Types of Configuration Section Containers
core
mod_version
mod_proxy
Directory
DirectoryMatch
Files
FilesMatch
If
IfDefine
IfModule
IfVersion
Location
LocationMatch
Proxy
ProxyMatch
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, IfModule, and
IfVersion
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
parameter 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.
The module must either be statically compiled in the server, or it
must be dynamically compiled and its LoadModule 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.
In the following example, the MimeMagicFile directive will be
applied only if mod_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 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.
<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 complex
restrictions.
Filesystem, Webspace, and Boolean Expressions
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 /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 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 /dir/
in the
webspace corresponds to the path
/usr/local/apache2/htdocs/dir/
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.
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 (as well as the files in those directories).
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>
Directives 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">
Require all denied
</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">
Require all denied
</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 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 particular
URL to an internal Apache HTTP Server handler provided by mod_status.
No file called server-status
needs to exist in the
filesystem.
<Location "/server-status">
SetHandler server-status
</Location>
Overlapping Webspace
In order to have two overlapping URLs one has to consider the order in which
certain sections or directives are evaluated. For
Location this would be:
<Location "/foo">
</Location>
<Location "/foo/bar">
</Location>
Aliases 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 the ProxyPass
directives:
ProxyPass "/special-area" "http://special.example.com" smax=5 max=10
ProxyPass "/" "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 in
fnmatch
from the C standard library. The character "*"
matches any sequence of characters, "?" matches any single character,
and "[seq]" matches any character in seq. The "/"
character will not be matched by any wildcard; it must be specified
explicitly.
If even more flexible matching is required, each
container has a regular expression (regex) counterpart DirectoryMatch, FilesMatch, and LocationMatch that allow
perl-compatible
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.
A non-regex wildcard section that changes the configuration of
all 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 files
at once:
<FilesMatch "\.(?i:gif|jpe?g|png)$">
Require all denied
</FilesMatch>
Regular expressions containing named groups and
backreferences are added to the environment with the
corresponding name in uppercase. This allows elements of filename paths
and URLs to be referenced from within expressions
and modules like mod_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 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/".
<If "!(%{HTTP_REFERER} -strmatch 'http://www.example.com/*')">
Require all denied
</If>
What to use When
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/">
Require all denied
</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.
Nesting of sections
Some section types can be nested inside other section types. On the one
hand, Files can be used
inside Directory. On
the other hand, If can
be used inside Directory,
Location, and Files sections. The regex
counterparts of the named section behave identically.
Nested sections are merged after non-nested sections of the same type.
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:
- Directory (except regular expressions)
and
.htaccess
done simultaneously (with
.htaccess
, if allowed, overriding
Directory)
- DirectoryMatch
(and
<Directory ~>
)
- Files and FilesMatch done
simultaneously
- Location
and LocationMatch done simultaneously
- If
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.
When the request is served by mod_proxy, the
Proxy
container takes the place of the Directory container in the processing
order.
Later sections override earlier ones, however each module is responsible
for interpreting what form this override takes. A later configuration section
with directives from a given module might cause a conceptual "merge" of some
directives, all directives, or a complete replacement of the modules
configuration with the module defaults and directives explicitly listed in
the later context.
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 A > B > C > D >
E.
<Location "/">
E
</Location>
<Files "f.html">
D
</Files>
<VirtualHost *>
<Directory "/a/emphasis role="bold"">
B
</Directory>
</VirtualHost>
<DirectoryMatch "^.*b$">
C
</DirectoryMatch>
<Directory "/a/b>
A
</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 "/">
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>