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<title>Environment Variables in Apache</title>

<summary>

<p>The Apache HTTP Server provides a mechanism for storing

information in named variables that are called <em>environment

variables</em>. 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.</p>

<p>Although these variables are referred to as <em>environment

variables</em>, 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.</p>

</summary>

<section id="setting">

<title>Setting Environment Variables</title>

<related>

<modulelist>

<module>mod_env</module>

<module>mod_rewrite</module>

<module>mod_setenvif</module>

<module>mod_unique_id</module>

</modulelist>

<directivelist>

<directive module="mod_setenvif">BrowserMatch</directive>

<directive module="mod_setenvif">BrowserMatchNoCase</directive>

<directive module="mod_env">PassEnv</directive>

<directive module="mod_rewrite">RewriteRule</directive>

<directive module="mod_env">SetEnv</directive>

<directive module="mod_setenvif">SetEnvIf</directive>

<directive module="mod_setenvif">SetEnvIfNoCase</directive>

<directive module="mod_env">UnsetEnv</directive>

</directivelist>

</related>

<section id="basic-manipulation">

<title>Basic Environment Manipulation</title>

<p>The most basic way to set an environment variable in Apache

is using the unconditional <directive module="mod_env"

>SetEnv</directive> directive. Variables may also be passed from

the environment of the shell which started the server using the

<directive module="mod_env">PassEnv</directive> directive.</p>

</section>

<section id="conditional">

<title>Conditional Per-Request Settings</title>

<p>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 <directive

module="mod_rewrite">RewriteRule</directive> which uses the

<code>[E=...]</code> option to set environment variables.</p>

</section>

<section id="unique-identifiers">

<title>Unique Identifiers</title>

<p>Finally, mod_unique_id sets the environment variable

<code>UNIQUE_ID</code> for each request to a value which is

guaranteed to be unique across "all" requests under very

specific conditions.</p>

</section>

<section id="standard-cgi">

<title>Standard CGI Variables</title>

<p>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 <a href="http://cgi-spec.golux.com/">CGI

specification</a>.</p>

</section>

<section id="caveats">

<title>Some Caveats</title>

<ul>

<li>It is not possible to override or change the standard CGI

variables using the environment manipulation directives.</li>

<li>When <program>suexec</program> is used to launch

CGI scripts, the environment will be cleaned down to a set of

<em>safe</em> variables before CGI scripts are launched. The

list of <em>safe</em> variables is defined at compile-time in

<code>suexec.c</code>.</li>

<li>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.</li>

</ul>

</section>

</section>

<section id="using">

<title>Using Environment Variables</title>

<related>

<modulelist>

<module>mod_authz_host</module>

<module>mod_cgi</module>

<module>mod_ext_filter</module>

<module>mod_headers</module>

<module>mod_include</module>

<module>mod_log_config</module>

<module>mod_rewrite</module>

</modulelist>

<directivelist>

<directive module="mod_authz_host">Allow</directive>

<directive module="mod_log_config">CustomLog</directive>

<directive module="mod_authz_host">Deny</directive>

<directive module="mod_ext_filter">ExtFilterDefine</directive>

<directive module="mod_headers">Header</directive>

<directive module="mod_log_config">LogFormat</directive>

<directive module="mod_rewrite">RewriteCond</directive>

<directive module="mod_rewrite">RewriteRule</directive>

</directivelist>

</related>

<section id="cgi-scripts">

<title>CGI Scripts</title>

<p>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

<a href="howto/cgi.html">CGI tutorial</a>.</p>

</section>

<section id="ssi-pages">

<title>SSI Pages</title>

<p>Server-parsed (SSI) documents processed by mod_include's

<code>INCLUDES</code> filter can print environment variables

using the <code>echo</code> 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 <a

href="howto/ssi.html">SSI tutorial</a>.</p>

</section>

<section id="access-control">

<title>Access Control</title>

<p>Access to the server can be controlled based on the value of

environment variables using the <code>allow from env=</code>

and <code>deny from env=</code> directives. In combination with

<directive module="mod_setenvif">SetEnvIf</directive>, 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).

</p>

</section>

<section id="logging">

<title>Conditional Logging</title>

<p>Environment variables can be logged in the access log using

the <directive module="mod_log_config">LogFormat</directive>

option <code>%e</code>. 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

<directive module="mod_log_config">CustomLog</directive>

directive. In combination with <directive module="mod_setenvif"

>SetEnvIf</directive> this allows for flexible control of which

requests are logged. For example, you can choose not to log

requests for filenames ending in <code>gif</code>, or you can

choose to only log requests from clients which are outside your

subnet.</p>

</section>

<section id="response-headers">

<title>Conditional Response Headers</title>

<p>The <directive module="mod_headers">Header</directive>

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.</p>

</section>

<section id="external-filter">

<title>External Filter Activation</title>

<p>External filters configured by <module>mod_ext_filter</module>

using the <directive

module="mod_ext_filter">ExtFilterDefine</directive> directive can

by activated conditional on an environment variable using the

<code>disableenv=</code> and <code>enableenv=</code> options.</p>

</section>

<section id="url-rewriting">

<title>URL Rewriting</title>

<p>The <code>%{ENV:...}</code> form of <em>TestString</em> in

the <directive module="mod_rewrite">RewriteCond</directive>

allows mod_rewrite's rewrite

engine to make decisions conditional on environment variables.

Note that the variables accessible in mod_rewrite without the

<code>ENV:</code> prefix are not actually environment

variables. Rather, they are variables special to mod_rewrite

which cannot be accessed from other modules.</p>

</section>

</section>

<section id="special">

<title>Special Purpose Environment Variables</title>

<p>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 <directive module="mod_setenvif"

>BrowserMatch</directive>, though <directive module="mod_env"

>SetEnv</directive> and <directive module="mod_env"

>PassEnv</directive> could also be used, for example.</p>

<section id="downgrade">

<title>downgrade-1.0</title>

<p>This forces the request to be treated as a HTTP/1.0 request

even if it was in a later dialect.</p>

</section>

<section id="force-gzip">

<title>force-gzip</title>

<p>If you have the <code>DEFLATE</code> filter activated, this

environment variable will ignore the accept-encoding setting of

your browser and will send compressed output unconditionally.</p>

</section>

<section id="force-no-vary">

<title>force-no-vary</title>

<p>This causes any <code>Vary</code> 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 <strong>force-response-1.0</strong>.</p>

</section>

<section id="force-response">

<title>force-response-1.0</title>

<p>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.</p>

</section>

<section id="gzip-only-text-html">

<title>gzip-only-text/html</title>

<p>When set to a value of "1", this variable disables the DEFLATE

output filter provided by <module>mod_deflate</module> for

content-types other than <code>text/html</code>. If you'd rather

use statically compressed files; <module>mod_negotiation</module>

evaluates the variable as well (not only for gzip, but for all

encodings that differ from "identity").</p>

</section>

<section id="no-gzip"><title>no-gzip</title>

<p>When set, the <code>DEFLATE</code> filter of

<module>mod_deflate</module> will be turned off and

<module>mod_negotiation</module> will refuse to deliver encoded

resources.</p>

</section>

<section id="nokeepalive">

<title>nokeepalive</title>

<p>This disables <directive module="core">KeepAlive</directive>

when set.</p>

</section>

<section id="prefer-language"><title>prefer-language</title>

<p>This influences <module>mod_negotiation</module>'s behaviour. If

it contains a language tag (such as <code>en</code>, <code>ja</code>

or <code>x-klingon</code>), <module>mod_negotiation</module> tries

to deliver a variant with that language. If there's no such variant,

the normal <a href="content-negotiation.html">negotiation</a> process

applies.</p>

</section>

<section id="redirect-carefully">

<title>redirect-carefully</title>

<p>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.</p>

</section>

<section id="suppress-error-charset">

<title>suppress-error-charset</title>

<p><em>Available in versions after 2.0.54</em></p>

<p>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.</p>

<p> 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.</p>

<p>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.</p>

</section>

<section id="proxy"><title>force-proxy-request-1.0, proxy-nokeepalive, proxy-sendchunked, proxy-sendcl</title>

<p>These directives alter the protocol behavior of

<module>mod_proxy</module>. See the <module>mod_proxy</module>

documentation for more details.</p>

</section>

</section>

<section id="examples">

<title>Examples</title>

<section id="misbehaving">

<title>Changing protocol behavior with misbehaving clients</title>

<p>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.</p>

<example><pre>

#

# 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</pre></example>

</section>

<section id="no-img-log">

<title>Do not log requests for images in the access log</title>

<p>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.</p>

<example><pre>

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</pre></example>

</section>

<section id="image-theft">

<title>Prevent "Image Theft"</title>

<p>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.</p>

<example><pre>

SetEnvIf Referer "^http://www\.example\.com/" local_referal

# Allow browsers that do not send Referer info

SetEnvIf Referer "^$" local_referal

&lt;Directory /web/images&gt;

Order Deny,Allow

Deny from all

Allow from env=local_referal

&lt;/Directory&gt;</pre></example>

<p>For more information about this technique, see the

ApacheToday tutorial " <a

href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">

Keeping Your Images from Adorning Other Sites</a>".</p>

</section>

</section>

</manualpage>