Environment Variables in Apache
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Apache HTTP Server
Environment Variables in Apache
The Apache HTTP Server provides a mechanism for storing
information in named variables that are called environment
variables. This information can be used to control various
operations such as logging or access control. The variables are
also used as a mechanism to communicate with external programs
such as CGI scripts. This document discusses different ways to
manipulate and use these variables.
Although these variables are referred to as environment
variables, they are not the same as the environment
variables controlled by the underlying operating system.
Instead, these variables are stored and manipulated in an
internal Apache structure. They only become actual operating
system environment variables when they are provided to CGI
scripts and Server Side Include scripts. If you wish to
manipulate the operating system environment under which the
server itself runs, you must use the standard environment
manipulation mechanisms provided by your operating system
shell.
Basic Environment Manipulation
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.
Conditional Per-Request Settings
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.
Unique Identifiers
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.
Standard CGI Variables
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.
Some Caveats
- It is not possible to override or change the standard CGI
variables using the environment manipulation directives.
- When suexec is used to launch
CGI scripts, the environment will be cleaned down to a set of
safe variables before CGI scripts are launched. The
list of safe variables is defined at compile-time in
suexec.c.
- For portability reasons, the names of environment
variables may contain only letters, numbers, and the
underscore character. In addition, the first character may
not be a number. Characters which do not match this
restriction will be replaced by an underscore when passed to
CGI scripts and SSI pages.
CGI Scripts
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.
SSI Pages
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 Control
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).
Conditional Logging
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.
URL Rewriting
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.
downgrade-1.0
This forces the request to be treated as a HTTP/1.0 request
even if it was in a later dialect.
force-no-vary
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.
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.
nokeepalive
This disables KeepAlive when set.
Changing protocol behavior with misbehaving clients
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
Do not log requests for images in the access log
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
Prevent "Image Theft"
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".
Apache HTTP Server
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