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23 <manualpage metafile="logs.xml.meta">
25 <title>Log Files</title>
28 <p>In order to effectively manage a web server, it is necessary
29 to get feedback about the activity and performance of the
30 server as well as any problems that may be occurring. The Apache HTTP Server
31 provides very comprehensive and flexible logging
32 capabilities. This document describes how to configure its
33 logging capabilities, and how to understand what the logs
37 <section id="overview">
38 <title>Overview</title>
42 <module>mod_log_config</module>
43 <module>mod_log_forensic</module>
44 <module>mod_logio</module>
45 <module>mod_cgi</module>
50 The Apache HTTP Server provides a variety of different mechanisms for
51 logging everything that happens on your server, from the initial
52 request, through the URL mapping process, to the final resolution of
53 the connection, including any errors that may have occurred in the
54 process. In addition to this, third-party modules may provide logging
55 capabilities, or inject entries into the existing log files, and
56 applications such as CGI programs, or PHP scripts, or other handlers,
57 may send messages to the server error log.
61 In this document we discuss the logging modules that are a standard
62 part of the http server.
67 <section id="security">
68 <title>Security Warning</title>
70 <p>Anyone who can write to the directory where Apache httpd is
71 writing a log file can almost certainly gain access to the uid
72 that the server is started as, which is normally root. Do
73 <em>NOT</em> give people write access to the directory the logs
74 are stored in without being aware of the consequences; see the
75 <a href="misc/security_tips.html">security tips</a> document
78 <p>In addition, log files may contain information supplied
79 directly by the client, without escaping. Therefore, it is
80 possible for malicious clients to insert control-characters in
81 the log files, so care must be taken in dealing with raw
85 <section id="errorlog">
86 <title>Error Log</title>
93 <directive module="core">ErrorLog</directive>
94 <directive module="core">ErrorLogFormat</directive>
95 <directive module="core">LogLevel</directive>
99 <p>The server error log, whose name and location is set by the
100 <directive module="core">ErrorLog</directive> directive, is the
101 most important log file. This is the place where Apache httpd
102 will send diagnostic information and record any errors that it
103 encounters in processing requests. It is the first place to
104 look when a problem occurs with starting the server or with the
105 operation of the server, since it will often contain details of
106 what went wrong and how to fix it.</p>
108 <p>The error log is usually written to a file (typically
109 <code>error_log</code> on Unix systems and
110 <code>error.log</code> on Windows and OS/2). On Unix systems it
111 is also possible to have the server send errors to
112 <code>syslog</code> or <a href="#piped">pipe them to a
115 <p>The format of the error log is defined by the <directive
116 module="core">ErrorLogFormat</directive> directive, with which you
117 can customize what values are logged. A default is format defined
118 if you don't specify one. A typical log message follows:</p>
121 [Fri Sep 09 10:42:29.902022 2011] [core:error] [pid 35708:tid 4328636416]
122 [client 72.15.99.187] File does not exist: /usr/local/apache2/htdocs/favicon.ico
125 <p>The first item in the log entry is the date and time of the
126 message. The next is the module producing the message (core, in this
127 case) and the severity level of that message. This is followed by
128 the process ID and, if appropriate, the thread ID, of the process
129 that experienced the condition. Next, we have the client address
130 that made the request. And finally is the detailed error message,
131 which in this case indicates a request for a file that did not
134 <p>A very wide variety of different messages can appear in the
135 error log. Most look similar to the example above. The error
136 log will also contain debugging output from CGI scripts. Any
137 information written to <code>stderr</code> by a CGI script will
138 be copied directly to the error log.</p>
140 <p>Putting a <code>%L</code> token in both the error log and the access
141 log will produce a log entry ID with which you can correlate the entry
142 in the error log with the entry in the access log. If
143 <module>mod_unique_id</module> is loaded, its unique request ID will be
144 used as the log entry ID, too.</p>
146 <p>During testing, it is often useful to continuously monitor
147 the error log for any problems. On Unix systems, you can
148 accomplish this using:</p>
155 <section id="permodule">
156 <title>Per-module logging</title>
158 <p>The <directive module="core">LogLevel</directive> directive
159 allows you to specify a log severity level on a per-module basis. In
160 this way, if you are troubleshooting a problem with just one
161 particular module, you can turn up its logging volume without also
162 getting the details of other modules that you're not interested in.
163 This is particularly useful for modules such as
164 <module>mod_proxy</module> or <module>mod_rewrite</module> where you
165 want to know details about what it's trying to do.</p>
167 <p>Do this by specifying the name of the module in your
168 <directive>LogLevel</directive> directive:</p>
170 <highlight language="config">
171 LogLevel info rewrite:trace5
174 <p>This sets the main <directive>LogLevel</directive> to info, but
175 turns it up to <code>trace5</code> for
176 <module>mod_rewrite</module>.</p>
178 <note>This replaces the per-module logging directives, such as
179 <code>RewriteLog</code>, that were present in earlier versions of
183 <section id="accesslog">
184 <title>Access Log</title>
188 <module>mod_log_config</module>
189 <module>mod_setenvif</module>
192 <directive module="mod_log_config">CustomLog</directive>
193 <directive module="mod_log_config">LogFormat</directive>
194 <directive module="mod_setenvif">SetEnvIf</directive>
198 <p>The server access log records all requests processed by the
199 server. The location and content of the access log are
200 controlled by the <directive module="mod_log_config">CustomLog</directive>
201 directive. The <directive module="mod_log_config">LogFormat</directive>
202 directive can be used to simplify the selection of
203 the contents of the logs. This section describes how to configure the server
204 to record information in the access log.</p>
206 <p>Of course, storing the information in the access log is only
207 the start of log management. The next step is to analyze this
208 information to produce useful statistics. Log analysis in
209 general is beyond the scope of this document, and not really
210 part of the job of the web server itself. For more information
211 about this topic, and for applications which perform log
212 analysis, check the <a
213 href="http://dmoz.org/Computers/Software/Internet/Site_Management/Log_Analysis/">
217 <p>Various versions of Apache httpd have used other modules and
218 directives to control access logging, including
219 mod_log_referer, mod_log_agent, and the
220 <code>TransferLog</code> directive. The <directive
221 module="mod_log_config">CustomLog</directive> directive now subsumes
222 the functionality of all the older directives.</p>
224 <p>The format of the access log is highly configurable. The format
225 is specified using a format string that looks much like a C-style
226 printf(1) format string. Some examples are presented in the next
227 sections. For a complete list of the possible contents of the
228 format string, see the <module>mod_log_config</module> <a
229 href="mod/mod_log_config.html#formats">format strings</a>.</p>
231 <section id="common">
232 <title>Common Log Format</title>
234 <p>A typical configuration for the access log might look as
237 <highlight language="config">
238 LogFormat "%h %l %u %t \"%r\" %>s %b" common
239 CustomLog logs/access_log common
242 <p>This defines the <em>nickname</em> <code>common</code> and
243 associates it with a particular log format string. The format
244 string consists of percent directives, each of which tell the
245 server to log a particular piece of information. Literal
246 characters may also be placed in the format string and will be
247 copied directly into the log output. The quote character
248 (<code>"</code>) must be escaped by placing a backslash before
249 it to prevent it from being interpreted as the end of the
250 format string. The format string may also contain the special
251 control characters "<code>\n</code>" for new-line and
252 "<code>\t</code>" for tab.</p>
254 <p>The <directive module="mod_log_config">CustomLog</directive>
255 directive sets up a new log file using the defined
256 <em>nickname</em>. The filename for the access log is relative to
257 the <directive module="core">ServerRoot</directive> unless it
258 begins with a slash.</p>
260 <p>The above configuration will write log entries in a format
261 known as the Common Log Format (CLF). This standard format can
262 be produced by many different web servers and read by many log
263 analysis programs. The log file entries produced in CLF will
264 look something like this:</p>
267 127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET
268 /apache_pb.gif HTTP/1.0" 200 2326
271 <p>Each part of this log entry is described below.</p>
274 <dt><code>127.0.0.1</code> (<code>%h</code>)</dt>
276 <dd>This is the IP address of the client (remote host) which
277 made the request to the server. If <directive
278 module="core">HostnameLookups</directive> is
279 set to <code>On</code>, then the server will try to determine
280 the hostname and log it in place of the IP address. However,
281 this configuration is not recommended since it can
282 significantly slow the server. Instead, it is best to use a
283 log post-processor such as <program>logresolve</program> to determine
284 the hostnames. The IP address reported here is not
285 necessarily the address of the machine at which the user is
286 sitting. If a proxy server exists between the user and the
287 server, this address will be the address of the proxy, rather
288 than the originating machine.</dd>
290 <dt><code>-</code> (<code>%l</code>)</dt>
292 <dd>The "hyphen" in the output indicates that the requested
293 piece of information is not available. In this case, the
294 information that is not available is the RFC 1413 identity of
295 the client determined by <code>identd</code> on the clients
296 machine. This information is highly unreliable and should
297 almost never be used except on tightly controlled internal
298 networks. Apache httpd will not even attempt to determine
299 this information unless <directive
300 module="mod_ident">IdentityCheck</directive> is set
301 to <code>On</code>.</dd>
303 <dt><code>frank</code> (<code>%u</code>)</dt>
305 <dd>This is the userid of the person requesting the document
306 as determined by HTTP authentication. The same value is
307 typically provided to CGI scripts in the
308 <code>REMOTE_USER</code> environment variable. If the status
309 code for the request (see below) is 401, then this value
310 should not be trusted because the user is not yet
311 authenticated. If the document is not password protected,
312 this part will be "<code>-</code>" just like the previous
315 <dt><code>[10/Oct/2000:13:55:36 -0700]</code>
316 (<code>%t</code>)</dt>
319 The time that the request was received.
323 <code>[day/month/year:hour:minute:second zone]<br />
325 month = 3*letter<br />
328 minute = 2*digit<br />
329 second = 2*digit<br />
330 zone = (`+' | `-') 4*digit</code>
332 <p>It is possible to have the time displayed in another format
333 by specifying <code>%{format}t</code> in the log format
334 string, where <code>format</code> is either as in
335 <code>strftime(3)</code> from the C standard library,
336 or one of the supported special tokens. For details see
337 the <module>mod_log_config</module> <a
338 href="mod/mod_log_config.html#formats">format strings</a>.</p>
341 <dt><code>"GET /apache_pb.gif HTTP/1.0"</code>
342 (<code>\"%r\"</code>)</dt>
344 <dd>The request line from the client is given in double
345 quotes. The request line contains a great deal of useful
346 information. First, the method used by the client is
347 <code>GET</code>. Second, the client requested the resource
348 <code>/apache_pb.gif</code>, and third, the client used the
349 protocol <code>HTTP/1.0</code>. It is also possible to log
350 one or more parts of the request line independently. For
351 example, the format string "<code>%m %U%q %H</code>" will log
352 the method, path, query-string, and protocol, resulting in
353 exactly the same output as "<code>%r</code>".</dd>
355 <dt><code>200</code> (<code>%>s</code>)</dt>
357 <dd>This is the status code that the server sends back to the
358 client. This information is very valuable, because it reveals
359 whether the request resulted in a successful response (codes
360 beginning in 2), a redirection (codes beginning in 3), an
361 error caused by the client (codes beginning in 4), or an
362 error in the server (codes beginning in 5). The full list of
363 possible status codes can be found in the <a
364 href="http://www.w3.org/Protocols/rfc2616/rfc2616.txt">HTTP
365 specification</a> (RFC2616 section 10).</dd>
367 <dt><code>2326</code> (<code>%b</code>)</dt>
369 <dd>The last part indicates the size of the object returned
370 to the client, not including the response headers. If no
371 content was returned to the client, this value will be
372 "<code>-</code>". To log "<code>0</code>" for no content, use
373 <code>%B</code> instead.</dd>
377 <section id="combined">
378 <title>Combined Log Format</title>
380 <p>Another commonly used format string is called the Combined
381 Log Format. It can be used as follows.</p>
383 <highlight language="config">
384 LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\"" combined
385 CustomLog log/access_log combined
388 <p>This format is exactly the same as the Common Log Format,
389 with the addition of two more fields. Each of the additional
390 fields uses the percent-directive
391 <code>%{<em>header</em>}i</code>, where <em>header</em> can be
392 any HTTP request header. The access log under this format will
396 127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET
397 /apache_pb.gif HTTP/1.0" 200 2326
398 "http://www.example.com/start.html" "Mozilla/4.08 [en]
402 <p>The additional fields are:</p>
405 <dt><code>"http://www.example.com/start.html"</code>
406 (<code>\"%{Referer}i\"</code>)</dt>
408 <dd>The "Referer" (sic) HTTP request header. This gives the
409 site that the client reports having been referred from. (This
410 should be the page that links to or includes
411 <code>/apache_pb.gif</code>).</dd>
413 <dt><code>"Mozilla/4.08 [en] (Win98; I ;Nav)"</code>
414 (<code>\"%{User-agent}i\"</code>)</dt>
416 <dd>The User-Agent HTTP request header. This is the
417 identifying information that the client browser reports about
422 <section id="multiple">
423 <title>Multiple Access Logs</title>
425 <p>Multiple access logs can be created simply by specifying
426 multiple <directive module="mod_log_config">CustomLog</directive>
427 directives in the configuration
428 file. For example, the following directives will create three
429 access logs. The first contains the basic CLF information,
430 while the second and third contain referer and browser
431 information. The last two <directive
432 module="mod_log_config">CustomLog</directive> lines show how
433 to mimic the effects of the <code>ReferLog</code> and <code
434 >AgentLog</code> directives.</p>
436 <highlight language="config">
437 LogFormat "%h %l %u %t \"%r\" %>s %b" common
438 CustomLog logs/access_log common
439 CustomLog logs/referer_log "%{Referer}i -> %U"
440 CustomLog logs/agent_log "%{User-agent}i"
443 <p>This example also shows that it is not necessary to define a
444 nickname with the <directive
445 module="mod_log_config">LogFormat</directive> directive. Instead,
446 the log format can be specified directly in the <directive
447 module="mod_log_config">CustomLog</directive> directive.</p>
450 <section id="conditional">
451 <title>Conditional Logs</title>
453 <p>There are times when it is convenient to exclude certain
454 entries from the access logs based on characteristics of the
455 client request. This is easily accomplished with the help of <a
456 href="env.html">environment variables</a>. First, an
457 environment variable must be set to indicate that the request
458 meets certain conditions. This is usually accomplished with
459 <directive module="mod_setenvif">SetEnvIf</directive>. Then the
460 <code>env=</code> clause of the <directive
461 module="mod_log_config">CustomLog</directive> directive is used to
462 include or exclude requests where the environment variable is
463 set. Some examples:</p>
465 <highlight language="config">
466 # Mark requests from the loop-back interface
467 SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog
468 # Mark requests for the robots.txt file
469 SetEnvIf Request_URI "^/robots\.txt$" dontlog
471 CustomLog logs/access_log common env=!dontlog
474 <p>As another example, consider logging requests from
475 english-speakers to one log file, and non-english speakers to a
476 different log file.</p>
478 <highlight language="config">
479 SetEnvIf Accept-Language "en" english
480 CustomLog logs/english_log common env=english
481 CustomLog logs/non_english_log common env=!english
484 <p>In a caching scenario one would want to know about
485 the efficiency of the cache. A very simple method to
486 find this out would be:</p>
488 <highlight language="config">
490 LogFormat "%h %l %u %t "%r " %>s %b %{CACHE_MISS}e" common-cache
491 CustomLog logs/access_log common-cache
494 <p><module>mod_cache</module> will run before
495 <module>mod_env</module> and, when successful, will deliver the
496 content without it. In that case a cache hit will log
497 <code>-</code>, while a cache miss will log <code>1</code>.</p>
499 <p>In addition to the <code>env=</code> syntax, <directive
500 module="mod_log_config">LogFormat</directive> supports logging values
501 conditional upon the HTTP response code:</p>
503 <highlight language="config">
504 LogFormat "%400,501{User-agent}i" browserlog
505 LogFormat "%!200,304,302{Referer}i" refererlog
508 <p>In the first example, the <code>User-agent</code> will be
509 logged if the HTTP status code is 400 or 501. In other cases, a
510 literal "-" will be logged instead. Likewise, in the second
511 example, the <code>Referer</code> will be logged if the HTTP
512 status code is <strong>not</strong> 200, 204, or 302. (Note the
513 "!" before the status codes.</p>
515 <p>Although we have just shown that conditional logging is very
516 powerful and flexible, it is not the only way to control the
517 contents of the logs. Log files are more useful when they
518 contain a complete record of server activity. It is often
519 easier to simply post-process the log files to remove requests
520 that you do not want to consider.</p>
524 <section id="rotation">
525 <title>Log Rotation</title>
527 <p>On even a moderately busy server, the quantity of
528 information stored in the log files is very large. The access
529 log file typically grows 1 MB or more per 10,000 requests. It
530 will consequently be necessary to periodically rotate the log
531 files by moving or deleting the existing logs. This cannot be
532 done while the server is running, because Apache httpd will continue
533 writing to the old log file as long as it holds the file open.
534 Instead, the server must be <a
535 href="stopping.html">restarted</a> after the log files are
536 moved or deleted so that it will open new log files.</p>
538 <p>By using a <em>graceful</em> restart, the server can be
539 instructed to open new log files without losing any existing or
540 pending connections from clients. However, in order to
541 accomplish this, the server must continue to write to the old
542 log files while it finishes serving old requests. It is
543 therefore necessary to wait for some time after the restart
544 before doing any processing on the log files. A typical
545 scenario that simply rotates the logs and compresses the old
546 logs to save space is:</p>
549 mv access_log access_log.old<br />
550 mv error_log error_log.old<br />
551 apachectl graceful<br />
553 gzip access_log.old error_log.old
556 <p>Another way to perform log rotation is using <a
557 href="#piped">piped logs</a> as discussed in the next
562 <title>Piped Logs</title>
564 <p>Apache httpd is capable of writing error and access log
565 files through a pipe to another process, rather than directly
566 to a file. This capability dramatically increases the
567 flexibility of logging, without adding code to the main server.
568 In order to write logs to a pipe, simply replace the filename
569 with the pipe character "<code>|</code>", followed by the name
570 of the executable which should accept log entries on its
571 standard input. The server will start the piped-log process when
572 the server starts, and will restart it if it crashes while the
573 server is running. (This last feature is why we can refer to
574 this technique as "reliable piped logging".)</p>
576 <p>Piped log processes are spawned by the parent Apache httpd
577 process, and inherit the userid of that process. This means
578 that piped log programs usually run as root. It is therefore
579 very important to keep the programs simple and secure.</p>
581 <p>One important use of piped logs is to allow log rotation
582 without having to restart the server. The Apache HTTP Server
583 includes a simple program called <program>rotatelogs</program>
584 for this purpose. For example, to rotate the logs every 24 hours, you
587 <highlight language="config">
588 CustomLog "|/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common
591 <p>Notice that quotes are used to enclose the entire command
592 that will be called for the pipe. Although these examples are
593 for the access log, the same technique can be used for the
596 <p>As with conditional logging, piped logs are a very powerful
597 tool, but they should not be used where a simpler solution like
598 off-line post-processing is available.</p>
600 <p>By default the piped log process is spawned without invoking
601 a shell. Use "<code>|$</code>" instead of "<code>|</code>"
602 to spawn using a shell (usually with <code>/bin/sh -c</code>):</p>
604 <highlight language="config">
605 # Invoke "rotatelogs" using a shell
606 CustomLog "|$/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common
609 <p>This was the default behaviour for Apache 2.2.
610 Depending on the shell specifics this might lead to
611 an additional shell process for the lifetime of the logging
612 pipe program and signal handling problems during restart.
613 For compatibility reasons with Apache 2.2 the notation
614 "<code>||</code>" is also supported and equivalent to using
615 "<code>|</code>".</p>
617 <note><title>Windows note</title>
618 <p>Note that on Windows, you may run into problems when running many piped
619 logger processes, especially when HTTPD is running as a service. This is
620 caused by running out of desktop heap space. The desktop heap space given
621 to each service is specified by the third argument to the
622 <code>SharedSection</code> parameter in the
623 HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SessionManager\SubSystems\Windows
624 registry value. <strong>Change this value with care</strong>; the normal
625 caveats for changing the Windows registry apply, but you might also exhaust
626 the desktop heap pool if the number is adjusted too high.</p>
630 <section id="virtualhost">
631 <title>Virtual Hosts</title>
633 <p>When running a server with many <a href="vhosts/">virtual
634 hosts</a>, there are several options for dealing with log
635 files. First, it is possible to use logs exactly as in a
636 single-host server. Simply by placing the logging directives
637 outside the <directive module="core"
638 type="section">VirtualHost</directive> sections in the
639 main server context, it is possible to log all requests in the
640 same access log and error log. This technique does not allow
641 for easy collection of statistics on individual virtual
644 <p>If <directive module="mod_log_config">CustomLog</directive>
645 or <directive module="core">ErrorLog</directive>
646 directives are placed inside a
647 <directive module="core" type="section">VirtualHost</directive>
648 section, all requests or errors for that virtual host will be
649 logged only to the specified file. Any virtual host which does
650 not have logging directives will still have its requests sent
651 to the main server logs. This technique is very useful for a
652 small number of virtual hosts, but if the number of hosts is
653 very large, it can be complicated to manage. In addition, it
654 can often create problems with <a
655 href="vhosts/fd-limits.html">insufficient file
658 <p>For the access log, there is a very good compromise. By
659 adding information on the virtual host to the log format
660 string, it is possible to log all hosts to the same log, and
661 later split the log into individual files. For example,
662 consider the following directives.</p>
664 <highlight language="config">
665 LogFormat "%v %l %u %t \"%r\" %>s %b" comonvhost
666 CustomLog logs/access_log comonvhost
669 <p>The <code>%v</code> is used to log the name of the virtual
670 host that is serving the request. Then a program like <a
671 href="programs/split-logfile.html">split-logfile</a> can be used to
672 post-process the access log in order to split it into one file
673 per virtual host.</p>
677 <title>Other Log Files</title>
681 <module>mod_logio</module>
682 <module>mod_log_config</module>
683 <module>mod_log_forensic</module>
684 <module>mod_cgi</module>
688 <directive module="mod_log_config">LogFormat</directive>
689 <directive module="mod_log_config">BufferedLogs</directive>
690 <directive module="mod_log_forensic">ForensicLog</directive>
691 <directive module="mpm_common">PidFile</directive>
692 <directive module="mod_cgi">ScriptLog</directive>
693 <directive module="mod_cgi">ScriptLogBuffer</directive>
694 <directive module="mod_cgi">ScriptLogLength</directive>
699 <title>Logging actual bytes sent and received</title>
701 <p><module>mod_logio</module> adds in two additional
702 <directive module="mod_log_config">LogFormat</directive> fields
703 (%I and %O) that log the actual number of bytes received and sent
708 <title>Forensic Logging</title>
710 <p><module>mod_log_forensic</module> provides for forensic logging of
711 client requests. Logging is done before and after processing a
712 request, so the forensic log contains two log lines for each
713 request. The forensic logger is very strict with no customizations.
714 It can be an invaluable debugging and security tool.</p>
717 <section id="pidfile">
718 <title>PID File</title>
720 <p>On startup, Apache httpd saves the process id of the parent
721 httpd process to the file <code>logs/httpd.pid</code>. This
722 filename can be changed with the <directive
723 module="mpm_common">PidFile</directive> directive. The
724 process-id is for use by the administrator in restarting and
725 terminating the daemon by sending signals to the parent
726 process; on Windows, use the -k command line option instead.
727 For more information see the <a href="stopping.html">Stopping
728 and Restarting</a> page.</p>
731 <section id="scriptlog">
732 <title>Script Log</title>
734 <p>In order to aid in debugging, the
735 <directive module="mod_cgi">ScriptLog</directive> directive
736 allows you to record the input to and output from CGI scripts.
737 This should only be used in testing - not for live servers.
738 More information is available in the <a
739 href="mod/mod_cgi.html">mod_cgi</a> documentation.</p>