Apache Module mod_access
Summary
The directives provided by mod_access are used
in <Directory>,
<Files>, and
<Location> sections
as well as .htaccess
files to control access to particular parts of the server. Access
can be controlled based on the client hostname, IP address, or
other characteristics of the client request, as captured in environment variables. The Allow and Deny directives are used to
specify which clients are or are not allowed access to the server,
while the Order
directive sets the default access state, and configures how the
Allow and Deny directives interact with each
other.
Both host-based access restrictions and password-based
authentication may be implemented simultaneously. In that case,
the Satisfy directive is used
to determine how the two sets of restrictions interact.
In general, access restriction directives apply to all
access methods (GET, PUT,
POST, etc). This is the desired behavior in most
cases. However, it is possible to restrict some methods, while
leaving other methods unrestricted, by enclosing the directives
in a <Limit> section.
The Allow directive affects which hosts can
access an area of the server. Access can be controlled by
hostname, IP Address, IP Address range, or by other
characteristics of the client request captured in environment
variables.
The first argument to this directive is always
from. The subsequent arguments can take three
different forms. If Allow from all is specified, then
all hosts are allowed access, subject to the configuration of the
Deny and Order directives as discussed
below. To allow only particular hosts or groups of hosts to access
the server, the host can be specified in any of the
following formats:
- A (partial) domain-name
-
Example:
Allow from apache.org
Hosts whose names match, or end in, this string are allowed
access. Only complete components are matched, so the above
example will match foo.apache.org but it will not
match fooapache.org. This configuration will cause
Apache to perform a double reverse DNS lookup on the client IP
address, regardless of the setting of the HostnameLookups directive. It will do
a reverse DNS lookup on the IP address to find the associated
hostname, and then do a forward lookup on the hostname to assure
that it matches the original IP address. Only if the forward
and reverse DNS are consistent and the hostname matches will
access be allowed.
- A full IP address
-
Example:
Allow from 10.1.2.3
An IP address of a host allowed access
- A partial IP address
-
The first 1 to 3 bytes of an IP address, for subnet
restriction.
- A network/netmask pair
-
Example:
Allow from 10.1.0.0/255.255.0.0
A network a.b.c.d, and a netmask w.x.y.z. For more
fine-grained subnet restriction.
- A network/nnn CIDR specification
-
Example:
Allow from 10.1.0.0/16
Similar to the previous case, except the netmask consists of
nnn high-order 1 bits.
Note that the last three examples above match exactly the
same set of hosts.
IPv6 addresses and IPv6 subnets can be specified as shown
below:
Allow from fe80::a00:20ff:fea7:ccea
Allow from fe80::a00:20ff:fea7:ccea/10
The third format of the arguments to the
Allow directive allows access to the server
to be controlled based on the existence of an environment variable. When Allow from
env=env-variable is specified, then the request is
allowed access if the environment variable env-variable
exists. The server provides the ability to set environment
variables in a flexible way based on characteristics of the client
request using the directives provided by
mod_setenvif. Therefore, this directive can be
used to allow access based on such factors as the clients
User-Agent (browser type), Referer, or
other HTTP request header fields.
Example:
SetEnvIf User-Agent ^KnockKnock/2\.0 let_me_in
<Directory /docroot>
Order Deny,Allow
Deny from all
Allow from env=let_me_in
</Directory>
In this case, browsers with a user-agent string beginning
with KnockKnock/2.0 will be allowed access, and all
others will be denied.
This directive allows access to the server to be restricted
based on hostname, IP address, or environment variables. The
arguments for the Deny directive are
identical to the arguments for the Allow directive.
The Order directive controls the default
access state and the order in which Allow and Deny directives are evaluated.
Ordering is one of
Deny,Allow- The
Deny directives
are evaluated before the Allow directives. Access is
allowed by default. Any client which does not match a
Deny directive or does
match an Allow
directive will be allowed access to the server.
Allow,Deny- The
Allow
directives are evaluated before the Deny directives. Access is denied
by default. Any client which does not match an Allow directive or does match a
Deny directive will be
denied access to the server.
Mutual-failure- Only those hosts which appear on the
Allow list and do not appear on
the Deny list are
granted access. This ordering has the same effect as Order
Allow,Deny and is deprecated in favor of that
configuration.
Keywords may only be separated by a comma; no whitespace is
allowed between them. Note that in all cases every Allow and Deny statement is evaluated.
In the following example, all hosts in the apache.org domain
are allowed access; all other hosts are denied access.
Order Deny,Allow
Deny from all
Allow from apache.org
In the next example, all hosts in the apache.org domain are
allowed access, except for the hosts which are in the
foo.apache.org subdomain, who are denied access. All hosts not
in the apache.org domain are denied access because the default
state is to deny access to the server.
Order Allow,Deny
Allow from apache.org
Deny from foo.apache.org
On the other hand, if the Order in the last
example is changed to Deny,Allow, all hosts will
be allowed access. This happens because, regardless of the
actual ordering of the directives in the configuration file,
the Allow from apache.org will be evaluated last
and will override the Deny from foo.apache.org.
All hosts not in the apache.org domain will also
be allowed access because the default state will change to
allow.
The presence of an Order directive can affect
access to a part of the server even in the absence of accompanying
Allow and Deny directives because of its effect
on the default access state. For example,
<Directory /www>
Order Allow,Deny
</Directory>
will deny all access to the /www directory
because the default access state will be set to
deny.
The Order directive controls the order of access
directive processing only within each phase of the server's
configuration processing. This implies, for example, that an
Allow or Deny directive occurring in a
<Location> section will
always be evaluated after an Allow or Deny directive occurring in a
<Directory> section or
.htaccess file, regardless of the setting of the
Order directive. For details on the merging
of configuration sections, see the documentation on How Directory, Location and Files sections
work.
