Major revision of sections.html

22 Aug 2002 22:58:44 -0000

One of the more complicated aspects of apache configuration (and also one of the more powerful aspects) are all those <directory>, <files>, <location>, etc sections. The existing documentation on this stuff is a little bit sketchy. Attached is both html and xml versions of a rough draft of a major revision to sections.html to make it more complete and a little less technical.

Feedback is very much appreciated. I think a few more examples could be added. I was also rather sketchy about how the <proxy> section fits into things. That's mainly because I really don't know myself how it works. Mostly I am interested in things that could make the docs simpler and clearer.

Fire away! (But don't bother with spelling mistakes; I haven't spell-checked it yet and my spelling is atroshus.)

Joshua. 
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
<?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>

<manualpage>
  <relativepath href="."/>

<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 restrict the application of other configuration directives.</p>
</summary>

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

<related>
<modulelist>
<module>core</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">IfDefine</directive>
<directive type="section" module="core">IfModule</directive>
<directive type="section" module="core">Location</directive>
<directive type="section" module="core">LocationMatch</directive>
<directive type="section" module="proxy">Proxy</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> and
<directive type="section" module="core">IfModule</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
paramter is defined on the <code>httpd</code> 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>

<example>
&lt;IfDefine ClosedForNow&gt;<br />
Redirect / http://otherserver.example.com/<br />
&lt;/IfDefine&gt;
</example>

<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.  This directive
should only be used if you need your configuration file to work with
or without certain modules installed.  It should not be used to
enclose directives that you want to work all the time, because it can
supress useful error messages about missing modules.</p>

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

<example>
&lt;IfModule mod_mime_magic.c&gt;<br />
MimeMagicFile conf/magic<br />
&lt;/IfModule&gt;
</example>

<p>Both <directive type="section" module="core">IfDefine</directive>
and <directive type="section" module="core">IfModule</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 and Webspace</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 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, 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 install on Unix.</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 regex 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.  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>

<example>
&lt;Directory /var/web/dir1&gt;<br />
Options +Indexes<br />
&lt;/Directory&gt;
</example>

<p>Directivies 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>

<example>
&lt;Files private.html&gt;<br />
Order allow,deny<br />
Deny from all<br />
&lt;/Files&gt;
</example>

<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>

<example>
&lt;Directory /var/web/dir1&gt;<br />
&lt;Files private.html&gt;<br />
Order allow,deny<br />
Deny from all<br />
&lt;/Files&gt;<br />
&lt;/Directory&gt;
</example>
</section>

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

<p>The <directive type="section" module="core">Location</directive>
directive and its regex 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 that string.</p>

<example>
&lt;Location /private&gt;<br />
Order Allow,Deny<br />
Deny from all<br />
&lt;/Location&gt;
</example>
</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 the wildcard characters "?" to match any
single character and "*" to match any set of characters.  This is
useful to apply the same configuration to a group of filesystem or
webspace locations. 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
regular expressions 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>
</section>

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

<example>
&lt;Location /dir/&gt;<br />
Order allow,deny<br />
Deny from all<br />
&lt;/Location&gt;
</example>

<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>

<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="proxy">Proxy</directive>
container applies 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 prevent the proxy server from being used to access the
<code>cnn.com</code> website.</p>

<example>
&lt;Proxy http://cnn.com/*&gt;<br />
Order allow,deny<br />
Deny from all<br />
&lt;/Proxy&gt;
</example>
</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>,
and
<directive type="section" module="proxy">Proxy</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="mergin"><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 .htaccess done simultaneously (with .htaccess, 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>
    </ol>

    <p>Apart from <directive type="section"
    module="core">Directory</directive>, each group is processed in
    the order that they appear in the configuration files.  <directive
    type="section" module="core">Directory</directive> (group 1 above)
    is processed in the order shortest directory component to longest.
    So for example, <code>&lt;Directory /var/web/dir&gt;</code> will
    be processed before <code>&lt;Directory
    /var/web/dir/subdir&gt;</code>.  If multiple <directive
    type="section" module="core">Directory</directive> sections apply
    to the same directory they are processed in the configuration file
    order. 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.</p>

    <p>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.</p>

    <p>Later sections override earlier ones.</p>

<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="merge-examples"><title>Some 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 E &gt; C &gt; D &gt; B &gt;
A.</p>

<example>
&lt;Location /&gt;<br />
A<br />
&lt;/Location&gt;<br />
<br />
&lt;Files f.html&gt;<br />
B<br />
&lt;/Files&gt;<br />
<br />
&lt;VirtualHost *&gt;<br />
&lt;Directory /a/b&gt;<br />
C<br />
&lt;/Directory&gt;<br />
&lt;/VirtualHost&gt;<br />
<br />
&lt;DirectoryMatch "^.*b$"&gt;<br />
D<br />
&lt;/DirectoryMatch&gt;<br />
<br />
&lt;Directory /a/b&gt;<br />
E<br />
&lt;/Directory&gt;<br />
<br />
</example>

<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>

<example>
&lt;Location /&gt;<br />
Order deny,allow<br />
Allow from all<br />
&lt;/Location&gt;<br />
<br />
&lt;Directory /&gt;<br />
Order allow,deny<br />
Allow from all<br />
Deny from badguy.example.com<br />
&lt;/Directory&gt;
</example>

</section>

</section>
</manualpage>
Title: Configuration Sections - Apache HTTP Server
[APACHE DOCUMENTATION]

Apache HTTP Server Version 2.0

Configuration Sections

Directives in the configuration files 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 .htaccess files to restrict the application of other configuration directives.

Types of Configuration Section Containers

Related Modules

core
mod_proxy
Related Directives

<Directory>
<DirectoryMatch>
<Files>
<FilesMatch>
<IfDefine>
<IfModule>
<Location>
<LocationMatch>
<Proxy>
<VirtualHost>

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 <IfDefine> and <IfModule> 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.

The <IfDefine> directive encloses directives that will only be applied if an appropriate paramter is defined on the httpd command line. For example, with the following configuration, all requests will be redirected to another site only if the server is started using httpd -DClosedForNow:

<IfDefine ClosedForNow>
Redirect / http://otherserver.example.com/
</IfDefine>

The <IfModule> directive is very similar, except it encloses directives that will only be applied if a particular module is available in the server. This directive should only be used if you need your configuration file to work with or without certain modules installed. It should not be used to enclose directives that you want to work all the time, because it can supress useful error messages about missing modules.

In the following example, the MimeMagicFiles directive will be applied only if mod_mime_magic is available.

<IfModule mod_mime_magic.c>
MimeMagicFile conf/magic
</IfModule>

Both <IfDefine> and <IfModule> can apply negative conditions by preceding their test with "!". Also, these sections can be nested to achieve more complex restrictions.

Filesystem and Webspace

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 resides at /usr/local/apache2 in the Unix filesystem or "c:/Program Files/Apache Group/Apache2" in the Windows filesystem. (Note that forward slashes should always be used as the path separator in Apache, 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 /dir/ in the webspace corresponds to the path /usr/local/apache2/htdocs/dir/ in the filesystem of a default Apache install on Unix.

Filesystem Containers

The <Directory> and <Files> directives, along with their regex counterparts, apply directives to parts of the filesystem. Directives enclosed in a <Directory> section apply to the named filesystem directory and all subdirectories of that directory. The same effect can be obtained using .htaccess files. For example, in the following configuration, directory indexes will be enabled for the /var/web/dir1 directory and all subdirectories.

<Directory /var/web/dir1>
Options +Indexes
</Directory>

Directivies enclosed in a <Files> 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 private.html regardless of where it is found.

<Files private.html>
Order allow,deny
Deny from all
</Files>

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

<Directory /var/web/dir1>
<Files private.html>
Order allow,deny
Deny from all
</Files>
</Directory>

Webspace Containers

The <Location> directive and its regex 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 http://yoursite.example.com/private, http://yoursite.example.com/private123, and http://yoursite.example.com/private/dir/file.html as well as any other requests starting with that string.

<Location /private>
Order Allow,Deny
Deny from all
</Location>

Wildcards and Regular Expressions

The <Directory>, <Files>, and <Location> directives can each use the wildcard characters "?" to match any single character and "*" to match any set of characters. This is useful to apply the same configuration to a group of filesystem or webspace locations. If even more flexible matching is required, each container has a regular-_expression_ (regex) counterpart <DirectoryMatch>, <FilesMatch>, and <LocationMatch> that allow regular expressions 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.

When to use What

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

It is important to never use <Location> 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:

<Location /dir/>
Order allow,deny
Deny from all
</Location>

This works fine if the request is for http://yoursite.example.com/dir/. But what if you are on a case-insensitive filesystem? Then your restriction could be easily circumvented by requesting http://yoursite.example.com/DIR/. The <Directory> 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 <Directory> 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 Options directive.)

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 <Location /> section is perfectly safe because this section will apply to all requests regardless of the specific URL.

Virtual Hosts

The <VirtualHost> 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 Virtual Host Documentation.

Proxy

The <Proxy> container applies enclosed configuration directives only to sites accessed through mod_proxy's proxy server that match the specified URL. For example, the following configuration will prevent the proxy server from being used to access the cnn.com website.

<Proxy http://cnn.com/*>
Order allow,deny
Deny from all
</Proxy>

What Directives are Allowed?

To find out what directives are allowed in what types of configuration sections, check the Context of the directive. Everything that is allowed in <Directory> sections is also syntactically allowed in <DirectoryMatch>, <Files>, <FilesMatch>, <Location>, <LocationMatch>, and <Proxy> sections. There are some exceptions, however.

How the sections are merged

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.

The order of merging is:

  1. <Directory> (except regular expressions) and .htaccess done simultaneously (with .htaccess, if allowed, overriding <Directory>)
  2. <DirectoryMatch> (and <Directory ~>)
  3. <Files> and <FilesMatch> done simultaneously
  4. <Location> and <LocationMatch> done simultaneously

Apart from <Directory>, each group is processed in the order that they appear in the configuration files. <Directory> (group 1 above) is processed in the order shortest directory component to longest. So for example, <Directory /var/web/dir> will be processed before <Directory /var/web/dir/subdir>. If multiple <Directory> sections apply to the same directory they are processed in the configuration file order. Configurations included via the Include directive will be treated as if they were inside the including file at the location of the Include directive.

Sections inside <VirtualHost> sections are applied after the corresponding sections outside the virtual host definition. This allows virtual hosts to override the main server configuration.

Later sections override earlier ones.

Technical Note

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

Some Examples

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 E > C > D > B > A.

<Location />
A
</Location>

<Files f.html>
B
</Files>

<VirtualHost *>
<Directory /a/b>
C
</Directory>
</VirtualHost>

<DirectoryMatch "^.*b$">
D
</DirectoryMatch>

<Directory /a/b>
E
</Directory>

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

<Location />
Order deny,allow
Allow from all
</Location>

<Directory />
Order allow,deny
Allow from all
Deny from badguy.example.com
</Directory>

Apache HTTP Server Version 2.0

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