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+**DRAFT**
+
+Overview
+========
+
+If a portion of a confined program needs different access permissions
+than other portions, the program can “change hats” via AppArmor's
+change\_hat() mechanism to a sub-profile, or hat. The mod\_apparmor
+Apache module uses change\_hat() to offer more fine-grained confinement
+of dynamic elements within Apache, such as individual php and perl
+scripts, while still allowing the performance benefits of using
+mod\_php and mod\_perl.
+
+To use mod\_apparmor with Apache, ensure you have a functioning
+AppArmor using 'aa-status' and that mod\_apparmor is loaded into Apache
+(using tools such as a2enmod and yast or manual editing of the Apache
+configuration files), then restart Apache.
+
+URI Requests
+============
+
+Once mod\_apparmor is loaded within Apache, all requests to Apache
+will cause mod\_apparmor to attempt to change into a hat named by
+the URI that is defined in /etc/apparmor.d/. For example:
+
+```
+ $ cat /etc/apparmor.d/usr.lib.cgi-bin.some.cgi
+ #include <tunables/global>
+ /usr/lib/cgi-bin/some.cgi {
+   ...
+ }
+```
+
+If no such hat is found, it will attempt to use the DEFAULT\_URI hat,
+if it exists. If this hat does not exist, mod\_apparmor will fall
+back to using the global Apache profile. Most static web pages can
+simply make use of the DEFAULT\_URI hat.
+
+Additionally, to allow even more control, before any requests come
+in to Apache, mod\_apparmor will attempt to change hat into the
+HANDLING\_UNTRUSTED\_INPUT hat while Apache is doing the initial
+parsing of a given http request, before the request is given to a
+specific handler (like mod\_php) for processing. This hat can be
+used to provide an even stricter confinement while processing the
+untrusted input. Eg:
+
+```
+ $ cat /etc/apparmor.d/usr.sbin.apache2
+ #include <tunables/global>
+ /usr/sbin/apache2 {
+   #
+   # Global Apache policy
+   #
+
+   #include <abstractions/base>
+   #include <abstractions/nameservice>
+
+   capability kill,
+   capability net_bind_service,
+   capability setgid,
+   capability setuid,
+   capability sys_tty_config,
+
+   /      r,
+   /**/   r,
+   /var/www/** r,
+
+   #
+   # Separate hat policies
+   #
+   ^DEFAULT_URI {
+     ...
+     /      r,
+     /**/   r,
+     /var/www/** r,
+   }
+
+   ^HANDLING_UNTRUSTED_INPUT {
+     ...
+     /      r,
+     /**/   r,
+   }
+
+   ^foo {
+     ...
+     /usr/lib/cgi-bin/foo.gci ixr,
+   }
+
+   ^bar {
+     ...
+     /var/lib/bar/bar.py{,c} ixr,
+   }
+ }
+```
+
+Because defining hats for every URI/URL often becomes tedious,
+mod\_apparmor provides the AAHatName and AADefaultHatName Apache
+configuration options.
+
+#### AAHatName
+
+AAHatName allows you to specify a hat to be used for a given Apache
+\<Directory\>, \<DirectoryMatch\>, \<Location\> or \<LocationMatch\>
+directive (see the Apache documentation for more details). Note
+that mod\_apparmor behavior can become confused if \<Directory\*\>
+and \<Location\*\> directives are intermingled and it is recommended
+to use one type of directive. If the hat specified by AAHatName does
+not exist in the Apache profile, then it falls back to the behavior
+described above.
+
+#### AADefaultHatName
+
+AADefaultHatName allows you to specify a default hat to be used for
+virtual hosts and other Apache server directives, so that you can have
+different defaults for different virtual hosts. This can be overridden
+by the AAHatName directive and is checked for only if there isn't a
+matching AAHatName or hat named by the URI. If the AADefaultHatName
+hat does not exist, it falls back to the DEFAULT\_URI hat if it exists
+(as described above).
+
+Using mod\_apparmor
+===================
+
+Due to the flexibility of mod\_apparmor, it might be unclear what
+mod\_apparmor is doing behind the scenes. Put simply, on each URI
+request, mod\_apparmor will:
+
+-   change\_hat() into ^HANDLING\_UNTRUSTED\_INPUT (if it exists) when performing the initial parsing of the request
+-   after performing the initial parsing of the request mod\_apparmor will:
+    1.  try to change\_hat() into a matching AAHatName hat if it exists and applies, otherwise
+    2.  try to change\_hat() into the URI itself, otherwise
+    3.  try to change\_hat() into an AADefaultHatName hat if it has been defined for the server/vhost, otherwise
+    4.  try to change\_hat() into the DEFAULT\_URI hat, if it exists, otherwise
+    5.  fallback to the global Apache policy
+
+Troublshooting
+--------------
+
+**TODO**: debugging options for seeing what AppArmor and Apache are doing
+
+Examples
+--------
+
+See ['Using mod\_apparmor with Apache to confine web
+applications'](mod_apparmor_example) for an example of how to use
+AppArmor to confine web applications with a trusted Apache daemon.
+
+Caveats
+=======
+
+mod\_apparmor() currently only supports apache2, and has only been
+tested with the prefork MPM configuration. Threaded configurations of
+Apache may not work correctly. Additionally, due to how change\_hat()
+is implemented, if there is a bug it is theoretically possible for
+a confined subprofile to break out of subprofile confinement (but
+still be confined by the global Apache profile).