Configuration Sections

Directives in theconfiguration files may apply to theentire server, or they may be restricted to apply only to particulardirectories, files, hosts, or URLs. This document describes how touse configuration section containers or.htaccess filesto change the scope of other configuration directives.

• Types of Configuration Section Containers
• Filesystem, Webspace, and Boolean Expressions
• Virtual Hosts
• Proxy
• What Directives are Allowed?
• How the sections are merged

See also

• Comments

Types of Configuration Section Containers

There are two basic types of containers. Most containers areevaluated for each request. The enclosed directives are applied onlyfor those requests that match the containers. The<IfDefine>,<IfModule>, and<IfVersion>containers, on the other hand, are evaluated only at server startupand restart. If their conditions are true at startup, then theenclosed directives will apply to all requests. If the conditions arenot true, the enclosed directives will be ignored.

The<IfDefine> directiveencloses directives that will only be applied if an appropriateparameter is defined on thehttpd command line. For example,with the following configuration, all requests will be redirectedto another site only if the server is started usinghttpd -DClosedForNow:

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

The<IfModule>directive is very similar, except it encloses directives that willonly be applied if a particular module is available in the server.The module must either be statically compiled in the server, or itmust be dynamically compiled and itsLoadModule line must be earlier in theconfiguration file. This directive should only be used if you needyour configuration file to work whether or not certain modules areinstalled. It should not be used to enclose directives that you wantto work all the time, because it can suppress useful error messagesabout missing modules.

In the following example, theMimeMagicFile directive will beapplied only ifmod_mime_magic is available.

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

The<IfVersion>directive is very similar to<IfDefine> and<IfModule>, except it encloses directives that willonly be applied if a particular version of the server is executing. Thismodule is designed for the use in test suites and large networks which have todeal with different httpd versions and different configurations.

<IfVersion >= 2.4>    # this happens only in versions greater or    # equal 2.4.0.</IfVersion>

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

Filesystem, Webspace, and Boolean Expressions

The most commonly used configuration section containers are theones that change the configuration of particular places in thefilesystem or webspace. First, it is important to understand thedifference between the two. The filesystem is the view of your disksas seen by your operating system. For example, in a default install,Apache httpd resides at/usr/local/apache2 in the Unixfilesystem or"c:/Program Files/Apache Group/Apache2" inthe Windows filesystem. (Note that forward slashes should always beused 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 serverand seen by the client. So the path/dir/ in thewebspace corresponds to the path/usr/local/apache2/htdocs/dir/ in the filesystem of adefault Apache httpd install on Unix. The webspace need not map directly tothe filesystem, since webpages may be generated dynamicallyfrom databases or other locations.

Filesystem Containers

The<Directory>and<Files>directives, along with theirregexcounterparts, apply directives toparts of the filesystem. Directives enclosed in a<Directory> section apply tothe named filesystem directory and all subdirectories of thatdirectory (as well as the files in those directories).The same effect can be obtained using.htaccess files. For example, in thefollowing configuration, directory indexes will be enabled for the/var/web/dir1 directory and all subdirectories.

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

Directives enclosed in a<Files> section apply to any file withthe 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 namedprivate.html regardlessof where it is found.

<Files "private.html">    Require all denied</Files>

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

<Directory "/var/web/dir1">    <Files "private.html">        Require all denied    </Files></Directory>

Webspace Containers

The<Location>directive and itsregex counterpart, onthe other hand, change theconfiguration for content in the webspace. For example, the followingconfiguration prevents access to any URL-path that begins in /private.In particular, it will apply to requests forhttp://yoursite.example.com/private,http://yoursite.example.com/private123, andhttp://yoursite.example.com/private/dir/file.html as wellas any other requests starting with the/private string.

<LocationMatch "^/private">    Require all denied</LocationMatch>

The<Location>directive need not have anything to do with the filesystem.For example, the following example shows how to map a particularURL to an internal Apache HTTP Server handler provided bymod_status.No file calledserver-status needs to exist in thefilesystem.

<Location "/server-status">    SetHandler server-status</Location>

Overlapping Webspace

In order to have two overlapping URLs one has to consider the order in whichcertain sections or directives are evaluated. For<Location> this would be:

<Location "/foo"></Location><Location "/foo/bar"></Location>

<Alias>es on the other hand,are mapped vice-versa:

Alias "/foo/bar" "/srv/www/uncommon/bar"Alias "/foo"     "/srv/www/common/foo"

The same is true for theProxyPassdirectives:

ProxyPass "/special-area" "http://special.example.com" smax=5 max=10ProxyPass "/" "balancer://mycluster/" stickysession=JSESSIONID|jsessionid nofailover=On

Wildcards and Regular Expressions

The<Directory>,<Files>, and<Location>directives can each use shell-style wildcard characters as infnmatch from the C standard library. The character "*"matches any sequence of characters, "?" matches any single character,and "[seq]" matches any character inseq. The "/"character will not be matched by any wildcard; it must be specifiedexplicitly.

If even more flexible matching is required, eachcontainer has a regular expression (regex) counterpart<DirectoryMatch>,<FilesMatch>, and<LocationMatch> that allowperl-compatibleregular expressionsto be used in choosing the matches. But see the section below onconfiguration merging to find out how using regex sections will changehow directives are applied.

A non-regex wildcard section that changes the configuration ofall user directories could look as follows:

<Directory "/home/*/public_html">    Options Indexes</Directory>

Using regex sections, we can deny access to many types of image filesat once:

<FilesMatch "\.(?i:gif|jpe?g|png)$">    Require all denied</FilesMatch>

Regular expressions containingnamed groups andbackreferences are added to the environment with thecorresponding name in uppercase. This allows elements of filename pathsand URLs to be referenced from withinexpressionsand modules likemod_rewrite.

<DirectoryMatch "^/var/www/combined/(?<SITENAME>[^/]+)">    Require ldap-group "cn=%{env:MATCH_SITENAME},ou=combined,o=Example"</DirectoryMatch>

Boolean expressions

The<If>directive change the configuration depending on a condition which can beexpressed by a boolean expression. For example, the following configurationdenies access if the HTTP Referer header does not start with"http://www.example.com/".

<If "!(%{HTTP_REFERER} -strmatch 'http://www.example.com/*')">    Require all denied</If>

What to use When

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

It is important to never use<Location> when trying to restrictaccess to objects in the filesystem. This is because manydifferent webspace locations (URLs) could map to the same filesystemlocation, allowing your restrictions to be circumvented.For example, consider the following configuration:

<Location "/dir/">    Require all denied</Location>

This works fine if the request is forhttp://yoursite.example.com/dir/. But what if you are ona case-insensitive filesystem? Then your restriction could be easilycircumvented by requestinghttp://yoursite.example.com/DIR/. The<Directory> directive, incontrast, 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 thefilesystem using symbolic links. The<Directory> directive will follow the symboliclink without resetting the pathname. Therefore, for the highest levelof security, symbolic links should be disabled with the appropriateOptions directive.)

If you are, perhaps, thinking that none of this applies to youbecause you use a case-sensitive filesystem, remember that there aremany other ways to map multiple webspace locations to the samefilesystem location. Therefore you should always use the filesystemcontainers when you can. There is, however, one exception to thisrule. Putting configuration restrictions in a<Location"/"> section is perfectly safe because this section will applyto all requests regardless of the specific URL.

Nesting of sections

Some section types can be nested inside other section types. On the onehand,<Files> can be usedinside<Directory>. Onthe other hand,<If> canbe used inside<Directory>,<Location>, and<Files> sections (but not inside another<If>). The regexcounterparts of the named section behave identically.

Nested sections are merged after non-nested sections of the same type.

Virtual Hosts

The<VirtualHost>container encloses directives that apply to specific hosts.This is useful when serving multiple hosts from the same machinewith a different configuration for each. For more information,see theVirtual Host Documentation.

Proxy

The<Proxy>and<ProxyMatch>containers apply enclosed configuration directives onlyto sites accessed throughmod_proxy's proxy serverthat match the specified URL. For example, the following configurationwill allow only a subset of clients to access thewww.example.com website using the proxy server:

<Proxy "http://www.example.com/*">    Require host yournetwork.example.com</Proxy>

What Directives are Allowed?

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

• TheAllowOverride directiveworks only in<Directory>sections.
• TheFollowSymLinks andSymLinksIfOwnerMatchOptions work only in<Directory> sections or.htaccess files.
• TheOptions directive cannotbe used in<Files>and<FilesMatch>sections.

How the sections are merged

The configuration sections are applied in a very particular order.Since this can have important effects on how configuration directivesare 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
5. <If> sections, even when they are enclosed in any of the preceding contexts.

Some important remarks:

• Apart from<Directory>, within each group the sections are processed in the order they appear in the configuration files. For example, a request for/foo/bar will match<Location "/foo/bar"> and<Location "/foo"> (group 4 in this case): both sections will be evaluated but in the order they appear in the configuration files.
• <Directory> (group 1 above) is processed in the order shortest directory component to longest. 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 theInclude directive will be treated as if they were inside the including file at the location of theInclude directive.
• Sections inside<VirtualHost> sections are appliedafter the corresponding sections outside the virtual host definition. This allows virtual hosts to override the main server configuration.
• When the request is served bymod_proxy, the<Proxy> container takes the place of the<Directory> container in the processing order.
• Caution should be exercised when mixing related configuration directives inside and outside of<If> because of the effect on merging order. Explicit use of<Else> can help.
• When<If> is used in.htaccess, the enclosed directives in a parent directory will be mergedafter non-enclosed directives in a subdirectory.

Relationship between modules and configuration sections

One question that often arises after reading how configuration sections are merged is related to how and when directives of specific modules likemod_rewrite 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.

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.

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.

An example can help to visualize the whole process. The following configuration uses theHeader directive ofmod_headers to set a specific HTTP header. What value will httpd set in theCustomHeaderName header for a request to/example/index.html ?

<Directory "/">    Header set CustomHeaderName one    <FilesMatch ".*">        Header set CustomHeaderName three    </FilesMatch></Directory><Directory "/example">    Header set CustomHeaderName two</Directory>

• Directory "/" matches and an initial configuration to set theCustomHeaderName header with the valueone is created.
• Directory "/example" matches, and sincemod_headers specifies in its code to override in case of a merge, a new configuration is created to set theCustomHeaderName header with the valuetwo.
• FilesMatch ".*" matches and another merge opportunity arises, causing theCustomHeaderName header to be set with the valuethree.
• Eventually during the next steps of the HTTP request processingmod_headers will be called and it will receive the configuration to set theCustomHeaderName header with the valuethree.mod_headers normally uses this configuration to perform 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..

This is true for .htaccess too since they have the same priority asDirectory in the merge order. The important concept to understand is that configuration sections likeDirectory andFilesMatch are not comparable to module specific directives likeHeader orRewriteRule because they operate on different levels.

Some useful examples

Below is an artificial example to show the order ofmerging. Assuming they all apply to the request, the directives inthis example will be applied in the order A > B > C > D >E.

<Location "/">    E</Location><Files "f.html">    D</Files><VirtualHost *>    <Directory "/a/b">        B    </Directory></VirtualHost><DirectoryMatch "^.*b$">    C</DirectoryMatch><Directory "/a/b">    A</Directory>

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

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