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.
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Related Modules mod_env mod_rewrite mod_setenvif mod_unique_id |
Related Directives BrowserMatch BrowserMatchNoCase PassEnv RewriteRule SetEnv SetEnvIf SetEnvIfNoCase UnsetEnv |
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 of the shell which started
the server using the PassEnv directive.
For additional flexibility, the directives provided by mod_setenvif
allow environment variables to be set on a per-request basis,
conditional 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 [sic] 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.
In addition to all environment variables set within the Apache configuration and passed from the shell, CGI scripts and SSI pages are provided with a set of environment variables containing meta-information about the request as required by the CGI specification.
suexec.c.|
Related Modules mod_access mod_cgi mod_include mod_log_config mod_rewrite |
Related Directives Allow CustomLog Deny LogFormat RewriteCond RewriteRule |
One of the primary uses of environment variables is to communicate information to CGI scripts. As discussed above, the environment passed to CGI scripts includes standard meta-information about the request in addition to any variables set within the Apache configuration. For more details, see the CGI tutorial.
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 to makes parts of a page conditional on
characteristics of a request. Apache also provides SSI pages with the
standard CGI environment variables as discussed above. For more
details, see the SSI tutorial.
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. Note that the
variables accessible in mod_rewrite without the ENV:
prefix are not actually environment variables. Rather, they
are variables special to mod_rewrite which cannot be accessed from
other modules.
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>
Note: spelling of 'referer' and 'referal' is intentional.
For more information about this technique, see the ApacheToday tutorial "Keeping Your Images from Adorning Other Sites".