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.
| 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_headers mod_include mod_log_config mod_rewrite |
Related Directives Allow CustomLog Deny Header 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
INCLUDES filter 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 Header directive can use the presence or
absence of an environment variable to determine whether or not
a certain HTTP header will be placed in the response to the
client. This allows, for example, a certain response header to
be sent only if a corresponding header is received in the
request from the client.
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.
This forces the server to be more careful when sending a redirect to the client. This is typically used when a client has a known problem handling redirects. This was originally implemented as a result of a problem with Microsoft's WebFolders software which has a problem handling redirects on directory resources via DAV methods.
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".