5.6. Order, Allow, and Deny
So far we have dealt with potential users on an individual basis. We can also allow access from or deny access to specific IP addresses, hostnames, or groups of addresses and hostnames. The commands are allow from and deny from.
The order in which the allow and deny commands are applied is not set by the order in which they appear in your file. The default order is deny then allow: if a client is excluded by deny, it is excluded unless it matches allow. If neither is matched, the client is granted access.
The order in which these commands is applied can be set by the order directive.
5.6.1. Allow from
allow from host host ... Directory, .htaccess
The allow directive controls access to a directory. The argument host can be one of the following:
- all
All hosts are allowed access.
- A (partial) domain name
All hosts whose names match or end in this string are allowed access.
- A full IP address
The first one to three bytes of an IP address, for subnet restriction.
- A network/netmask pair
Network a.b.c.d and netmask w.x.y.z, to give finer-grained subnet control. For instance, 10.1.0.0/255.255.0.0.
- A network CIDR specification
The netmask consists of nnn high-order 1-bits. For instance, 10.1.0.0/16 is the same as 10.1.0.0/255.255.0.0.
5.6.2. Allow from env
allow from env=variablename ... Directory, .htaccess
The allow from env directive controls access by the existence of a named environment variable. For instance:
BrowserMatch ^KnockKnock/2.0 let_me_in <Directory /docroot> order deny,allow deny from all allow from env=let_me_in </Directory>
Access by a browser called KnockKnock v2.0 sets an environment variable let_me_in, which in turn triggers allow from.
5.6.3. Deny from
deny from host host ... Directory, .htaccess
The deny from directive controls access by host. The argument host can be one of the following:
- all
All hosts are denied access.
- A (partial) domain name
All hosts whose names match or end in this string are denied access.
- A full IP address
The first one to three bytes of an IP address, for subnet restriction.
- A network/netmask pair
Network a.b.c.d and netmask w.x.y.z, to give finer-grained subnet control. For instance, 10.1.0.0/255.255.0.0.
- A network CIDR specification
The netmask consists of nnn high-order 1-bits. For instance, 10.1.0.0/16 is the same as 10.1.0.0/255.255.0.0.
5.6.4. Deny from env
deny from env=variablename ... Directory, .htaccess
The deny from env directive controls access by the existence of a named environment variable. For instance:
BrowserMatch ^BadRobot/0.9 go_away <Directory /docroot> order allow,deny allow from all deny from env=go_away </Directory>
Access by a browser called BadRobot v0.9 sets an environment variable go_away, which in turn triggers deny from.
5.6.5. Order
order ordering Directory, .htaccess
The ordering argument is one word (i.e., it is not allowed to contain a space) and controls the order in which the foregoing directives are applied. If two order directives apply to the same host, the last one to be evaluated prevails:
- deny,allow
The deny directives are evaluated before the allow directives.
- allow,deny
The allow directives are evaluated before the denys.
- mutual-failure
Hosts that appear on the allow list and do not appear on the deny list are allowed access.
We could say:
allow from all
which lets everyone in and is hardly worth writing, or we could say:
allow from 123.156 deny from all
As it stands, this denies everyone except those whose IP addresses happen to start with 123.156. In other words, allow is applied last and carries the day. If, however, we changed the default order by saying:
order allow,deny allow from 123.156 deny from all
we effectively close the site because deny is now applied last. It is also possible to use domain names, so that instead of:
deny from 123.156.3.5
you could say:
deny from badguys.com
Although this has the advantage of keeping up with the Bad Guys as they move from one IP address to another, it also allows access by people who control the reverse-DNS mapping for their IP addresses.
A URL can be partial. In this case, the match is done on whole words from the right. That is, allow from fred.com allows fred.com and abc.fred.com, but not notfred.com.
Good intentions, however, are not enough: before conferring any trust in a set of access rules, you want to test those rules thoroughly in the privacy of the boudoir.[46]
[46]Boudoir is French for "a place where you pout" -- you may have reason to do so before you've finished with all this.
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