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23 <manualpage metafile="caching.xml.meta">
25 <title>Caching Guide</title>
28 <p>This document supplements the <module>mod_cache</module>,
29 <module>mod_cache_disk</module>, <module>mod_file_cache</module> and <a
30 href="programs/htcacheclean.html">htcacheclean</a> reference documentation.
31 It describes how to use the Apache HTTP Server's caching features to accelerate web and
32 proxy serving, while avoiding common problems and misconfigurations.</p>
35 <section id="introduction">
36 <title>Introduction</title>
38 <p>The Apache HTTP server offers a range of caching features that
39 are designed to improve the performance of the server in various
43 <dt>Three-state RFC2616 HTTP caching</dt>
45 <module>mod_cache</module>
46 and its provider modules
47 <module>mod_cache_disk</module>
48 provide intelligent, HTTP-aware caching. The content itself is stored
49 in the cache, and mod_cache aims to honor all of the various HTTP
50 headers and options that control the cacheability of content
52 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">Section
54 <module>mod_cache</module>
55 is aimed at both simple and complex caching configurations, where
56 you are dealing with proxied content, dynamic local content or
57 have a need to speed up access to local files on a potentially
61 <dt>Two-state key/value shared object caching</dt>
63 The <a href="socache.html">shared object cache API</a> (socache)
64 and its provider modules provide a
65 server wide key/value based shared object cache. These modules
66 are designed to cache low level data such as SSL sessions and
67 authentication credentials. Backends allow the data to be stored
68 server wide in shared memory, or datacenter wide in a cache such
69 as memcache or distcache.
72 <dt>Specialized file caching</dt>
74 <module>mod_file_cache</module>
75 offers the ability to pre-load
76 files into memory on server startup, and can improve access
77 times and save file handles on files that are accessed often,
78 as there is no need to go to disk on each request.
82 <p>To get the most from this document, you should be familiar with
83 the basics of HTTP, and have read the Users' Guides to
84 <a href="urlmapping.html">Mapping URLs to the Filesystem</a> and
85 <a href="content-negotiation.html">Content negotiation</a>.</p>
89 <section id="http-caching">
91 <title>Three-state RFC2616 HTTP caching</title>
95 <module>mod_cache</module>
96 <module>mod_cache_disk</module>
99 <directive module="mod_cache">CacheEnable</directive>
100 <directive module="mod_cache">CacheDisable</directive>
101 <directive module="core">UseCanonicalName</directive>
102 <directive module="mod_negotiation">CacheNegotiatedDocs</directive>
106 <p>The HTTP protocol contains built in support for an in-line caching
108 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
109 described by section 13 of RFC2616</a>, and the
110 <module>mod_cache</module> module can be used to take advantage of
113 <p>Unlike a simple two state key/value cache where the content
114 disappears completely when no longer fresh, an HTTP cache includes
115 a mechanism to retain stale content, and to ask the origin server
116 whether this stale content has changed and if not, make it fresh
119 <p>An entry in an HTTP cache exists in one of three states:</p>
124 If the content is new enough (younger than its <strong>freshness
125 lifetime</strong>), it is considered <strong>fresh</strong>. An
126 HTTP cache is free to serve fresh content without making any
127 calls to the origin server at all.
131 <p>If the content is too old (older than its <strong>freshness
132 lifetime</strong>), it is considered <strong>stale</strong>. An
133 HTTP cache should contact the origin server and check whether
134 the content is still fresh before serving stale content to a
135 client. The origin server will either respond with replacement
136 content if not still valid, or ideally, the origin server will
137 respond with a code to tell the cache the content is still
138 fresh, without the need to generate or send the content again.
139 The content becomes fresh again and the cycle continues.</p>
141 <p>The HTTP protocol does allow the cache to serve stale data
142 under certain circumstances, such as when an attempt to freshen
143 the data with an origin server has failed with a 5xx error, or
144 when another request is already in the process of freshening
145 the given entry. In these cases a <code>Warning</code> header
146 is added to the response.</p>
148 <dt>Non Existent</dt>
150 If the cache gets full, it reserves the option to delete content
151 from the cache to make space. Content can be deleted at any time,
152 and can be stale or fresh. The <a
153 href="programs/htcacheclean.html">htcacheclean</a> tool can be
154 run on a once off basis, or deployed as a daemon to keep the size
155 of the cache within the given size, or the given number of inodes.
156 The tool attempts to delete stale content before attempting to
157 delete fresh content.
161 <p>Full details of how HTTP caching works can be found in
162 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
163 Section 13 of RFC2616</a>.</p>
166 <title>Interaction with the Server</title>
168 <p>The <module>mod_cache</module> module hooks into the server in two
169 possible places depending on the value of the
170 <directive module="mod_cache">CacheQuickHandler</directive> directive:
174 <dt>Quick handler phase</dt>
176 <p>This phase happens very early on during the request processing,
177 just after the request has been parsed. If the content is
178 found within the cache, it is served immediately and almost
179 all request processing is bypassed.</p>
181 <p>In this scenario, the cache behaves as if it has been "bolted
182 on" to the front of the server.</p>
184 <p>This mode offers the best performance, as the majority of
185 server processing is bypassed. This mode however also bypasses the
186 authentication and authorization phases of server processing, so
187 this mode should be chosen with care when this is important.</p>
189 <dt>Normal handler phase</dt>
191 <p>This phase happens late in the request processing, after all
192 the request phases have completed.</p>
194 <p>In this scenario, the cache behaves as if it has been "bolted
195 on" to the back of the server.</p>
197 <p>This mode offers the most flexibility, as the potential exists
198 for caching to occur at a precisely controlled point in the filter
199 chain, and cached content can be filtered or personalized before
200 being sent to the client.</p>
204 <p>If the URL is not found within the cache, <module>mod_cache</module>
205 will add a <a href="filter.html">filter</a> to the filter stack in order
206 to record the response to the cache, and then stand down, allowing normal
207 request processing to continue. If the content is determined to be
208 cacheable, the content will be saved to the cache for future serving,
209 otherwise the content will be ignored.</p>
211 <p>If the content found within the cache is stale, the
212 <module>mod_cache</module> module converts the request into a
213 <strong>conditional request</strong>. If the origin server responds with
214 a normal response, the normal response is cached, replacing the content
215 already cached. If the origin server responds with a 304 Not Modified
216 response, the content is marked as fresh again, and the cached content
217 is served by the filter instead of saving it.</p>
221 <title>Improving Cache Hits</title>
223 <p>When a virtual host is known by one of many different server aliases,
224 ensuring that <directive module="core">UseCanonicalName</directive> is
225 set to <code>On</code> can dramatically improve the ratio of cache hits.
226 This is because the hostname of the virtual-host serving the content is
227 used within the cache key. With the setting set to <code>On</code>
228 virtual-hosts with multiple server names or aliases will not produce
229 differently cached entities, and instead content will be cached as
230 per the canonical hostname.</p>
235 <title>Freshness Lifetime</title>
237 <p>Well formed content that is intended to be cached should declare an
238 explicit freshness lifetime with the <code>Cache-Control</code>
239 header's <code>max-age</code> or <code>s-maxage</code> fields, or
240 by including an <code>Expires</code> header.</p>
242 <p>At the same time, the origin server defined freshness lifetime can
243 be overridden by a client when the client presents their own
244 <code>Cache-Control</code> header within the request. In this case,
245 the lowest freshness lifetime between request and response wins.</p>
247 <p>When this freshness lifetime is missing from the request or the
248 response, a default freshness lifetime is applied. The default
249 freshness lifetime for cached entities is one hour, however
250 this can be easily over-ridden by using the <directive
251 module="mod_cache">CacheDefaultExpire</directive> directive.</p>
253 <p>If a response does not include an <code>Expires</code> header but does
254 include a <code>Last-Modified</code> header, <module>mod_cache</module>
255 can infer a freshness lifetime based on a heuristic, which can be
256 controlled through the use of the <directive
257 module="mod_cache">CacheLastModifiedFactor</directive> directive.</p>
259 <p>For local content, or for remote content that does not define its own
260 <code>Expires</code> header, <module>mod_expires</module> may be used to
261 fine-tune the freshness lifetime by adding <code>max-age</code> and
262 <code>Expires</code>.</p>
264 <p>The maximum freshness lifetime may also be controlled by using the
265 <directive module="mod_cache">CacheMaxExpire</directive>.</p>
270 <title>A Brief Guide to Conditional Requests</title>
272 <p>When content expires from the cache and becomes stale, rather than
273 pass on the original request, httpd will modify the request to make
274 it conditional instead.</p>
276 <p>When an <code>ETag</code> header exists in the original cached
277 response, <module>mod_cache</module> will add an
278 <code>If-None-Match</code> header to the request to the origin server.
279 When a <code>Last-Modified</code> header exists in the original
280 cached response, <module>mod_cache</module> will add an
281 <code>If-Modified-Since</code> header to the request to the origin
282 server. Performing either of these actions makes the request
283 <strong>conditional</strong>.</p>
285 <p>When a conditional request is received by an origin server, the
286 origin server should check whether the ETag or the Last-Modified
287 parameter has changed, as appropriate for the request. If not, the
288 origin should respond with a terse "304 Not Modified" response. This
289 signals to the cache that the stale content is still fresh should be
290 used for subsequent requests until the content's new freshness lifetime
291 is reached again.</p>
293 <p>If the content has changed, then the content is served as if the
294 request were not conditional to begin with.</p>
296 <p>Conditional requests offer two benefits. Firstly, when making such
297 a request to the origin server, if the content from the origin
298 matches the content in the cache, this can be determined easily and
299 without the overhead of transferring the entire resource.</p>
301 <p>Secondly, a well designed origin server will be designed in such
302 a way that conditional requests will be significantly cheaper to
303 produce than a full response. For static files, typically all that is
304 involved is a call to <code>stat()</code> or similar system call, to
305 see if the file has changed in size or modification time. As such, even
306 local content may still be served faster from the cache if it has not
309 <p>Origin servers should make every effort to support conditional
310 requests as is practical, however if conditional requests are not
311 supported, the origin will respond as if the request was not
312 conditional, and the cache will respond as if the content had changed
313 and save the new content to the cache. In this case, the cache will
314 behave like a simple two state cache, where content is effectively
315 either fresh or deleted.</p>
319 <title>What Can be Cached?</title>
321 <p>The full definition of which responses can be cached by an HTTP
323 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4">
324 RFC2616 Section 13.4 Response Cacheability</a>, and can be summed up as
328 <li>Caching must be enabled for this URL. See the <directive
329 module="mod_cache">CacheEnable</directive> and <directive
330 module="mod_cache">CacheDisable</directive> directives.</li>
332 <li>The response must have a HTTP status code of 200, 203, 300, 301 or
335 <li>The request must be a HTTP GET request.</li>
337 <li>If the response contains an "Authorization:" header, it must
338 also contain an "s-maxage", "must-revalidate" or "public" option
339 in the "Cache-Control:" header, or it won't be cached.</li>
341 <li>If the URL included a query string (e.g. from a HTML form GET
342 method) it will not be cached unless the response specifies an
343 explicit expiration by including an "Expires:" header or the max-age
344 or s-maxage directive of the "Cache-Control:" header, as per RFC2616
345 sections 13.9 and 13.2.1.</li>
347 <li>If the response has a status of 200 (OK), the response must
348 also include at least one of the "Etag", "Last-Modified" or
349 the "Expires" headers, or the max-age or s-maxage directive of
350 the "Cache-Control:" header, unless the
351 <directive module="mod_cache">CacheIgnoreNoLastMod</directive>
352 directive has been used to require otherwise.</li>
354 <li>If the response includes the "private" option in a "Cache-Control:"
355 header, it will not be stored unless the
356 <directive module="mod_cache">CacheStorePrivate</directive> has been
357 used to require otherwise.</li>
359 <li>Likewise, if the response includes the "no-store" option in a
360 "Cache-Control:" header, it will not be stored unless the
361 <directive module="mod_cache">CacheStoreNoStore</directive> has been
364 <li>A response will not be stored if it includes a "Vary:" header
365 containing the match-all "*".</li>
370 <title>What Should Not be Cached?</title>
372 <p>It should be up to the client creating the request, or the origin
373 server constructing the response to decide whether or not the content
374 should be cacheable or not by correctly setting the
375 <code>Cache-Control</code> header, and <module>mod_cache</module> should
376 be left alone to honor the wishes of the client or server as appropriate.
379 <p>Content that is time sensitive, or which varies depending on the
380 particulars of the request that are not covered by HTTP negotiation,
381 should not be cached. This content should declare itself uncacheable
382 using the <code>Cache-Control</code> header.</p>
384 <p>If content changes often, expressed by a freshness lifetime of minutes
385 or seconds, the content can still be cached, however it is highly
386 desirable that the origin server supports
387 <strong>conditional requests</strong> correctly to ensure that
388 full responses do not have to be generated on a regular basis.</p>
390 <p>Content that varies based on client provided request headers can be
391 cached through intelligent use of the <code>Vary</code> response
397 <title>Variable/Negotiated Content</title>
399 <p>When the origin server is designed to respond with different content
400 based on the value of headers in the request, for example to serve
401 multiple languages at the same URL, HTTP's caching mechanism makes it
402 possible to cache multiple variants of the same page at the same URL.</p>
404 <p>This is done by the origin server adding a <code>Vary</code> header
405 to indicate which headers must be taken into account by a cache when
406 determining whether two variants are different from one another.</p>
408 <p>If for example, a response is received with a vary header such as;</p>
411 Vary: negotiate,accept-language,accept-charset
414 <p><module>mod_cache</module> will only serve the cached content to
415 requesters with accept-language and accept-charset headers
416 matching those of the original request.</p>
418 <p>Multiple variants of the content can be cached side by side,
419 <module>mod_cache</module> uses the <code>Vary</code> header and the
420 corresponding values of the request headers listed by <code>Vary</code>
421 to decide on which of many variants to return to the client.</p>
425 <title>Caching to Disk</title>
427 <p>The <module>mod_cache</module> module relies on specific backend store
428 implementations in order to manage the cache, and for caching to disk
429 <module>mod_cache_disk</module> is provided to support this.</p>
431 <p>Typically the module will be configured as so;</p>
434 CacheRoot /var/cache/apache/<br />
435 CacheEnable disk /<br />
436 CacheDirLevels 2<br />
440 <p>Importantly, as the cached files are locally stored, operating system
441 in-memory caching will typically be applied to their access also. So
442 although the files are stored on disk, if they are frequently accessed
443 it is likely the operating system will ensure that they are actually
444 served from memory.</p>
449 <title>Understanding the Cache-Store</title>
451 <p>To store items in the cache, <module>mod_cache_disk</module> creates
452 a 22 character hash of the URL being requested. This hash incorporates
453 the hostname, protocol, port, path and any CGI arguments to the URL,
454 as well as elements defined by the Vary header to ensure that multiple
455 URLs do not collide with one another.</p>
457 <p>Each character may be any one of 64-different characters, which mean
458 that overall there are 64^22 possible hashes. For example, a URL might
459 be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used
460 as a prefix for the naming of the files specific to that URL within
461 the cache, however first it is split up into directories as per
462 the <directive module="mod_cache_disk">CacheDirLevels</directive> and
463 <directive module="mod_cache_disk">CacheDirLength</directive>
466 <p><directive module="mod_cache_disk">CacheDirLevels</directive>
467 specifies how many levels of subdirectory there should be, and
468 <directive module="mod_cache_disk">CacheDirLength</directive>
469 specifies how many characters should be in each directory. With
470 the example settings given above, the hash would be turned into
472 <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p>
474 <p>The overall aim of this technique is to reduce the number of
475 subdirectories or files that may be in a particular directory,
476 as most file-systems slow down as this number increases. With
478 <directive module="mod_cache_disk">CacheDirLength</directive>
479 there can at most be 64 subdirectories at any particular level.
480 With a setting of 2 there can be 64 * 64 subdirectories, and so on.
481 Unless you have a good reason not to, using a setting of "1"
482 for <directive module="mod_cache_disk">CacheDirLength</directive>
486 <directive module="mod_cache_disk">CacheDirLevels</directive>
487 depends on how many files you anticipate to store in the cache.
488 With the setting of "2" used in the above example, a grand
489 total of 4096 subdirectories can ultimately be created. With
490 1 million files cached, this works out at roughly 245 cached
491 URLs per directory.</p>
493 <p>Each URL uses at least two files in the cache-store. Typically
494 there is a ".header" file, which includes meta-information about
495 the URL, such as when it is due to expire and a ".data" file
496 which is a verbatim copy of the content to be served.</p>
498 <p>In the case of a content negotiated via the "Vary" header, a
499 ".vary" directory will be created for the URL in question. This
500 directory will have multiple ".data" files corresponding to the
501 differently negotiated content.</p>
505 <title>Maintaining the Disk Cache</title>
507 <p>The <module>mod_cache_disk</module> module makes no attempt to
508 regulate the amount of disk space used by the cache, although it
509 will gracefully stand down on any disk error and behave as if the
510 cache was never present.</p>
512 <p>Instead, provided with httpd is the <a
513 href="programs/htcacheclean.html">htcacheclean</a> tool which allows you
514 to clean the cache periodically. Determining how frequently to run <a
515 href="programs/htcacheclean.html">htcacheclean</a> and what target size to
516 use for the cache is somewhat complex and trial and error may be needed to
517 select optimal values.</p>
519 <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of
520 operation. It can be run as persistent daemon, or periodically from
522 href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour
523 or more to process very large (tens of gigabytes) caches and if you are
524 running it from cron it is recommended that you determine how long a typical
525 run takes, to avoid running more than one instance at a time.</p>
527 <p>It is also recommended that an appropriate "nice" level is chosen for
528 htcacheclean so that the tool does not cause excessive disk io while the
529 server is running.</p>
532 <img src="images/caching_fig1.gif" alt="" width="600"
533 height="406" /><br />
534 <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical
535 cache growth / clean sequence.</p>
537 <p>Because <module>mod_cache_disk</module> does not itself pay attention
538 to how much space is used you should ensure that
539 <a href="programs/htcacheclean.html">htcacheclean</a> is configured to
540 leave enough "grow room" following a clean.</p>
545 <section id="socache-caching">
547 <title>Two-state Key/Value Shared Object Caching</title>
551 <module>mod_authn_socache</module>
552 <module>mod_socache_dbm</module>
553 <module>mod_socache_dc</module>
554 <module>mod_socache_memcache</module>
555 <module>mod_socache_shmcb</module>
556 <module>mod_ssl</module>
559 <directive module="mod_authn_socache">AuthnCacheSOCache</directive>
560 <directive module="mod_ssl">SSLSessionCache</directive>
561 <directive module="mod_ssl">SSLStaplingCache</directive>
565 <p>The Apache HTTP server offers a low level shared object cache for
566 caching information such as SSL sessions, or authentication credentials,
567 within the <a href="socache.html">socache</a> interface.</p>
569 <p>Additional modules are provided for each implementation, offering the
570 following backends:</p>
573 <dt><module>mod_socache_dbm</module></dt>
574 <dd>DBM based shared object cache.</dd>
575 <dt><module>mod_socache_dc</module></dt>
576 <dd>Distcache based shared object cache.</dd>
577 <dt><module>mod_socache_memcache</module></dt>
578 <dd>Memcache based shared object cache.</dd>
579 <dt><module>mod_socache_shmcb</module></dt>
580 <dd>Shared memory based shared object cache.</dd>
583 <section id="mod_authn_socache-caching">
584 <title>Caching Authentication Credentials</title>
588 <module>mod_authn_socache</module>
591 <directive module="mod_authn_socache">AuthnCacheSOCache</directive>
595 <p>The <module>mod_authn_socache</module> module allows the result of
596 authentication to be cached, relieving load on authentication backends.</p>
600 <section id="mod_ssl-caching">
601 <title>Caching SSL Sessions</title>
605 <module>mod_ssl</module>
608 <directive module="mod_ssl">SSLSessionCache</directive>
609 <directive module="mod_ssl">SSLStaplingCache</directive>
613 <p>The <module>mod_ssl</module> module uses the <code>socache</code> interface
614 to provide a session cache and a stapling cache.</p>
620 <section id="file-caching">
622 <title>Specialized File Caching</title>
626 <module>mod_file_cache</module>
629 <directive module="mod_file_cache">CacheFile</directive>
630 <directive module="mod_file_cache">MMapFile</directive>
634 <p>On platforms where a filesystem might be slow, or where file
635 handles are expensive, the option exists to pre-load files into
636 memory on startup.</p>
638 <p>On systems where opening files is slow, the option exists to
639 open the file on startup and cache the file handle. These
640 options can help on systems where access to static files is
643 <section id="filehandle">
644 <title>File-Handle Caching</title>
646 <p>The act of opening a file can itself be a source of delay, particularly
647 on network filesystems. By maintaining a cache of open file descriptors
648 for commonly served files, httpd can avoid this delay. Currently httpd
649 provides one implementation of File-Handle Caching.</p>
652 <title>CacheFile</title>
654 <p>The most basic form of caching present in httpd is the file-handle
655 caching provided by <module>mod_file_cache</module>. Rather than caching
656 file-contents, this cache maintains a table of open file descriptors. Files
657 to be cached in this manner are specified in the configuration file using
658 the <directive module="mod_file_cache">CacheFile</directive>
662 <directive module="mod_file_cache">CacheFile</directive> directive
663 instructs httpd to open the file when it is started and to re-use
664 this file-handle for all subsequent access to this file.</p>
667 CacheFile /usr/local/apache2/htdocs/index.html
670 <p>If you intend to cache a large number of files in this manner, you
671 must ensure that your operating system's limit for the number of open
672 files is set appropriately.</p>
674 <p>Although using <directive module="mod_file_cache">CacheFile</directive>
675 does not cause the file-contents to be cached per-se, it does mean
676 that if the file changes while httpd is running these changes will
677 not be picked up. The file will be consistently served as it was
678 when httpd was started.</p>
680 <p>If the file is removed while httpd is running, it will continue
681 to maintain an open file descriptor and serve the file as it was when
682 httpd was started. This usually also means that although the file
683 will have been deleted, and not show up on the filesystem, extra free
684 space will not be recovered until httpd is stopped and the file
685 descriptor closed.</p>
690 <section id="inmemory">
691 <title>In-Memory Caching</title>
693 <p>Serving directly from system memory is universally the fastest method
694 of serving content. Reading files from a disk controller or, even worse,
695 from a remote network is orders of magnitude slower. Disk controllers
696 usually involve physical processes, and network access is limited by
697 your available bandwidth. Memory access on the other hand can take mere
700 <p>System memory isn't cheap though, byte for byte it's by far the most
701 expensive type of storage and it's important to ensure that it is used
702 efficiently. By caching files in memory you decrease the amount of
703 memory available on the system. As we'll see, in the case of operating
704 system caching, this is not so much of an issue, but when using
705 httpd's own in-memory caching it is important to make sure that you
706 do not allocate too much memory to a cache. Otherwise the system
707 will be forced to swap out memory, which will likely degrade
711 <title>Operating System Caching</title>
713 <p>Almost all modern operating systems cache file-data in memory managed
714 directly by the kernel. This is a powerful feature, and for the most
715 part operating systems get it right. For example, on Linux, let's look at
716 the difference in the time it takes to read a file for the first time
717 and the second time;</p>
720 colm@coroebus:~$ time cat testfile > /dev/null
724 colm@coroebus:~$ time cat testfile > /dev/null
730 <p>Even for this small file, there is a huge difference in the amount
731 of time it takes to read the file. This is because the kernel has cached
732 the file contents in memory.</p>
734 <p>By ensuring there is "spare" memory on your system, you can ensure
735 that more and more file-contents will be stored in this cache. This
736 can be a very efficient means of in-memory caching, and involves no
737 extra configuration of httpd at all.</p>
739 <p>Additionally, because the operating system knows when files are
740 deleted or modified, it can automatically remove file contents from the
741 cache when necessary. This is a big advantage over httpd's in-memory
742 caching which has no way of knowing when a file has changed.</p>
745 <p>Despite the performance and advantages of automatic operating system
746 caching there are some circumstances in which in-memory caching may be
747 better performed by httpd.</p>
750 <title>MMapFile Caching</title>
752 <p><module>mod_file_cache</module> provides the
753 <directive module="mod_file_cache">MMapFile</directive> directive, which
754 allows you to have httpd map a static file's contents into memory at
755 start time (using the mmap system call). httpd will use the in-memory
756 contents for all subsequent accesses to this file.</p>
759 MMapFile /usr/local/apache2/htdocs/index.html
763 <directive module="mod_file_cache">CacheFile</directive> directive, any
764 changes in these files will not be picked up by httpd after it has
767 <p> The <directive module="mod_file_cache">MMapFile</directive>
768 directive does not keep track of how much memory it allocates, so
769 you must ensure not to over-use the directive. Each httpd child
770 process will replicate this memory, so it is critically important
771 to ensure that the files mapped are not so large as to cause the
772 system to swap memory.</p>
778 <section id="security">
779 <title>Security Considerations</title>
782 <title>Authorization and Access Control</title>
784 <p>Using <module>mod_cache</module> in its default state where
785 <directive module="mod_cache">CacheQuickHandler</directive> is set to
786 <code>On</code> is very much like having a caching reverse-proxy bolted
787 to the front of the server. Requests will be served by the caching module
788 unless it determines that the origin server should be queried just as an
789 external cache would, and this drastically changes the security model of
792 <p>As traversing a filesystem hierarchy to examine potential
793 <code>.htaccess</code> files would be a very expensive operation,
794 partially defeating the point of caching (to speed up requests),
795 <module>mod_cache</module> makes no decision about whether a cached
796 entity is authorised for serving. In other words; if
797 <module>mod_cache</module> has cached some content, it will be served
798 from the cache as long as that content has not expired.</p>
800 <p>If, for example, your configuration permits access to a resource by IP
801 address you should ensure that this content is not cached. You can do this
802 by using the <directive module="mod_cache">CacheDisable</directive>
803 directive, or <module>mod_expires</module>. Left unchecked,
804 <module>mod_cache</module> - very much like a reverse proxy - would cache
805 the content when served and then serve it to any client, on any IP
808 <p>When the <directive module="mod_cache">CacheQuickHandler</directive>
809 directive is set to <code>Off</code>, the full set of request processing
810 phases are executed and the security model remains unchanged.</p>
814 <title>Local exploits</title>
816 <p>As requests to end-users can be served from the cache, the cache
817 itself can become a target for those wishing to deface or interfere with
818 content. It is important to bear in mind that the cache must at all
819 times be writable by the user which httpd is running as. This is in
820 stark contrast to the usually recommended situation of maintaining
821 all content unwritable by the Apache user.</p>
823 <p>If the Apache user is compromised, for example through a flaw in
824 a CGI process, it is possible that the cache may be targeted. When
825 using <module>mod_cache_disk</module>, it is relatively easy to
826 insert or modify a cached entity.</p>
828 <p>This presents a somewhat elevated risk in comparison to the other
829 types of attack it is possible to make as the Apache user. If you are
830 using <module>mod_cache_disk</module> you should bear this in mind -
831 ensure you upgrade httpd when security upgrades are announced and
832 run CGI processes as a non-Apache user using <a
833 href="suexec.html">suEXEC</a> if possible.</p>
838 <title>Cache Poisoning</title>
840 <p>When running httpd as a caching proxy server, there is also the
841 potential for so-called cache poisoning. Cache Poisoning is a broad
842 term for attacks in which an attacker causes the proxy server to
843 retrieve incorrect (and usually undesirable) content from the origin
846 <p>For example if the DNS servers used by your system running httpd
847 are vulnerable to DNS cache poisoning, an attacker may be able to control
848 where httpd connects to when requesting content from the origin server.
849 Another example is so-called HTTP request-smuggling attacks.</p>
851 <p>This document is not the correct place for an in-depth discussion
852 of HTTP request smuggling (instead, try your favourite search engine)
853 however it is important to be aware that it is possible to make
854 a series of requests, and to exploit a vulnerability on an origin
855 webserver such that the attacker can entirely control the content
856 retrieved by the proxy.</p>
860 <title>Denial of Service / Cachebusting</title>
862 <p>The Vary mechanism allows multiple variants of the same URL to be
863 cached side by side. Depending on header values provided by the client,
864 the cache will select the correct variant to return to the client. This
865 mechanism can become a problem when an attempt is made to vary on a
866 header that is known to contain a wide range of possible values under
867 normal use, for example the <code>User-Agent</code> header. Depending
868 on the popularity of the particular web site thousands or millions of
869 duplicate cache entries could be created for the same URL, crowding
870 out other entries in the cache.</p>
872 <p>In other cases, there may be a need to change the URL of a particular
873 resource on every request, usually by adding a "cachebuster" string to
874 the URL. If this content is declared cacheable by a server for a
875 significant freshness lifetime, these entries can crowd out
876 legitimate entries in a cache. While <module>mod_cache</module>
878 <directive module="mod_cache">CacheIgnoreURLSessionIdentifiers</directive>
879 directive, this directive should be used with care to ensure that
880 downstream proxy or browser caches aren't subjected to the same denial
881 of service issue.</p>