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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/">
214 Open Directory</a> or <a
215 href="http://dir.yahoo.com/Computers_and_Internet/Software/Internet/World_Wide_Web/Servers/Log_Analysis_Tools/">
218 <p>Various versions of Apache httpd have used other modules and
219 directives to control access logging, including
220 mod_log_referer, mod_log_agent, and the
221 <code>TransferLog</code> directive. The <directive
222 module="mod_log_config">CustomLog</directive> directive now subsumes
223 the functionality of all the older directives.</p>
225 <p>The format of the access log is highly configurable. The format
226 is specified using a format string that looks much like a C-style
227 printf(1) format string. Some examples are presented in the next
228 sections. For a complete list of the possible contents of the
229 format string, see the <module>mod_log_config</module> <a
230 href="mod/mod_log_config.html#formats">format strings</a>.</p>
232 <section id="common">
233 <title>Common Log Format</title>
235 <p>A typical configuration for the access log might look as
238 <highlight language="config">
239 LogFormat "%h %l %u %t \"%r\" %>s %b" common
240 CustomLog "logs/access_log" common
243 <p>This defines the <em>nickname</em> <code>common</code> and
244 associates it with a particular log format string. The format
245 string consists of percent directives, each of which tell the
246 server to log a particular piece of information. Literal
247 characters may also be placed in the format string and will be
248 copied directly into the log output. The quote character
249 (<code>"</code>) must be escaped by placing a backslash before
250 it to prevent it from being interpreted as the end of the
251 format string. The format string may also contain the special
252 control characters "<code>\n</code>" for new-line and
253 "<code>\t</code>" for tab.</p>
255 <p>The <directive module="mod_log_config">CustomLog</directive>
256 directive sets up a new log file using the defined
257 <em>nickname</em>. The filename for the access log is relative to
258 the <directive module="core">ServerRoot</directive> unless it
259 begins with a slash.</p>
261 <p>The above configuration will write log entries in a format
262 known as the Common Log Format (CLF). This standard format can
263 be produced by many different web servers and read by many log
264 analysis programs. The log file entries produced in CLF will
265 look something like this:</p>
268 127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET
269 /apache_pb.gif HTTP/1.0" 200 2326
272 <p>Each part of this log entry is described below.</p>
275 <dt><code>127.0.0.1</code> (<code>%h</code>)</dt>
277 <dd>This is the IP address of the client (remote host) which
278 made the request to the server. If <directive
279 module="core">HostnameLookups</directive> is
280 set to <code>On</code>, then the server will try to determine
281 the hostname and log it in place of the IP address. However,
282 this configuration is not recommended since it can
283 significantly slow the server. Instead, it is best to use a
284 log post-processor such as <program>logresolve</program> to determine
285 the hostnames. The IP address reported here is not
286 necessarily the address of the machine at which the user is
287 sitting. If a proxy server exists between the user and the
288 server, this address will be the address of the proxy, rather
289 than the originating machine.</dd>
291 <dt><code>-</code> (<code>%l</code>)</dt>
293 <dd>The "hyphen" in the output indicates that the requested
294 piece of information is not available. In this case, the
295 information that is not available is the RFC 1413 identity of
296 the client determined by <code>identd</code> on the clients
297 machine. This information is highly unreliable and should
298 almost never be used except on tightly controlled internal
299 networks. Apache httpd will not even attempt to determine
300 this information unless <directive
301 module="mod_ident">IdentityCheck</directive> is set
302 to <code>On</code>.</dd>
304 <dt><code>frank</code> (<code>%u</code>)</dt>
306 <dd>This is the userid of the person requesting the document
307 as determined by HTTP authentication. The same value is
308 typically provided to CGI scripts in the
309 <code>REMOTE_USER</code> environment variable. If the status
310 code for the request (see below) is 401, then this value
311 should not be trusted because the user is not yet
312 authenticated. If the document is not password protected,
313 this part will be "<code>-</code>" just like the previous
316 <dt><code>[10/Oct/2000:13:55:36 -0700]</code>
317 (<code>%t</code>)</dt>
320 The time that the request was received.
324 <code>[day/month/year:hour:minute:second zone]<br />
326 month = 3*letter<br />
329 minute = 2*digit<br />
330 second = 2*digit<br />
331 zone = (`+' | `-') 4*digit</code>
333 <p>It is possible to have the time displayed in another format
334 by specifying <code>%{format}t</code> in the log format
335 string, where <code>format</code> is either as in
336 <code>strftime(3)</code> from the C standard library,
337 or one of the supported special tokens. For details see
338 the <module>mod_log_config</module> <a
339 href="mod/mod_log_config.html#formats">format strings</a>.</p>
342 <dt><code>"GET /apache_pb.gif HTTP/1.0"</code>
343 (<code>\"%r\"</code>)</dt>
345 <dd>The request line from the client is given in double
346 quotes. The request line contains a great deal of useful
347 information. First, the method used by the client is
348 <code>GET</code>. Second, the client requested the resource
349 <code>/apache_pb.gif</code>, and third, the client used the
350 protocol <code>HTTP/1.0</code>. It is also possible to log
351 one or more parts of the request line independently. For
352 example, the format string "<code>%m %U%q %H</code>" will log
353 the method, path, query-string, and protocol, resulting in
354 exactly the same output as "<code>%r</code>".</dd>
356 <dt><code>200</code> (<code>%>s</code>)</dt>
358 <dd>This is the status code that the server sends back to the
359 client. This information is very valuable, because it reveals
360 whether the request resulted in a successful response (codes
361 beginning in 2), a redirection (codes beginning in 3), an
362 error caused by the client (codes beginning in 4), or an
363 error in the server (codes beginning in 5). The full list of
364 possible status codes can be found in the <a
365 href="http://www.w3.org/Protocols/rfc2616/rfc2616.txt">HTTP
366 specification</a> (RFC2616 section 10).</dd>
368 <dt><code>2326</code> (<code>%b</code>)</dt>
370 <dd>The last part indicates the size of the object returned
371 to the client, not including the response headers. If no
372 content was returned to the client, this value will be
373 "<code>-</code>". To log "<code>0</code>" for no content, use
374 <code>%B</code> instead.</dd>
378 <section id="combined">
379 <title>Combined Log Format</title>
381 <p>Another commonly used format string is called the Combined
382 Log Format. It can be used as follows.</p>
384 <highlight language="config">
385 LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\"" combined
386 CustomLog "log/access_log" combined
389 <p>This format is exactly the same as the Common Log Format,
390 with the addition of two more fields. Each of the additional
391 fields uses the percent-directive
392 <code>%{<em>header</em>}i</code>, where <em>header</em> can be
393 any HTTP request header. The access log under this format will
397 127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET
398 /apache_pb.gif HTTP/1.0" 200 2326
399 "http://www.example.com/start.html" "Mozilla/4.08 [en]
403 <p>The additional fields are:</p>
406 <dt><code>"http://www.example.com/start.html"</code>
407 (<code>\"%{Referer}i\"</code>)</dt>
409 <dd>The "Referer" (sic) HTTP request header. This gives the
410 site that the client reports having been referred from. (This
411 should be the page that links to or includes
412 <code>/apache_pb.gif</code>).</dd>
414 <dt><code>"Mozilla/4.08 [en] (Win98; I ;Nav)"</code>
415 (<code>\"%{User-agent}i\"</code>)</dt>
417 <dd>The User-Agent HTTP request header. This is the
418 identifying information that the client browser reports about
423 <section id="multiple">
424 <title>Multiple Access Logs</title>
426 <p>Multiple access logs can be created simply by specifying
427 multiple <directive module="mod_log_config">CustomLog</directive>
428 directives in the configuration
429 file. For example, the following directives will create three
430 access logs. The first contains the basic CLF information,
431 while the second and third contain referer and browser
432 information. The last two <directive
433 module="mod_log_config">CustomLog</directive> lines show how
434 to mimic the effects of the <code>ReferLog</code> and <code
435 >AgentLog</code> directives.</p>
437 <highlight language="config">
438 LogFormat "%h %l %u %t \"%r\" %>s %b" common
439 CustomLog "logs/access_log" common
440 CustomLog "logs/referer_log" "%{Referer}i -> %U"
441 CustomLog "logs/agent_log" "%{User-agent}i"
444 <p>This example also shows that it is not necessary to define a
445 nickname with the <directive
446 module="mod_log_config">LogFormat</directive> directive. Instead,
447 the log format can be specified directly in the <directive
448 module="mod_log_config">CustomLog</directive> directive.</p>
451 <section id="conditional">
452 <title>Conditional Logs</title>
454 <p>There are times when it is convenient to exclude certain
455 entries from the access logs based on characteristics of the
456 client request. This is easily accomplished with the help of <a
457 href="env.html">environment variables</a>. First, an
458 environment variable must be set to indicate that the request
459 meets certain conditions. This is usually accomplished with
460 <directive module="mod_setenvif">SetEnvIf</directive>. Then the
461 <code>env=</code> clause of the <directive
462 module="mod_log_config">CustomLog</directive> directive is used to
463 include or exclude requests where the environment variable is
464 set. Some examples:</p>
466 <highlight language="config">
467 # Mark requests from the loop-back interface
468 SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog
469 # Mark requests for the robots.txt file
470 SetEnvIf Request_URI "^/robots\.txt$" dontlog
472 CustomLog "logs/access_log" common env=!dontlog
475 <p>As another example, consider logging requests from
476 english-speakers to one log file, and non-english speakers to a
477 different log file.</p>
479 <highlight language="config">
480 SetEnvIf Accept-Language "en" english<br />
481 CustomLog "logs/english_log" common env=english<br />
482 CustomLog "logs/non_english_log" common env=!english
485 <p>In a caching scenario one would want to know about
486 the efficiency of the cache. A very simple method to
487 find this out would be:</p>
489 <highlight language="config">
491 LogFormat "%h %l %u %t "%r " %>s %b %{CACHE_MISS}e" common-cache
492 CustomLog "logs/access_log" common-cache
495 <p><module>mod_cache</module> will run before
496 <module>mod_env</module> and, when successful, will deliver the
497 content without it. In that case a cache hit will log
498 <code>-</code>, while a cache miss will log <code>1</code>.</p>
500 <p>In addition to the <code>env=</code> syntax, <directive
501 module="mod_log_config">LogFormat</directive> supports logging values
502 conditional upon the HTTP response code:</p>
504 <highlight language="config">
505 LogFormat "%400,501{User-agent}i" browserlog
506 LogFormat "%!200,304,302{Referer}i" refererlog
509 <p>In the first example, the <code>User-agent</code> will be
510 logged if the HTTP status code is 400 or 501. In other cases, a
511 literal "-" will be logged instead. Likewise, in the second
512 example, the <code>Referer</code> will be logged if the HTTP
513 status code is <strong>not</strong> 200, 204, or 302. (Note the
514 "!" before the status codes.</p>
516 <p>Although we have just shown that conditional logging is very
517 powerful and flexible, it is not the only way to control the
518 contents of the logs. Log files are more useful when they
519 contain a complete record of server activity. It is often
520 easier to simply post-process the log files to remove requests
521 that you do not want to consider.</p>
525 <section id="rotation">
526 <title>Log Rotation</title>
528 <p>On even a moderately busy server, the quantity of
529 information stored in the log files is very large. The access
530 log file typically grows 1 MB or more per 10,000 requests. It
531 will consequently be necessary to periodically rotate the log
532 files by moving or deleting the existing logs. This cannot be
533 done while the server is running, because Apache httpd will continue
534 writing to the old log file as long as it holds the file open.
535 Instead, the server must be <a
536 href="stopping.html">restarted</a> after the log files are
537 moved or deleted so that it will open new log files.</p>
539 <p>By using a <em>graceful</em> restart, the server can be
540 instructed to open new log files without losing any existing or
541 pending connections from clients. However, in order to
542 accomplish this, the server must continue to write to the old
543 log files while it finishes serving old requests. It is
544 therefore necessary to wait for some time after the restart
545 before doing any processing on the log files. A typical
546 scenario that simply rotates the logs and compresses the old
547 logs to save space is:</p>
550 mv access_log access_log.old<br />
551 mv error_log error_log.old<br />
552 apachectl graceful<br />
554 gzip access_log.old error_log.old
557 <p>Another way to perform log rotation is using <a
558 href="#piped">piped logs</a> as discussed in the next
563 <title>Piped Logs</title>
565 <p>Apache httpd is capable of writing error and access log
566 files through a pipe to another process, rather than directly
567 to a file. This capability dramatically increases the
568 flexibility of logging, without adding code to the main server.
569 In order to write logs to a pipe, simply replace the filename
570 with the pipe character "<code>|</code>", followed by the name
571 of the executable which should accept log entries on its
572 standard input. The server will start the piped-log process when
573 the server starts, and will restart it if it crashes while the
574 server is running. (This last feature is why we can refer to
575 this technique as "reliable piped logging".)</p>
577 <p>Piped log processes are spawned by the parent Apache httpd
578 process, and inherit the userid of that process. This means
579 that piped log programs usually run as root. It is therefore
580 very important to keep the programs simple and secure.</p>
582 <p>One important use of piped logs is to allow log rotation
583 without having to restart the server. The Apache HTTP Server
584 includes a simple program called <program>rotatelogs</program>
585 for this purpose. For example, to rotate the logs every 24 hours, you
588 <highlight language="config">
589 CustomLog "|/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common
592 <p>Notice that quotes are used to enclose the entire command
593 that will be called for the pipe. Although these examples are
594 for the access log, the same technique can be used for the
597 <p>As with conditional logging, piped logs are a very powerful
598 tool, but they should not be used where a simpler solution like
599 off-line post-processing is available.</p>
601 <p>By default the piped log process is spawned without invoking
602 a shell. Use "<code>|$</code>" instead of "<code>|</code>"
603 to spawn using a shell (usually with <code>/bin/sh -c</code>):</p>
605 <highlight language="config">
606 # Invoke "rotatelogs" using a shell
607 CustomLog "|$/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common
610 <p>This was the default behaviour for Apache 2.2.
611 Depending on the shell specifics this might lead to
612 an additional shell process for the lifetime of the logging
613 pipe program and signal handling problems during restart.
614 For compatibility reasons with Apache 2.2 the notation
615 "<code>||</code>" is also supported and equivalent to using
616 "<code>|</code>".</p>
618 <note><title>Windows note</title>
619 <p>Note that on Windows, you may run into problems when running many piped
620 logger processes, especially when HTTPD is running as a service. This is
621 caused by running out of desktop heap space. The desktop heap space given
622 to each service is specified by the third argument to the
623 <code>SharedSection</code> parameter in the
624 HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SessionManager\SubSystems\Windows
625 registry value. <strong>Change this value with care</strong>; the normal
626 caveats for changing the Windows registry apply, but you might also exhaust
627 the desktop heap pool if the number is adjusted too high.</p>
631 <section id="virtualhost">
632 <title>Virtual Hosts</title>
634 <p>When running a server with many <a href="vhosts/">virtual
635 hosts</a>, there are several options for dealing with log
636 files. First, it is possible to use logs exactly as in a
637 single-host server. Simply by placing the logging directives
638 outside the <directive module="core"
639 type="section">VirtualHost</directive> sections in the
640 main server context, it is possible to log all requests in the
641 same access log and error log. This technique does not allow
642 for easy collection of statistics on individual virtual
645 <p>If <directive module="mod_log_config">CustomLog</directive>
646 or <directive module="core">ErrorLog</directive>
647 directives are placed inside a
648 <directive module="core" type="section">VirtualHost</directive>
649 section, all requests or errors for that virtual host will be
650 logged only to the specified file. Any virtual host which does
651 not have logging directives will still have its requests sent
652 to the main server logs. This technique is very useful for a
653 small number of virtual hosts, but if the number of hosts is
654 very large, it can be complicated to manage. In addition, it
655 can often create problems with <a
656 href="vhosts/fd-limits.html">insufficient file
659 <p>For the access log, there is a very good compromise. By
660 adding information on the virtual host to the log format
661 string, it is possible to log all hosts to the same log, and
662 later split the log into individual files. For example,
663 consider the following directives.</p>
665 <highlight language="config">
666 LogFormat "%v %l %u %t \"%r\" %>s %b" comonvhost
667 CustomLog "logs/access_log" comonvhost
670 <p>The <code>%v</code> is used to log the name of the virtual
671 host that is serving the request. Then a program like <a
672 href="programs/split-logfile.html">split-logfile</a> can be used to
673 post-process the access log in order to split it into one file
674 per virtual host.</p>
678 <title>Other Log Files</title>
682 <module>mod_logio</module>
683 <module>mod_log_config</module>
684 <module>mod_log_forensic</module>
685 <module>mod_cgi</module>
689 <directive module="mod_log_config">LogFormat</directive>
690 <directive module="mod_log_config">BufferedLogs</directive>
691 <directive module="mod_log_forensic">ForensicLog</directive>
692 <directive module="mpm_common">PidFile</directive>
693 <directive module="mod_cgi">ScriptLog</directive>
694 <directive module="mod_cgi">ScriptLogBuffer</directive>
695 <directive module="mod_cgi">ScriptLogLength</directive>
700 <title>Logging actual bytes sent and received</title>
702 <p><module>mod_logio</module> adds in two additional
703 <directive module="mod_log_config">LogFormat</directive> fields
704 (%I and %O) that log the actual number of bytes received and sent
709 <title>Forensic Logging</title>
711 <p><module>mod_log_forensic</module> provides for forensic logging of
712 client requests. Logging is done before and after processing a
713 request, so the forensic log contains two log lines for each
714 request. The forensic logger is very strict with no customizations.
715 It can be an invaluable debugging and security tool.</p>
718 <section id="pidfile">
719 <title>PID File</title>
721 <p>On startup, Apache httpd saves the process id of the parent
722 httpd process to the file <code>logs/httpd.pid</code>. This
723 filename can be changed with the <directive
724 module="mpm_common">PidFile</directive> directive. The
725 process-id is for use by the administrator in restarting and
726 terminating the daemon by sending signals to the parent
727 process; on Windows, use the -k command line option instead.
728 For more information see the <a href="stopping.html">Stopping
729 and Restarting</a> page.</p>
732 <section id="scriptlog">
733 <title>Script Log</title>
735 <p>In order to aid in debugging, the
736 <directive module="mod_cgi">ScriptLog</directive> directive
737 allows you to record the input to and output from CGI scripts.
738 This should only be used in testing - not for live servers.
739 More information is available in the <a
740 href="mod/mod_cgi.html">mod_cgi</a> documentation.</p>