Apache HTTP Server

Environment Variables in Apache

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.

• Setting Environment Variables
• Using Environment Variables
• Special Purpose Environment Variables
• Examples

Setting Environment Variables

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.

Using Environment Variables

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.

Special Purpose 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.

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.

Examples

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