Many operating systems provide a facility for storage and transmission of information called environment variables. Apache uses environment variables in many ways to control operations and to communicate with other programs like CGI scripts. This document explains some of the ways to use environment variables in Apache.
The most basic way to set an environment variable in Apache is
using the unconditional SetEnv
directive. Variables
may also be passed from the environment when Apache is started
using the PassEnv
directive.
The directives provided by mod_setenvif allow environment variables
to be set on a per-request basis based on characteristics of particular
requests. For example, a variable could be set only when a specific
browser (User-Agent) is making a request, or only when a specific
Referer header is found. Even more flexibility is available through the
mod_rewrite's RewriteRule
which uses the
[E=...]
option to set environment variables.
Finally, mod_unique_id sets the environment variable
UNIQUE_ID
for each request to a value which is guaranteed
to be unique across "all" requests under very specific conditions.
One of the primary uses of environment variables is to communicate
information to CGI scripts. In addition to all environment variables
set within Apache, CGI scripts are provided with a set of
meta-information about the request as provided for in the CGI specification. If you are using
Suexec to execute CGI scripts under
different userids, note that the environment will be cleaned down to a
set of safe environment variables before the CGI script is
executed. The set of safe environment variables is defined at
compile time in suexec.c
.
Server-parsed (SSI) documents processed by mod_include's
server-parsed
handler can print environment variables
using the echo
element, and can use environment variables
in flow control elements.
Access to the server can be controlled based on the value of
environment variables using the allow from env=
and
deny from env=
directives. In combination with
SetEnvIf
, this allows for flexible control of access to
the server based on characteristics of the client. For example, you
can use these directives to deny access to a particular browser
(User-Agent).
Environment variables can be logged in the access log using the
LogFormat
option %e
. In addition, the
decision on whether or not to log requests can be made based on the
status of environment variables using the conditional form of the
CustomLog
directive. In combination with
SetEnvIf
this allows for flexible control of which
requests are logged. For example, you can choose not to log requests
for filenames ending in gif
, or you can choose to only
log requests from clients which are outside your subnet.
The %{ENV:...}
form of TestString in the
RewriteCond
allows mod_rewrite's rewrite engine to make
decisions conditional on environment variables.
Interoperability problems have led to the introduction of mechanisms to modify the way Apache behaves when talking to particular clients. To make these mechanisms as flexible as possible, they are invoked by defining environment variables, typically with BrowserMatch, though SetEnv and PassEnv could also be used, for example.
This forces the request to be treated as a HTTP/1.0 request even if it was in a later dialect.
This causes any Vary
fields to be removed from the response
header before it is sent back to the client. Some clients don't
interpret this field correctly (see the
known client problems
page); setting this variable can work around this problem. Setting
this variable also implies force-response-1.0.
This forces an HTTP/1.0 response when set. It was originally implemented as a result of a problem with AOL's proxies. Some clients may not behave correctly when given an HTTP/1.1 response, and this can be used to interoperate with them.
This disables KeepAlive when set.
We recommend that the following lines be included in httpd.conf to deal with known client problems.
# # The following directives modify normal HTTP response behavior. # The first directive disables keepalive for Netscape 2.x and browsers that # spoof it. There are known problems with these browser implementations. # The second directive is for Microsoft Internet Explorer 4.0b2 # which has a broken HTTP/1.1 implementation and does not properly # support keepalive when it is used on 301 or 302 (redirect) responses. # BrowserMatch "Mozilla/2" nokeepalive BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0 # # The following directive disables HTTP/1.1 responses to browsers which # are in violation of the HTTP/1.0 spec by not being able to grok a # basic 1.1 response. # BrowserMatch "RealPlayer 4\.0" force-response-1.0 BrowserMatch "Java/1\.0" force-response-1.0 BrowserMatch "JDK/1\.0" force-response-1.0
This example keeps requests for images from appearing in the access log. It can be easily modified to prevent logging of particular directories, or to prevent logging of requests coming from particular hosts.
SetEnvIf Request_URI \.gif image-request SetEnvIf Request_URI \.jpg image-request SetEnvIf Request_URI \.png image-request CustomLog logs/access_log env=!image-request
This example shows how to keep people not on your server from using images on your server as inline-images on their pages. This is not a recommended configuration, but it can work in limited circumstances. We assume that all your images are in a directory called /web/images.
SetEnvIf Referer "^http://www.example.com/" local_referal # Allow browsers that do not send Referer info SetEnvIf Referer "^$" local_referal <Directory /web/images> Order Deny,Allow Deny from all Allow from env=local_referal </Directory>
For more information about this technique, see the ApacheToday tutorial "Keeping Your Images from Adorning Other Sites".