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7 --><title>Environment Variables in Apache - Apache HTTP Server</title><link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link href="./images/favicon.ico" rel="shortcut icon" /></head><body id="manual-page"><div id="page-header"><p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.0</p><img alt="" src="./images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="./">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>Environment Variables in Apache</h1>
8 <p>The Apache HTTP Server provides a mechanism for storing
9 information in named variables that are called <em>environment
10 variables</em>. This information can be used to control various
11 operations such as logging or access control. The variables are
12 also used as a mechanism to communicate with external programs
13 such as CGI scripts. This document discusses different ways to
14 manipulate and use these variables.</p>
16 <p>Although these variables are referred to as <em>environment
17 variables</em>, they are not the same as the environment
18 variables controlled by the underlying operating system.
19 Instead, these variables are stored and manipulated in an
20 internal Apache structure. They only become actual operating
21 system environment variables when they are provided to CGI
22 scripts and Server Side Include scripts. If you wish to
23 manipulate the operating system environment under which the
24 server itself runs, you must use the standard environment
25 manipulation mechanisms provided by your operating system
27 </div><div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#setting">Setting Environment Variables</a></li><li><img alt="" src="./images/down.gif" /> <a href="#using">Using Environment Variables</a></li><li><img alt="" src="./images/down.gif" /> <a href="#special">Special Purpose Environment Variables</a></li><li><img alt="" src="./images/down.gif" /> <a href="#examples">Examples</a></li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div><div class="section"><h2><a name="setting" id="setting">Setting Environment Variables</a></h2>
29 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_env.html">mod_env</a></code></li><li><code class="module"><a href="./mod/mod_rewrite.html">mod_rewrite</a></code></li><li><code class="module"><a href="./mod/mod_setenvif.html">mod_setenvif</a></code></li><li><code class="module"><a href="./mod/mod_unique_id.html">mod_unique_id</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_setenvif.html#browsermatch">BrowserMatch</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#browsermatchnocase">BrowserMatchNoCase</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#setenvifnocase">SetEnvIfNoCase</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#unsetenv">UnsetEnv</a></code></li></ul></td></tr></table>
31 <h3><a name="basic-manipulation" id="basic-manipulation">Basic Environment Manipulation</a></h3>
34 <p>The most basic way to set an environment variable in Apache
35 is using the unconditional <code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code> directive. Variables may also be passed from
36 the environment of the shell which started the server using the
37 <code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code> directive.</p>
40 <h3><a name="conditional" id="conditional">Conditional Per-Request Settings</a></h3>
43 <p>For additional flexibility, the directives provided by
44 mod_setenvif allow environment variables to be set on a
45 per-request basis, conditional on characteristics of particular
46 requests. For example, a variable could be set only when a
47 specific browser (User-Agent) is making a request, or only when
48 a specific Referer [sic] header is found. Even more flexibility
49 is available through the mod_rewrite's <code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> which uses the
50 <code>[E=...]</code> option to set environment variables.</p>
53 <h3><a name="unique-identifiers" id="unique-identifiers">Unique Identifiers</a></h3>
56 <p>Finally, mod_unique_id sets the environment variable
57 <code>UNIQUE_ID</code> for each request to a value which is
58 guaranteed to be unique across "all" requests under very
59 specific conditions.</p>
62 <h3><a name="standard-cgi" id="standard-cgi">Standard CGI Variables</a></h3>
65 <p>In addition to all environment variables set within the
66 Apache configuration and passed from the shell, CGI scripts and
67 SSI pages are provided with a set of environment variables
68 containing meta-information about the request as required by
69 the <a href="http://cgi-spec.golux.com/">CGI
70 specification</a>.</p>
73 <h3><a name="caveats" id="caveats">Some Caveats</a></h3>
77 <li>It is not possible to override or change the standard CGI
78 variables using the environment manipulation directives.</li>
80 <li>When <a href="suexec.html">suexec</a> is used to launch
81 CGI scripts, the environment will be cleaned down to a set of
82 <em>safe</em> variables before CGI scripts are launched. The
83 list of <em>safe</em> variables is defined at compile-time in
84 <code>suexec.c</code>.</li>
86 <li>For portability reasons, the names of environment
87 variables may contain only letters, numbers, and the
88 underscore character. In addition, the first character may
89 not be a number. Characters which do not match this
90 restriction will be replaced by an underscore when passed to
91 CGI scripts and SSI pages.</li>
94 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div><div class="section"><h2><a name="using" id="using">Using Environment Variables</a></h2>
97 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_authz_host.html">mod_authz_host</a></code></li><li><code class="module"><a href="./mod/mod_cgi.html">mod_cgi</a></code></li><li><code class="module"><a href="./mod/mod_ext_filter.html">mod_ext_filter</a></code></li><li><code class="module"><a href="./mod/mod_headers.html">mod_headers</a></code></li><li><code class="module"><a href="./mod/mod_include.html">mod_include</a></code></li><li><code class="module"><a href="./mod/mod_log_config.html">mod_log_config</a></code></li><li><code class="module"><a href="./mod/mod_rewrite.html">mod_rewrite</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_authz_host.html#allow">Allow</a></code></li><li><code class="directive"><a href="./mod/mod_log_config.html#customlog">CustomLog</a></code></li><li><code class="directive"><a href="./mod/mod_authz_host.html#deny">Deny</a></code></li><li><code class="directive"><a href="./mod/mod_ext_filter.html#extfilterdefine">ExtFilterDefine</a></code></li><li><code class="directive"><a href="./mod/mod_headers.html#header">Header</a></code></li><li><code class="directive"><a href="./mod/mod_log_config.html#logformat">LogFormat</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewritecond">RewriteCond</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code></li></ul></td></tr></table>
99 <h3><a name="cgi-scripts" id="cgi-scripts">CGI Scripts</a></h3>
102 <p>One of the primary uses of environment variables is to
103 communicate information to CGI scripts. As discussed above, the
104 environment passed to CGI scripts includes standard
105 meta-information about the request in addition to any variables
106 set within the Apache configuration. For more details, see the
107 <a href="howto/cgi.html">CGI tutorial</a>.</p>
110 <h3><a name="ssi-pages" id="ssi-pages">SSI Pages</a></h3>
113 <p>Server-parsed (SSI) documents processed by mod_include's
114 <code>INCLUDES</code> filter can print environment variables
115 using the <code>echo</code> element, and can use environment
116 variables in flow control elements to makes parts of a page
117 conditional on characteristics of a request. Apache also
118 provides SSI pages with the standard CGI environment variables
119 as discussed above. For more details, see the <a href="howto/ssi.html">SSI tutorial</a>.</p>
122 <h3><a name="access-control" id="access-control">Access Control</a></h3>
125 <p>Access to the server can be controlled based on the value of
126 environment variables using the <code>allow from env=</code>
127 and <code>deny from env=</code> directives. In combination with
128 <code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code>, this
129 allows for flexible control of access to the server based on
130 characteristics of the client. For example, you can use these
131 directives to deny access to a particular browser (User-Agent).
135 <h3><a name="logging" id="logging">Conditional Logging</a></h3>
138 <p>Environment variables can be logged in the access log using
139 the <code class="directive"><a href="./mod/mod_log_config.html#logformat">LogFormat</a></code>
140 option <code>%e</code>. In addition, the decision on whether
141 or not to log requests can be made based on the status of
142 environment variables using the conditional form of the
143 <code class="directive"><a href="./mod/mod_log_config.html#customlog">CustomLog</a></code>
144 directive. In combination with <code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code> this allows for flexible control of which
145 requests are logged. For example, you can choose not to log
146 requests for filenames ending in <code>gif</code>, or you can
147 choose to only log requests from clients which are outside your
151 <h3><a name="response-headers" id="response-headers">Conditional Response Headers</a></h3>
154 <p>The <code class="directive"><a href="./mod/mod_headers.html#header">Header</a></code>
155 directive can use the presence or
156 absence of an environment variable to determine whether or not
157 a certain HTTP header will be placed in the response to the
158 client. This allows, for example, a certain response header to
159 be sent only if a corresponding header is received in the
160 request from the client.</p>
164 <h3><a name="external-filter" id="external-filter">External Filter Activation</a></h3>
167 <p>External filters configured by <code class="module"><a href="./mod/mod_ext_filter.html">mod_ext_filter</a></code>
168 using the <code class="directive"><a href="./mod/mod_ext_filter.html#extfilterdefine">ExtFilterDefine</a></code> directive can
169 by activated conditional on an environment variable using the
170 <code>disableenv=</code> and <code>enableenv=</code> options.</p>
173 <h3><a name="url-rewriting" id="url-rewriting">URL Rewriting</a></h3>
176 <p>The <code>%{ENV:...}</code> form of <em>TestString</em> in
177 the <code class="directive"><a href="./mod/mod_rewrite.html#rewritecond">RewriteCond</a></code>
178 allows mod_rewrite's rewrite
179 engine to make decisions conditional on environment variables.
180 Note that the variables accessible in mod_rewrite without the
181 <code>ENV:</code> prefix are not actually environment
182 variables. Rather, they are variables special to mod_rewrite
183 which cannot be accessed from other modules.</p>
185 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div><div class="section"><h2><a name="special" id="special">Special Purpose Environment Variables</a></h2>
188 <p>Interoperability problems have led to the introduction of
189 mechanisms to modify the way Apache behaves when talking to
190 particular clients. To make these mechanisms as flexible as
191 possible, they are invoked by defining environment variables,
192 typically with <code class="directive"><a href="./mod/mod_setenvif.html#browsermatch">BrowserMatch</a></code>,
193 though <code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code> and
194 <code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code> could also be used,
197 <h3><a name="downgrade" id="downgrade">downgrade-1.0</a></h3>
200 <p>This forces the request to be treated as a HTTP/1.0 request
201 even if it was in a later dialect.</p>
204 <h3><a name="force-no-vary" id="force-no-vary">force-no-vary</a></h3>
207 <p>This causes any <code>Vary</code> fields to be removed from
208 the response header before it is sent back to the client. Some
209 clients don't interpret this field correctly (see the <a href="misc/known_client_problems.html">known client
210 problems</a> page); setting this variable can work around this
211 problem. Setting this variable also implies
212 <strong>force-response-1.0</strong>.</p>
215 <h3><a name="force-response" id="force-response">force-response-1.0</a></h3>
218 <p>This forces an HTTP/1.0 response when set. It was originally
219 implemented as a result of a problem with AOL's proxies. Some
220 clients may not behave correctly when given an HTTP/1.1
221 response, and this can be used to interoperate with them.</p>
224 <h3><a name="gzip-only-text-html" id="gzip-only-text-html">gzip-only-text/html</a></h3>
227 <p>When set to a value of "1", this variable disables the DEFLATE
228 output filter provided by <code class="module"><a href="./mod/mod_deflate.html">mod_deflate</a></code> for
229 content-types other than <code>text/html</code>.</p>
232 <h3><a name="nokeepalive" id="nokeepalive">nokeepalive</a></h3>
235 <p>This disables <code class="directive"><a href="./mod/core.html#keepalive">KeepAlive</a></code> when set.</p>
238 <h3><a name="redirect-carefully" id="redirect-carefully">redirect-carefully</a></h3>
241 <p>This forces the server to be more careful when sending a redirect
242 to the client. This is typically used when a client has a known
243 problem handling redirects. This was originally implemented as a
244 result of a problem with Microsoft's WebFolders software which has
245 a problem handling redirects on directory resources via DAV
250 <h3><a name="suppress-error-charset" id="suppress-error-charset">suppress-error-charset</a></h3>
253 <p><em>Available in versions after 2.0.40</em></p>
255 <p>When Apache issues a redirect in response to a client request,
256 the response includes some actual text to be displayed in case
257 the client can't (or doesn't) automatically follow the redirection.
258 Apache ordinarily labels this text according to the character set
259 which it uses, which is ISO-8859-1.</p>
260 <p> However, if the redirection is to a page that uses a different
261 character set, some broken browser versions will try to use the
262 character set from the redirection text rather than the actual page.
263 This can result in Greek, for instance, being incorrectly rendered.</p>
264 <p>Setting this environment variable causes Apache to omit the character
265 set for the redirection text, and these broken browsers will then correctly
266 use that of the destination page.</p>
270 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div><div class="section"><h2><a name="examples" id="examples">Examples</a></h2>
273 <h3><a name="misbehaving" id="misbehaving">Changing protocol behavior with misbehaving clients</a></h3>
276 <p>We recommend that the following lines be included in
277 httpd.conf to deal with known client problems.</p>
278 <div class="example"><pre>
280 # The following directives modify normal HTTP response behavior.
281 # The first directive disables keepalive for Netscape 2.x and browsers that
282 # spoof it. There are known problems with these browser implementations.
283 # The second directive is for Microsoft Internet Explorer 4.0b2
284 # which has a broken HTTP/1.1 implementation and does not properly
285 # support keepalive when it is used on 301 or 302 (redirect) responses.
287 BrowserMatch "Mozilla/2" nokeepalive
288 BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0
291 # The following directive disables HTTP/1.1 responses to browsers which
292 # are in violation of the HTTP/1.0 spec by not being able to grok a
293 # basic 1.1 response.
295 BrowserMatch "RealPlayer 4\.0" force-response-1.0
296 BrowserMatch "Java/1\.0" force-response-1.0
297 BrowserMatch "JDK/1\.0" force-response-1.0</pre></div>
300 <h3><a name="no-img-log" id="no-img-log">Do not log requests for images in the access log</a></h3>
303 <p>This example keeps requests for images from appearing in the
304 access log. It can be easily modified to prevent logging of
305 particular directories, or to prevent logging of requests
306 coming from particular hosts.</p>
307 <div class="example"><pre>
308 SetEnvIf Request_URI \.gif image-request
309 SetEnvIf Request_URI \.jpg image-request
310 SetEnvIf Request_URI \.png image-request
311 CustomLog logs/access_log common env=!image-request</pre></div>
314 <h3><a name="image-theft" id="image-theft">Prevent "Image Theft"</a></h3>
317 <p>This example shows how to keep people not on your server
318 from using images on your server as inline-images on their
319 pages. This is not a recommended configuration, but it can work
320 in limited circumstances. We assume that all your images are in
321 a directory called /web/images.</p>
322 <div class="example"><pre>
323 SetEnvIf Referer "^http://www.example.com/" local_referal
324 # Allow browsers that do not send Referer info
325 SetEnvIf Referer "^$" local_referal
326 <Directory /web/images>
329 Allow from env=local_referal
330 </Directory></pre></div>
332 <p>For more information about this technique, see the
333 ApacheToday tutorial " <a href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">
334 Keeping Your Images from Adorning Other Sites</a>".</p>
336 </div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p></div></body></html>
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