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.
Setting Environment Variables
Related Modules | Related Directives |
---|---|
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 insuexec.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.
- The
SetEnv
directive runs late during request processing meaning that directives such asSetEnvIf
andRewriteCond
will not see the variables set with it.
Using Environment Variables
Related Modules | Related Directives |
---|---|
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
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 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.
Conditional Response Headers
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.
External Filter Activation
External filters configured by mod_ext_filter
using the ExtFilterDefine
directive can
by activated conditional on an environment variable using the
disableenv=
and enableenv=
options.
URL Rewriting
The %{ENV:variable}
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 to clients making an HTTP/1.0 request. It was originally implemented as a result of a problem with AOL's proxies. Some HTTP/1.0 clients may not behave correctly when given an HTTP/1.1 response, and this can be used to interoperate with them.
gzip-only-text/html
When set to a value of "1", this variable disables the
DEFLATE
output filter provided by
mod_deflate
for content-types other than
text/html
.
no-gzip
When set, the DEFLATE
filter of
mod_deflate
will be turned off.
nokeepalive
This disables KeepAlive
when set.
prefer-language
This influences mod_negotiation
's behaviour. If
it contains a language tag (such as en
, ja
or x-klingon
), mod_negotiation
tries
to deliver a variant with that language. If there's no such variant,
the normal negotiation process
applies.
redirect-carefully
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.
suppress-error-charset
Available in versions after 2.0.54
When Apache issues a redirect in response to a client request, the response includes some actual text to be displayed in case the client can't (or doesn't) automatically follow the redirection. Apache ordinarily labels this text according to the character set which it uses, which is ISO-8859-1.
However, if the redirection is to a page that uses a different character set, some broken browser versions will try to use the character set from the redirection text rather than the actual page. This can result in Greek, for instance, being incorrectly rendered.
Setting this environment variable causes Apache to omit the character set for the redirection text, and these broken browsers will then correctly use that of the destination page.
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 common 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>
For more information about this technique, see the "Keeping Your Images from Adorning Other Sites" tutorial on ServerWatch.