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23 <manualpage metafile="env.xml.meta">
25 <title>Environment Variables in Apache</title>
28 <p>There are two kinds of environment variables that affect
29 the Apache HTTP Server.</p>
31 <p>First, there are the environment variables controlled by
32 the underlying operating system. These are set before the
33 server starts. They can be used in expansions in configuration
34 files, and can optionally be passed to CGI scripts and SSI
35 using the PassEnv directive.</p>
37 <p>Second, the Apache HTTP Server provides a mechanism for storing
38 information in named variables that are also called <em>environment
39 variables</em>. This information can be used to control various
40 operations such as logging or access control. The variables are
41 also used as a mechanism to communicate with external programs
42 such as CGI scripts. This document discusses different ways to
43 manipulate and use these variables.</p>
45 <p>Although these variables are referred to as <em>environment
46 variables</em>, they are not the same as the environment
47 variables controlled by the underlying operating system.
48 Instead, these variables are stored and manipulated in an
49 internal Apache structure. They only become actual operating
50 system environment variables when they are provided to CGI
51 scripts and Server Side Include scripts. If you wish to
52 manipulate the operating system environment under which the
53 server itself runs, you must use the standard environment
54 manipulation mechanisms provided by your operating system
58 <section id="setting">
59 <title>Setting Environment Variables</title>
62 <module>mod_cache</module>
63 <module>mod_env</module>
64 <module>mod_rewrite</module>
65 <module>mod_setenvif</module>
66 <module>mod_unique_id</module>
69 <directive module="mod_setenvif">BrowserMatch</directive>
70 <directive module="mod_setenvif">BrowserMatchNoCase</directive>
71 <directive module="mod_env">PassEnv</directive>
72 <directive module="mod_rewrite">RewriteRule</directive>
73 <directive module="mod_env">SetEnv</directive>
74 <directive module="mod_setenvif">SetEnvIf</directive>
75 <directive module="mod_setenvif">SetEnvIfNoCase</directive>
76 <directive module="mod_env">UnsetEnv</directive>
80 <section id="basic-manipulation">
81 <title>Basic Environment Manipulation</title>
83 <p>The most basic way to set an environment variable in Apache
84 is using the unconditional <directive module="mod_env"
85 >SetEnv</directive> directive. Variables may also be passed from
86 the environment of the shell which started the server using the
87 <directive module="mod_env">PassEnv</directive> directive.</p>
90 <section id="conditional">
91 <title>Conditional Per-Request Settings</title>
93 <p>For additional flexibility, the directives provided by
94 <module>mod_setenvif</module> allow environment variables to be set
95 on a per-request basis, conditional on characteristics of particular
96 requests. For example, a variable could be set only when a
97 specific browser (User-Agent) is making a request, or only when
98 a specific Referer [sic] header is found. Even more flexibility
99 is available through the <module>mod_rewrite</module>'s <directive
100 module="mod_rewrite">RewriteRule</directive> which uses the
101 <code>[E=...]</code> option to set environment variables.</p>
104 <section id="unique-identifiers">
105 <title>Unique Identifiers</title>
107 <p>Finally, <module>mod_unique_id</module> sets the environment
108 variable <code>UNIQUE_ID</code> for each request to a value which is
109 guaranteed to be unique across "all" requests under very
110 specific conditions.</p>
113 <section id="standard-cgi">
114 <title>Standard CGI Variables</title>
116 <p>In addition to all environment variables set within the
117 Apache configuration and passed from the shell, CGI scripts and
118 SSI pages are provided with a set of environment variables
119 containing meta-information about the request as required by
120 the <a href="http://www.ietf.org/rfc/rfc3875">CGI
121 specification</a>.</p>
124 <section id="caveats">
125 <title>Some Caveats</title>
128 <li>It is not possible to override or change the standard CGI
129 variables using the environment manipulation directives.</li>
131 <li>When <program>suexec</program> is used to launch
132 CGI scripts, the environment will be cleaned down to a set of
133 <em>safe</em> variables before CGI scripts are launched. The
134 list of <em>safe</em> variables is defined at compile-time in
135 <code>suexec.c</code>.</li>
137 <li>For portability reasons, the names of environment
138 variables may contain only letters, numbers, and the
139 underscore character. In addition, the first character may
140 not be a number. Characters which do not match this
141 restriction will be replaced by an underscore when passed to
142 CGI scripts and SSI pages.</li>
144 <li>A special case are HTTP headers which are passed to CGI
145 scripts and the like via environment variables (see below).
146 They are converted to uppercase and only dashes are replaced with
147 underscores; if the header contains any other (invalid) character,
148 the whole header is silently dropped. See <a href="#fixheader">
149 below</a> for a workaround.</li>
151 <li>The <directive module="mod_env">SetEnv</directive> directive runs
152 late during request processing meaning that directives such as
153 <directive module="mod_setenvif">SetEnvIf</directive> and <directive
154 module="mod_rewrite">RewriteCond</directive> will not see the
155 variables set with it.</li>
157 <li>When the server looks up a path via an internal
158 <glossary ref="subrequest">subrequest</glossary> such as looking
159 for a <directive module="mod_dir" >DirectoryIndex</directive>
160 or generating a directory listing with <module>mod_autoindex</module>,
161 per-request environment variables are <em>not</em> inherited in the
162 subrequest. Additionally,
163 <directive module="mod_setenvif">SetEnvIf</directive> directives
164 are not separately evaluated in the subrequest due to the API phases
165 <module>mod_setenvif</module> takes action in.</li>
170 <title>Using Environment Variables</title>
174 <module>mod_authz_host</module>
175 <module>mod_cgi</module>
176 <module>mod_ext_filter</module>
177 <module>mod_headers</module>
178 <module>mod_include</module>
179 <module>mod_log_config</module>
180 <module>mod_rewrite</module>
183 <directive module="mod_authz_core">Require</directive>
184 <directive module="mod_log_config">CustomLog</directive>
185 <directive module="mod_access_compat">Deny</directive>
186 <directive module="mod_ext_filter">ExtFilterDefine</directive>
187 <directive module="mod_headers">Header</directive>
188 <directive module="mod_log_config">LogFormat</directive>
189 <directive module="mod_rewrite">RewriteCond</directive>
190 <directive module="mod_rewrite">RewriteRule</directive>
194 <section id="cgi-scripts">
195 <title>CGI Scripts</title>
197 <p>One of the primary uses of environment variables is to
198 communicate information to CGI scripts. As discussed above, the
199 environment passed to CGI scripts includes standard
200 meta-information about the request in addition to any variables
201 set within the Apache configuration. For more details, see the
202 <a href="howto/cgi.html">CGI tutorial</a>.</p>
205 <section id="ssi-pages">
206 <title>SSI Pages</title>
208 <p>Server-parsed (SSI) documents processed by
209 <module>mod_include</module>'s
210 <code>INCLUDES</code> filter can print environment variables
211 using the <code>echo</code> element, and can use environment
212 variables in flow control elements to makes parts of a page
213 conditional on characteristics of a request. Apache also
214 provides SSI pages with the standard CGI environment variables
215 as discussed above. For more details, see the <a
216 href="howto/ssi.html">SSI tutorial</a>.</p>
219 <section id="access-control">
220 <title>Access Control</title>
222 <p>Access to the server can be controlled based on the value of
223 environment variables using the <code>allow from env=</code>
224 and <code>deny from env=</code> directives. In combination with
225 <directive module="mod_setenvif">SetEnvIf</directive>, this
226 allows for flexible control of access to the server based on
227 characteristics of the client. For example, you can use these
228 directives to deny access to a particular browser (User-Agent).
232 <section id="logging">
233 <title>Conditional Logging</title>
235 <p>Environment variables can be logged in the access log using
236 the <directive module="mod_log_config">LogFormat</directive>
237 option <code>%e</code>. In addition, the decision on whether
238 or not to log requests can be made based on the status of
239 environment variables using the conditional form of the
240 <directive module="mod_log_config">CustomLog</directive>
241 directive. In combination with <directive module="mod_setenvif"
242 >SetEnvIf</directive> this allows for flexible control of which
243 requests are logged. For example, you can choose not to log
244 requests for filenames ending in <code>gif</code>, or you can
245 choose to only log requests from clients which are outside your
249 <section id="response-headers">
250 <title>Conditional Response Headers</title>
252 <p>The <directive module="mod_headers">Header</directive>
253 directive can use the presence or
254 absence of an environment variable to determine whether or not
255 a certain HTTP header will be placed in the response to the
256 client. This allows, for example, a certain response header to
257 be sent only if a corresponding header is received in the
258 request from the client.</p>
262 <section id="external-filter">
263 <title>External Filter Activation</title>
265 <p>External filters configured by <module>mod_ext_filter</module>
267 module="mod_ext_filter">ExtFilterDefine</directive> directive can
268 by activated conditional on an environment variable using the
269 <code>disableenv=</code> and <code>enableenv=</code> options.</p>
272 <section id="url-rewriting">
273 <title>URL Rewriting</title>
275 <p>The <code>%{ENV:<em>variable</em>}</code> form of
276 <em>TestString</em> in the <directive module="mod_rewrite"
277 >RewriteCond</directive> allows <module>mod_rewrite</module>'s rewrite
278 engine to make decisions conditional on environment variables.
279 Note that the variables accessible in <module>mod_rewrite</module>
280 without the <code>ENV:</code> prefix are not actually environment
281 variables. Rather, they are variables special to
282 <module>mod_rewrite</module> which cannot be accessed from other
287 <section id="special">
288 <title>Special Purpose Environment Variables</title>
290 <p>Interoperability problems have led to the introduction of
291 mechanisms to modify the way Apache behaves when talking to
292 particular clients. To make these mechanisms as flexible as
293 possible, they are invoked by defining environment variables,
294 typically with <directive module="mod_setenvif"
295 >BrowserMatch</directive>, though <directive module="mod_env"
296 >SetEnv</directive> and <directive module="mod_env"
297 >PassEnv</directive> could also be used, for example.</p>
299 <section id="downgrade">
300 <title>downgrade-1.0</title>
302 <p>This forces the request to be treated as a HTTP/1.0 request
303 even if it was in a later dialect.</p>
306 <section id="force-gzip">
307 <title>force-gzip</title>
308 <p>If you have the <code>DEFLATE</code> filter activated, this
309 environment variable will ignore the accept-encoding setting of
310 your browser and will send compressed output unconditionally.</p>
312 <section id="force-no-vary">
313 <title>force-no-vary</title>
315 <p>This causes any <code>Vary</code> fields to be removed from
316 the response header before it is sent back to the client. Some
317 clients don't interpret this field correctly; setting this
318 variable can work around this problem. Setting this variable
319 also implies <strong>force-response-1.0</strong>.</p>
322 <section id="force-response">
323 <title>force-response-1.0</title>
325 <p>This forces an HTTP/1.0 response to clients making an HTTP/1.0
326 request. It was originally
327 implemented as a result of a problem with AOL's proxies. Some
328 HTTP/1.0 clients may not behave correctly when given an HTTP/1.1
329 response, and this can be used to interoperate with them.</p>
333 <section id="gzip-only-text-html">
334 <title>gzip-only-text/html</title>
336 <p>When set to a value of "1", this variable disables the
337 <code>DEFLATE</code> output filter provided by
338 <module>mod_deflate</module> for content-types other than
339 <code>text/html</code>. If you'd rather
340 use statically compressed files, <module>mod_negotiation</module>
341 evaluates the variable as well (not only for gzip, but for all
342 encodings that differ from "identity").</p>
345 <section id="no-gzip"><title>no-gzip</title>
347 <p>When set, the <code>DEFLATE</code> filter of
348 <module>mod_deflate</module> will be turned off and
349 <module>mod_negotiation</module> will refuse to deliver encoded
354 <section id="no-cache"><title>no-cache</title>
355 <p><em>Available in versions 2.2.12 and later</em></p>
357 <p>When set, <module>mod_cache</module> will not save an otherwise
358 cacheable response. This environment variable does not influence
359 whether a response already in the cache will be served for the current
364 <section id="nokeepalive">
365 <title>nokeepalive</title>
367 <p>This disables <directive module="core">KeepAlive</directive>
372 <section id="prefer-language"><title>prefer-language</title>
374 <p>This influences <module>mod_negotiation</module>'s behaviour. If
375 it contains a language tag (such as <code>en</code>, <code>ja</code>
376 or <code>x-klingon</code>), <module>mod_negotiation</module> tries
377 to deliver a variant with that language. If there's no such variant,
378 the normal <a href="content-negotiation.html">negotiation</a> process
383 <section id="redirect-carefully">
384 <title>redirect-carefully</title>
386 <p>This forces the server to be more careful when sending a redirect
387 to the client. This is typically used when a client has a known
388 problem handling redirects. This was originally implemented as a
389 result of a problem with Microsoft's WebFolders software which has
390 a problem handling redirects on directory resources via DAV
395 <section id="suppress-error-charset">
396 <title>suppress-error-charset</title>
398 <p><em>Available in versions after 2.0.54</em></p>
400 <p>When Apache issues a redirect in response to a client request,
401 the response includes some actual text to be displayed in case
402 the client can't (or doesn't) automatically follow the redirection.
403 Apache ordinarily labels this text according to the character set
404 which it uses, which is ISO-8859-1.</p>
406 <p> However, if the redirection is to a page that uses a different
407 character set, some broken browser versions will try to use the
408 character set from the redirection text rather than the actual page.
409 This can result in Greek, for instance, being incorrectly rendered.</p>
411 <p>Setting this environment variable causes Apache to omit the character
412 set for the redirection text, and these broken browsers will then correctly
413 use that of the destination page.</p>
415 <note type="warning">
416 <title>Security note</title>
418 <p>Sending error pages without a specified character set may
419 allow a cross-site-scripting attack for existing browsers (MSIE)
420 which do not follow the HTTP/1.1 specification and attempt to
421 "guess" the character set from the content. Such browsers can
422 be easily fooled into using the UTF-7 character set, and UTF-7
423 content from input data (such as the request-URI) will not be
424 escaped by the usual escaping mechanisms designed to prevent
425 cross-site-scripting attacks.</p>
430 <section id="proxy"><title>force-proxy-request-1.0, proxy-nokeepalive, proxy-sendchunked,
431 proxy-sendcl, proxy-chain-auth, proxy-interim-response, proxy-initial-not-pooled</title>
433 <p>These directives alter the protocol behavior of
434 <module>mod_proxy</module>. See the <module>mod_proxy</module> and <module>mod_proxy_http</module>
435 documentation for more details.</p>
440 <section id="examples">
441 <title>Examples</title>
443 <section id="fixheader">
444 <title>Passing broken headers to CGI scripts</title>
446 <p>Starting with version 2.4, Apache is more strict about how HTTP
447 headers are converted to environment variables in <module>mod_cgi
448 </module> and other modules: Previously any invalid characters
449 in header names were simply translated to underscores. This allowed
450 for some potential cross-site-scripting attacks via header injection
451 (see <a href="http://events.ccc.de/congress/2007/Fahrplan/events/2212.en.html">
452 Unusual Web Bugs</a>, slide 19/20).</p>
454 <p>If you have to support a client which sends broken headers and
455 which can't be fixed, a simple workaround involving <module>mod_setenvif
456 </module> and <module>mod_headers</module> allows you to still accept
459 <highlight language="config">
461 # The following works around a client sending a broken Accept_Encoding
464 SetEnvIfNoCase ^Accept.Encoding$ ^(.*)$ fix_accept_encoding=$1
465 RequestHeader set Accept-Encoding %{fix_accept_encoding}e env=fix_accept_encoding
470 <section id="misbehaving">
471 <title>Changing protocol behavior with misbehaving clients</title>
473 <p>Earlier versions recommended that the following lines be included in
474 httpd.conf to deal with known client problems. Since the affected clients
475 are no longer seen in the wild, this configuration is likely no-longer
477 <highlight language="config">
479 # The following directives modify normal HTTP response behavior.
480 # The first directive disables keepalive for Netscape 2.x and browsers that
481 # spoof it. There are known problems with these browser implementations.
482 # The second directive is for Microsoft Internet Explorer 4.0b2
483 # which has a broken HTTP/1.1 implementation and does not properly
484 # support keepalive when it is used on 301 or 302 (redirect) responses.
486 BrowserMatch "Mozilla/2" nokeepalive
487 BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0
490 # The following directive disables HTTP/1.1 responses to browsers which
491 # are in violation of the HTTP/1.0 spec by not being able to understand a
492 # basic 1.1 response.
494 BrowserMatch "RealPlayer 4\.0" force-response-1.0
495 BrowserMatch "Java/1\.0" force-response-1.0
496 BrowserMatch "JDK/1\.0" force-response-1.0
500 <section id="no-img-log">
501 <title>Do not log requests for images in the access log</title>
503 <p>This example keeps requests for images from appearing in the
504 access log. It can be easily modified to prevent logging of
505 particular directories, or to prevent logging of requests
506 coming from particular hosts.</p>
508 <highlight language="config">
509 SetEnvIf Request_URI \.gif image-request
510 SetEnvIf Request_URI \.jpg image-request
511 SetEnvIf Request_URI \.png image-request
512 CustomLog logs/access_log common env=!image-request
516 <section id="image-theft">
517 <title>Prevent "Image Theft"</title>
519 <p>This example shows how to keep people not on your server
520 from using images on your server as inline-images on their
521 pages. This is not a recommended configuration, but it can work
522 in limited circumstances. We assume that all your images are in
523 a directory called <code>/web/images</code>.</p>
525 <highlight language="config">
526 SetEnvIf Referer "^http://www\.example\.com/" local_referal
527 # Allow browsers that do not send Referer info
528 SetEnvIf Referer "^$" local_referal
529 <Directory "/web/images">
530 Require env local_referal
534 <p>For more information about this technique, see the
535 "<a href="http://www.serverwatch.com/tutorials/article.php/1132731"
536 >Keeping Your Images from Adorning Other Sites</a>"
537 tutorial on ServerWatch.</p>