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Apache HTTP Server Version 2.4
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 calledenvironment 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 asenvironment 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 VariablesUsing Environment VariablesSpecial Purpose Environment VariablesExamples
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The most basic way to set an environment variable in Apache is using the unconditionalSetEnv directive. Variables may also be passed from the environment of the shell which started the server using thePassEnv directive.
For additional flexibility, the directives provided bymod_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 themod_rewrite'sRewriteRule which uses the[E=...] option to set environment variables.
Finally,mod_unique_id sets the environment variableUNIQUE_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 bytheCGI specification.
suexec is used to launch CGI scripts, the environment will be cleaned down to a set ofsafe variables before CGI scripts are launched. The list ofsafe variables is defined at compile-time insuexec.c.SetEnv directive runs late during request processing meaning that directives such asSetEnvIf andRewriteCond will not see the variables set with it.DirectoryIndex or generating a directory listing withmod_autoindex, per-request environment variables arenot inherited in the subrequest. Additionally,SetEnvIf directives are not separately evaluated in the subrequest due to the API phasesmod_setenvif takes action in.| Related Modules | Related Directives |
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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 theCGI tutorial.
Server-parsed (SSI) documents processed bymod_include'sINCLUDES filter can print environment variables using theecho 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 theSSI tutorial.
Access to the server can be controlled based on environment variables using theRequire env andRequire not env directives. In combination withSetEnvIf, 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 theLogFormat 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 theCustomLog directive. In combination withSetEnvIf this allows for flexible control of which requests are logged. For example, you can choose not to log requests for filenames ending ingif, or you can choose to only log requests from clients which are outside your subnet.
TheHeader 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 filters configured bymod_ext_filter using theExtFilterDefine directive can by activated conditional on an environment variable using thedisableenv= andenableenv= options.
The%{ENV:variable} form ofTestString in theRewriteCond allowsmod_rewrite's rewrite engine to make decisions conditional on environment variables. Note that the variables accessible inmod_rewrite without theENV: prefix are not actually environment variables. Rather, they are variables special tomod_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 withBrowserMatch, thoughSetEnv andPassEnv 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.
If you have theDEFLATE filter activated, this environment variable will ignore the accept-encoding setting of your browser and will send compressed output unconditionally.
This causes anyVary 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 impliesforce-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.
When set to a value of "1", this variable disables theDEFLATE output filter provided bymod_deflate for content-types other thantext/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").
When set, theDEFLATE filter ofmod_deflate will be turned off andmod_negotiation will refuse to deliver encoded resources.
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.
This disablesKeepAlive when set.
This influencesmod_negotiation's behaviour. If it contains a language tag (such asen,ja orx-klingon),mod_negotiation tries to deliver a variant with that language. If there's no such variant, the normalnegotiation process applies.
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.
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.
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.
These directives alter the protocol behavior ofmod_proxy. See themod_proxy andmod_proxy_http documentation for more details.
Available in 2.4.59 and later
This variable allows a script running in CGI-like module to supply it's own Content-Length HTTP response header. It should only be set on configuration sections that contain trusted scripts.
Starting with version 2.4, Apache is more strict about how HTTP headers are converted to environment variables inmod_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 involvingmod_setenvif andmod_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=$1RequestHeader set Accept-Encoding %{fix_accept_encoding}e env=fix_accept_encodingEarlier versions recommended that the following lines be included in httpd.conf to deal with known client problems. Since the affected clients are no longer seen in the wild, this configuration is likely no-longer necessary.
## The following directives modify normal HTTP response behavior.# The first directive disables keepalive for Netscape 2.x and browsers that# spoof it. There are known problems with these browser implementations.# The second directive is for Microsoft Internet Explorer 4.0b2# which has a broken HTTP/1.1 implementation and does not properly# support keepalive when it is used on 301 or 302 (redirect) responses.#BrowserMatch "Mozilla/2" nokeepaliveBrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0## The following directive disables HTTP/1.1 responses to browsers which# are in violation of the HTTP/1.0 spec by not being able to understand a# basic 1.1 response.#BrowserMatch "RealPlayer 4\.0" force-response-1.0BrowserMatch "Java/1\.0" force-response-1.0BrowserMatch "JDK/1\.0" force-response-1.0
This example keeps requests for images from appearing in the access log. It can be easily modified to prevent logging of particular directories, or to prevent logging of requests coming from particular hosts.
SetEnvIf Request_URI \.gif image-requestSetEnvIf Request_URI \.jpg image-requestSetEnvIf Request_URI \.png image-requestCustomLog "logs/access_log" common env=!image-request
This example shows how to keep people not on your server from using images on your server as inline-images on their pages. This is not a recommended configuration, but it can work in limited circumstances. We assume that all your images are in a directory called/web/images.
SetEnvIf Referer "^http://www\.example\.com/" local_referal# Allow browsers that do not send Referer infoSetEnvIf Referer "^$" 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.
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Licensed under theApache License, Version 2.0.