Environment Variables in Apache
There are two kinds of environment variables that affect the Apache HTTP Server.
First, there are the environment variables controlled by the underlying operating system. These are set before the server starts. They can be used in expansions in configuration files, and can optionally be passed to CGI scripts and SSI using the PassEnv directive.
Second, the Apache HTTP Server provides a mechanism for storing information in named variables that are also 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
suexecis 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.
- A special case are HTTP headers which are passed to CGI scripts and the like via environment variables (see below). They are converted to uppercase and only dashes are replaced with underscores; if the header contains any other (invalid) character, the whole header is silently dropped. See below for a workaround.
- The
SetEnvdirective runs late during request processing meaning that directives such asSetEnvIfandRewriteCondwill not see the variables set with it. - When the server looks up a path via an internal
subrequest such as looking
for a
DirectoryIndexor generating a directory listing withmod_autoindex, per-request environment variables are not inherited in the subrequest. Additionally,SetEnvIfdirectives are not separately evaluated in the subrequest due to the API phasesmod_setenviftakes action in.
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-gzip
If you have the DEFLATE filter activated, this
environment variable will ignore the accept-encoding setting of
your browser and will send compressed output unconditionally.
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; 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. If you'd rather
use statically compressed files, mod_negotiation
evaluates the variable as well (not only for gzip, but for all
encodings that differ from "identity").
no-gzip
When set, the DEFLATE filter of
mod_deflate will be turned off and
mod_negotiation will refuse to deliver encoded
resources.
no-cache
Available in versions 2.2.12 and later
When set, mod_cache will not save an otherwise
cacheable response. This environment variable does not influence
whether a response already in the cache will be served for the current
request.
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.
Security note
Sending error pages without a specified character set may allow a cross-site-scripting attack for existing browsers (MSIE) which do not follow the HTTP/1.1 specification and attempt to "guess" the character set from the content. Such browsers can be easily fooled into using the UTF-7 character set, and UTF-7 content from input data (such as the request-URI) will not be escaped by the usual escaping mechanisms designed to prevent cross-site-scripting attacks.
force-proxy-request-1.0, proxy-nokeepalive, proxy-sendchunked, proxy-sendcl, proxy-chain-auth, proxy-interim-response, proxy-initial-not-pooled
These directives alter the protocol behavior of
mod_proxy. See the mod_proxy and mod_proxy_http
documentation for more details.
Examples
Passing broken headers to CGI scripts
Starting with version 2.4, Apache is more strict about how HTTP
headers are converted to environment variables in mod_cgi
and other modules: Previously any invalid characters
in header names were simply translated to underscores. This allowed
for some potential cross-site-scripting attacks via header injection
(see
Unusual Web Bugs, slide 19/20).
If you have to support a client which sends broken headers and
which can't be fixed, a simple workaround involving mod_setenvif
and mod_headers allows you to still accept
these headers:
#
# The following works around a client sending a broken Accept_Encoding
# header.
#
SetEnvIfNoCase ^Accept.Encoding$ ^(.*)$ fix_accept_encoding=$1
RequestHeader set Accept-Encoding %{fix_accept_encoding}e env=fix_accept_encoding
Changing protocol behavior with misbehaving clients
Earlier versions recommended that the following lines be included in httpd.conf to deal with known client problems. Since the affected clients are no longer seen in the wild, this configuration is likely no-longer necessary.
# # 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 understand 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">
Require env local_referal
</Directory>
For more information about this technique, see the "Keeping Your Images from Adorning Other Sites" tutorial on ServerWatch.