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The Apache HTTP Server Reference Manual
by Apache Software Foundation
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3.19  Apache Module mod_authz_host



Description:

Group authorizations based on host (name or IP address)

Status:

Base

Module Identifier:

authz_host_module

Source File:

mod_authz_host.c

Compatibility:

Available in Apache 2.1 and later



Summary

The directives provided by mod_authz_host are used in <Directory>, <Files>, and <Location> sections as well as .htaccess (p. 185) 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 (p. 1471). 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.

Directives:

Allow

Deny

Order

See also:

Allow Directive

Description:

Controls which hosts can access an area of the server

Syntax:

Allow from all|host|env=[!]env-variable [host|env=[!]env-variable]

Context:

directory, .htaccess

Override:

Limit

Status:

Base

Module:

mod_authz_host

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
Allow from .net example.edu

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
Allow from 192.168.1.104 192.168.1.205

An IP address of a host allowed access

A partial IP address

Example:

Allow from 10.1
Allow from 10 172.20 192.168.2

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 2001:db8::a00:20ff:fea7:ccea
Allow from 2001:db8::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 (p. 1471). When Allow from env=env-variable is specified, then the request is allowed access if the environment variable env-variable exists. When Allow from env=!env-variable is specified, then the request is allowed access if the environment variable env-variable doesn’t exist. 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.

Deny Directive

Description:

Controls which hosts are denied access to the server

Syntax:

Deny from all|host|env=[!]env-variable [host|env=[!]env-variable]

Context:

directory, .htaccess

Override:

Limit

Status:

Base

Module:

mod_authz_host

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.

Order Directive

Description:

Controls the default access state and the order in which Allow and Deny are evaluated.

Syntax:

Order ordering

Default:

Order Deny,Allow

Context:

directory, .htaccess

Override:

Limit

Status:

Base

Module:

mod_authz_host

The Order directive, along with the Allow and Deny directives, controls a three-pass access control system. The first pass processes either all Allow or all Deny directives, as specified by the Order directive. The second pass parses the rest of the directives (Deny or Allow). The third pass applies to all requests which do not match either of the first two.

Note that all Allow and Deny directives are processed, unlike a typical firewall, where only the first match is used. The last match is effective (also unlike a typical firewall). Additionally, the order in which lines appear in the configuration files is not significant – all Allow lines are processed as one group, all Deny lines are considered as another, and the default state is considered by itself.

Ordering is one of:

Allow,Deny

First, all Allow directives are evaluated; at least one must match, or the request is rejected. Next, all Deny directives are evaluated. If any matches, the request is rejected. Last, any requests which do not match an Allow or a Deny directive are denied by default.

Deny,Allow

First, all Deny directives are evaluated; if any match, the request is denied unless it also matches an Allow directive. Any requests which do not match any Allow or Deny directives are permitted.

Mutual-failure

This order has the same effect as Order Allow,Deny and is deprecated in its favor.

Keywords may only be separated by a comma; no whitespace is allowed between them.




Match

Allow,Deny result

Deny,Allow result




Match Allow only

Request allowed

Request allowed

Match Deny only

Request denied

Request denied

No match

Default to second directive: Denied

Default to second directive: Allowed

Match both Allow & Deny

Final match controls: Denied

Final match controls: Allowed




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 is 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 is 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 (p. 110).

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