the section below on
configuration merging to find out how using regex sections will change
how directives are applied.</p>

<p>A non-regex wildcard section that changes the configuration of
all user directories could look as follows:</p>

<highlight language="config">
&lt;Directory "/home/*/public_html"&gt;
    Options Indexes
&lt;/Directory&gt;
</highlight>

<p>Using regex sections, we can deny access to many types of image files
at once:</p>
<highlight language="config">
&lt;FilesMatch "\.(?i:gif|jpe?g|png)$"&gt;
    Require all denied
&lt;/FilesMatch&gt;
</highlight>

<p>Regular expressions containing <strong>named groups and
backreferences</strong> are added to the environment with the
corresponding name in uppercase. This allows elements of filename paths
and URLs to be referenced from within <a href="expr.html">expressions</a>
and modules like <module>mod_rewrite</module>.</p>

<highlight language="config">
&lt;DirectoryMatch "^/var/www/combined/(?&lt;SITENAME&gt;[^/]+)"&gt;
    require ldap-group cn=%{env:MATCH_SITENAME},ou=combined,o=Example
&lt;/DirectoryMatch&gt;
</highlight>

</section>

<section id="expressions"><title>Boolean expressions</title>
<p>The <directive type="section" module="core">If</directive>
directive change the configuration depending on a condition which can be
expressed by a boolean expression. For example, the following configuration
denies access if the HTTP Referer header does not start with
"http://www.example.com/".</p>
<highlight language="config">
&lt;If "!(%{HTTP_REFERER} -strmatch 'http://www.example.com/*')"&gt;
    Require all denied
&lt;/If&gt;
</highlight>

</section>

<section id="whichwhen"><title>What to use When</title>

<p>Choosing between filesystem containers and webspace containers is
actually quite easy.  When applying directives to objects that reside
in the filesystem always use <directive type="section"
module="core">Directory</directive> or <directive type="section"
module="core">Files</directive>.  When applying directives to objects
that do not reside in the filesystem (such as a webpage generated from
a database), use <directive type="section"
module="core">Location</directive>.</p>

<p>It is important to never use <directive type="section"
module="core">Location</directive> when trying to restrict
access to objects in the filesystem.  This is because many
different webspace locations (URLs) could map to the same filesystem
location, allowing your restrictions to be circumvented.
For example, consider the following configuration:</p>

<highlight language="config">
&lt;Location "/dir/"&gt;
    Require all denied
&lt;/Location&gt;
</highlight>

<p>This works fine if the request is for
<code>http://yoursite.example.com/dir/</code>.  But what if you are on
a case-insensitive filesystem?  Then your restriction could be easily
circumvented by requesting
<code>http://yoursite.example.com/DIR/</code>.  The <directive
type="section" module="core">Directory</directive> directive, in
contrast, will apply to any content served from that location,
regardless of how it is called.  (An exception is filesystem links.
The same directory can be placed in more than one part of the
filesystem using symbolic links.  The <directive type="section"
module="core">Directory</directive> directive will follow the symbolic
link without resetting the pathname.  Therefore, for the highest level
of security, symbolic links should be disabled with the appropriate
<directive module="core">Options</directive> directive.)</p>

<p>If you are, perhaps, thinking that none of this applies to you
because you use a case-sensitive filesystem, remember that there are
many other ways to map multiple webspace locations to the same
filesystem location.  Therefore you should always use the filesystem
containers when you can.  There is, however, one exception to this
rule.  Putting configuration restrictions in a <code>&lt;Location
"/"&gt;</code> section is perfectly safe because this section will apply
to all requests regardless of the specific URL.</p>
</section>

<section id="nesting"><title>Nesting of sections</title>

<p>Some section types can be nested inside other section types. On the one
hand, <directive type="section" module="core">Files</directive> can be used
inside <directive type="section" module="core">Directory</directive>.  On
the other hand, <directive type="section" module="core">If</directive> can
be used inside <directive type="section" module="core">Directory</directive>,
<directive type="section" module="core">Location</directive>, and <directive
type="section" module="core">Files</directive> sections (but not inside another
<directive type="section" module="core">If</directive>). The regex
counterparts of the named section behave identically.</p>

<p>Nested sections are merged after non-nested sections of the same type.</p>

</section>

</section>

<section id="virtualhost"><title>Virtual Hosts</title>

<p>The <directive type="section" module="core">VirtualHost</directive>
container encloses directives that apply to specific hosts.
This is useful when serving multiple hosts from the same machine
with a different configuration for each.  For more information,
see the <a href="vhosts/">Virtual Host Documentation</a>.</p>
</section>

<section id="proxy"><title>Proxy</title>

<p>The <directive type="section" module="mod_proxy">Proxy</directive>
and <directive type="section" module="mod_proxy">ProxyMatch</directive>
containers apply enclosed configuration directives only
to sites accessed through <module>mod_proxy</module>'s proxy server
that match the specified URL. For example, the following configuration
will allow only a subset of clients to access the
<code>www.example.com</code> website using the proxy server:</p>

<highlight language="config">
&lt;Proxy http://www.example.com/*&gt;
    Require host yournetwork.example.com
&lt;/Proxy&gt;
</highlight>
</section>

<section id="whatwhere"><title>What Directives are Allowed?</title>

<p>To find out what directives are allowed in what types of
configuration sections, check the <a
href="mod/directive-dict.html#Context">Context</a> of the directive.
Everything that is allowed in
<directive type="section" module="core">Directory</directive>
sections is also syntactically allowed in
<directive type="section" module="core">DirectoryMatch</directive>,
<directive type="section" module="core">Files</directive>,
<directive type="section" module="core">FilesMatch</directive>,
<directive type="section" module="core">Location</directive>,
<directive type="section" module="core">LocationMatch</directive>,
<directive type="section" module="mod_proxy">Proxy</directive>,
and <directive type="section" module="mod_proxy">ProxyMatch</directive>
sections.  There are some exceptions, however:</p>

<ul>
<li>The <directive module="core">AllowOverride</directive> directive
works only in <directive type="section" module="core">Directory</directive>
sections.</li>

<li>The <code>FollowSymLinks</code> and
<code>SymLinksIfOwnerMatch</code> <directive
module="core">Options</directive> work only in <directive
type="section" module="core">Directory</directive> sections or
<code>.htaccess</code> files.</li>

<li>The <directive module="core">Options</directive> directive cannot
be used in <directive type="section" module="core">Files</directive>
and <directive type="section" module="core">FilesMatch</directive>
sections.</li>
</ul>
</section>

<section id="merging"><title>How the sections are merged</title>

<p>The configuration sections are applied in a very particular order.
Since this can have important effects on how configuration directives
are interpreted, it is important to understand how this works.</p>

    <p>The order of merging is:</p>

    <ol>
      <li> <directive type="section"
      module="core">Directory</directive> (except regular expressions)
      and <code>.htaccess</code> done simultaneously (with
      <code>.htaccess</code>, if allowed, overriding
      <directive type="section" module="core">Directory</directive>)</li>

      <li><directive type="section" module="core">DirectoryMatch</directive>
      (and <code>&lt;Directory ~&gt;</code>)</li>

      <li><directive type="section"
      module="core">Files</directive> and <directive
      type="section" module="core">FilesMatch</directive> done
      simultaneously</li>

      <li><directive type="section" module="core">Location</directive>
      and <directive type="section"
      module="core">LocationMatch</directive> done simultaneously</li>

      <li><directive type="section" module="core">If</directive>
      </li>

    </ol>

    <p>Some important remarks:</p>
    <ul>
        <li>Apart from <directive type="section"
        module="core">Directory</directive>, within each group the sections are
        processed in the order they appear in the configuration files.
        For example, a request for <em>/foo</em> will match
        <code>&lt;Location "/foo/bar"&gt;</code> and 
        <code>&lt;Location "/foo"&gt;</code> (group 4 in this case):
        both sections will be evaluated
        but in the order they appear in the configuration files.</li>
        <li><directive type="section" module="core">Directory</directive>
        (group 1 above) is processed in the order shortest directory
        component to longest. For example,
        <code>&lt;Directory "/var/web/dir"&gt;</code> will be processed before
        <code>&lt;Directory "/var/web/dir/subdir"&gt;</code>.</li>
        <li>If multiple <directive
        type="section" module="core">Directory</directive> sections apply
        to the same directory they are processed in the configuration file
        order.</li>
        <li>Configurations included via the <directive
        module="core">Include</directive> directive will be treated as if
        they were inside the including file at the location of the
        <directive module="core">Include</directive> directive.</li>
        <li>Sections inside <directive type="section"
        module="core">VirtualHost</directive> sections
        are applied <em>after</em> the corresponding sections outside
        the virtual host definition. This allows virtual hosts to
        override the main server configuration.</li>
        <li>When the request is served by <module>mod_proxy</module>, the
        <directive module="mod_proxy" type="section">Proxy</directive>
        container takes the place of the <directive module="core"
        type="section">Directory</directive> container in the processing
        order.</li>
    </ul>

    <note><title>Technical Note</title>
      There is actually a
      <code>&lt;Location&gt;</code>/<code>&lt;LocationMatch&gt;</code>
      sequence performed just before the name translation phase
      (where <code>Aliases</code> and <code>DocumentRoots</code>
      are used to map URLs to filenames). The results of this
      sequence are completely thrown away after the translation has
      completed.
    </note>

<section id="relationship-module-configuration"><title>Relationship between modules and configuration sections</title>
    <p>One question that often arises after reading how configuration sections are
    merged is related to how and when directives of specific modules like <module>mod_rewrite</module>
    are processed. The answer is not trivial and needs a bit of background. 
    Each httpd module manages its own configuration, and each of its directives in httpd.conf specify one piece 
    of configuration in a particular context. httpd does not execute a command as it is read.</p>
    <p>At runtime, the core of httpd iterates over the defined configuration sections in the order
    described above to determine which ones apply to the current request. When the first section matches, 
    it is considered the current configuration for this request. If a subsequent section matches too, 
    then each module with a directive in either of the sections is given a chance to merge its configuration between the two sections. The result is a third configuration, and the process goes on until all the configuration sections
    are evaluated.</p>
    <p>After the above step, the "real" processing of the HTTP request begins: each module has a chance to run 
    and perform whatever tasks they like. They can retrieve their own final merged configuration from the core
    of the httpd to determine how they should act.</p>
    <p>An example can help to visualize the whole process. The following configuration uses the 
        <directive module="mod_headers">Header</directive> directive of <module>mod_headers</module> to set
        a specific HTTP header. What value will httpd set in the <code>CustomHeaderName</code> header for a request to
        <code>/example/index.html</code> ?
    </p>
    <highlight language="config">

&lt;Directory "/"&gt;
    Header set CustomHeaderName one
    &lt;FilesMatch ".*"&gt;
        Header set CustomHeaderName three
    &lt;/FilesMatch&gt;
&lt;/Directory&gt;

&lt;Directory "/example"&gt;
    Header set CustomHeaderName two
&lt;/Directory&gt;
     
    </highlight>    
    <ul>
        <li><directive>Directory</directive> "/" matches and an initial configuration to set the <code>CustomHeaderName</code> header with the value <code>one</code> is created.</li>
        <li><directive>Directory</directive> "/example" matches, and since <module>mod_headers</module> specifies in its code to override in case of a merge, a new configuration is created to set the <code>CustomHeaderName</code> header with the value <code>two</code>.</li>
        <li><directive>FilesMatch</directive> ".*" matches and another merge opportunity arises, causing the <code>CustomHeaderName</code> header to be set with the value <code>three</code>.</li>
        <li>Eventually during the next steps of the HTTP request processing <module>mod_headers</module> will be called and it will receive the configuration to set the <code>CustomHeaderName</code> header with the value <code>three</code>. <module>mod_headers</module> normally uses this configuration to perfom its job, namely setting the foo header. This does not mean that a module can't perform a more complex action like discarding directives because not needed or deprecated, etc..</li>
    </ul>

    <p>This is true for .htaccess too since they have the same priority as <directive>Directory</directive> in the merge order. The important concept to understand is that configuration sections like  <directive>Directory</directive> and <directive>FilesMatch</directive> are not comparable to module specific directives like <directive module="mod_headers">Header</directive> or <directive module="mod_rewrite">RewriteRule</directive> because they operate on different levels.
    </p>
</section>

<section id="merge-examples"><title>Some useful examples</title>

<p>Below is an artificial example to show the order of
merging. Assuming they all apply to the request, the directives in
this example will be applied in the order A &gt; B &gt; C &gt; D &gt;
E.</p>

<highlight language="config">
&lt;Location "/"&gt;
    E
&lt;/Location&gt;

&lt;Files "f.html"&gt;
    D
&lt;/Files&gt;

&lt;VirtualHost *&gt;
    &lt;Directory "/a/"&gt;
        B
    &lt;/Directory&gt;
&lt;/VirtualHost&gt;

&lt;DirectoryMatch "^.*b$"&gt;
    C
&lt;/DirectoryMatch&gt;

&lt;Directory "/a/b"&gt;
    A
&lt;/Directory&gt;

</highlight>


<p>For a more concrete example, consider the following.  Regardless of
any access restrictions placed in <directive module="core"
type="section">Directory</directive> sections, the <directive
module="core" type="section">Location</directive> section will be
evaluated last and will allow unrestricted access to the server.  In
other words, order of merging is important, so be careful!</p>

<highlight language="config">
&lt;Location "/"&gt;
    Require all granted
&lt;/Location&gt;

# Whoops!  This &lt;Directory&gt; section will have no effect
&lt;Directory "/"&gt;
    &lt;RequireAll&gt;
        Require all granted
        Require not host badguy.example.com
    &lt;/RequireAll&gt;
&lt;/Directory&gt;
</highlight>

</section>

</section>
</manualpage>