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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>
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
239 LogFormat "%h %l %u %t \"%r\" %>s %b" common<br />
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="core">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>
385 LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\"
386 \"%{User-agent}i\"" combined<br />
387 CustomLog log/access_log combined
390 <p>This format is exactly the same as the Common Log Format,
391 with the addition of two more fields. Each of the additional
392 fields uses the percent-directive
393 <code>%{<em>header</em>}i</code>, where <em>header</em> can be
394 any HTTP request header. The access log under this format will
398 127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET
399 /apache_pb.gif HTTP/1.0" 200 2326
400 "http://www.example.com/start.html" "Mozilla/4.08 [en]
404 <p>The additional fields are:</p>
407 <dt><code>"http://www.example.com/start.html"</code>
408 (<code>\"%{Referer}i\"</code>)</dt>
410 <dd>The "Referer" (sic) HTTP request header. This gives the
411 site that the client reports having been referred from. (This
412 should be the page that links to or includes
413 <code>/apache_pb.gif</code>).</dd>
415 <dt><code>"Mozilla/4.08 [en] (Win98; I ;Nav)"</code>
416 (<code>\"%{User-agent}i\"</code>)</dt>
418 <dd>The User-Agent HTTP request header. This is the
419 identifying information that the client browser reports about
424 <section id="multiple">
425 <title>Multiple Access Logs</title>
427 <p>Multiple access logs can be created simply by specifying
428 multiple <directive module="mod_log_config">CustomLog</directive>
429 directives in the configuration
430 file. For example, the following directives will create three
431 access logs. The first contains the basic CLF information,
432 while the second and third contain referer and browser
433 information. The last two <directive
434 module="mod_log_config">CustomLog</directive> lines show how
435 to mimic the effects of the <code>ReferLog</code> and <code
436 >AgentLog</code> directives.</p>
439 LogFormat "%h %l %u %t \"%r\" %>s %b" common<br />
440 CustomLog logs/access_log common<br />
441 CustomLog logs/referer_log "%{Referer}i -> %U"<br />
442 CustomLog logs/agent_log "%{User-agent}i"
445 <p>This example also shows that it is not necessary to define a
446 nickname with the <directive
447 module="mod_log_config">LogFormat</directive> directive. Instead,
448 the log format can be specified directly in the <directive
449 module="mod_log_config">CustomLog</directive> directive.</p>
452 <section id="conditional">
453 <title>Conditional Logs</title>
455 <p>There are times when it is convenient to exclude certain
456 entries from the access logs based on characteristics of the
457 client request. This is easily accomplished with the help of <a
458 href="env.html">environment variables</a>. First, an
459 environment variable must be set to indicate that the request
460 meets certain conditions. This is usually accomplished with
461 <directive module="mod_setenvif">SetEnvIf</directive>. Then the
462 <code>env=</code> clause of the <directive
463 module="mod_log_config">CustomLog</directive> directive is used to
464 include or exclude requests where the environment variable is
465 set. Some examples:</p>
468 # Mark requests from the loop-back interface<br />
469 SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog<br />
470 # Mark requests for the robots.txt file<br />
471 SetEnvIf Request_URI "^/robots\.txt$" dontlog<br />
472 # Log what remains<br />
473 CustomLog logs/access_log common env=!dontlog
476 <p>As another example, consider logging requests from
477 english-speakers to one log file, and non-english speakers to a
478 different log file.</p>
481 SetEnvIf Accept-Language "en" english<br />
482 CustomLog logs/english_log common env=english<br />
483 CustomLog logs/non_english_log common env=!english
486 <p>In a caching scenario one would want to know about
487 the efficiency of the cache. A very simple method to
488 find this out would be:</p>
491 SetEnv CACHE_MISS 1<br />
492 LogFormat "%h %l %u %t "%r " %>s %b %{CACHE_MISS}e" common-cache<br />
493 CustomLog logs/access_log common-cache
496 <p><module>mod_cache</module> will run before
497 <module>mod_env</module> and when successfull will deliver the
498 content without it. In that case a cache hit will log
499 <code>-</code>, while a cache miss will log <code>1</code>.</p>
501 <p>In addition to the <code>env=</code> syntax, <directive
502 module="mod_log_config">LogFormat</directive> supports logging values
503 conditional upon the HTTP response code:</p>
506 LogFormat "%400,501{User-agent}i" browserlog<br />
507 LogFormat "%!200,304,302{Referer}i" refererlog
510 <p>In the first example, the <code>User-agent</code> will be
511 logged if the HTTP status code is 400 or 501. In other cases, a
512 literal "-" will be logged instead. Likewise, in the second
513 example, the <code>Referer</code> will be logged if the HTTP
514 status code is <strong>not</strong> 200, 204, or 302. (Note the
515 "!" before the status codes.</p>
517 <p>Although we have just shown that conditional logging is very
518 powerful and flexible, it is not the only way to control the
519 contents of the logs. Log files are more useful when they
520 contain a complete record of server activity. It is often
521 easier to simply post-process the log files to remove requests
522 that you do not want to consider.</p>
526 <section id="rotation">
527 <title>Log Rotation</title>
529 <p>On even a moderately busy server, the quantity of
530 information stored in the log files is very large. The access
531 log file typically grows 1 MB or more per 10,000 requests. It
532 will consequently be necessary to periodically rotate the log
533 files by moving or deleting the existing logs. This cannot be
534 done while the server is running, because Apache httpd will continue
535 writing to the old log file as long as it holds the file open.
536 Instead, the server must be <a
537 href="stopping.html">restarted</a> after the log files are
538 moved or deleted so that it will open new log files.</p>
540 <p>By using a <em>graceful</em> restart, the server can be
541 instructed to open new log files without losing any existing or
542 pending connections from clients. However, in order to
543 accomplish this, the server must continue to write to the old
544 log files while it finishes serving old requests. It is
545 therefore necessary to wait for some time after the restart
546 before doing any processing on the log files. A typical
547 scenario that simply rotates the logs and compresses the old
548 logs to save space is:</p>
551 mv access_log access_log.old<br />
552 mv error_log error_log.old<br />
553 apachectl graceful<br />
555 gzip access_log.old error_log.old
558 <p>Another way to perform log rotation is using <a
559 href="#piped">piped logs</a> as discussed in the next
564 <title>Piped Logs</title>
566 <p>Apache httpd is capable of writing error and access log
567 files through a pipe to another process, rather than directly
568 to a file. This capability dramatically increases the
569 flexibility of logging, without adding code to the main server.
570 In order to write logs to a pipe, simply replace the filename
571 with the pipe character "<code>|</code>", followed by the name
572 of the executable which should accept log entries on its
573 standard input. The server will start the piped-log process when
574 the server starts, and will restart it if it crashes while the
575 server is running. (This last feature is why we can refer to
576 this technique as "reliable piped logging".)</p>
578 <p>Piped log processes are spawned by the parent Apache httpd
579 process, and inherit the userid of that process. This means
580 that piped log programs usually run as root. It is therefore
581 very important to keep the programs simple and secure.</p>
583 <p>One important use of piped logs is to allow log rotation
584 without having to restart the server. The Apache HTTP Server
585 includes a simple program called <program>rotatelogs</program>
586 for this purpose. For example, to rotate the logs every 24 hours, you
590 CustomLog "|/usr/local/apache/bin/rotatelogs
591 /var/log/access_log 86400" common
594 <p>Notice that quotes are used to enclose the entire command
595 that will be called for the pipe. Although these examples are
596 for the access log, the same technique can be used for the
599 <p>A similar but much more flexible log rotation program
600 called <a href="http://www.cronolog.org/">cronolog</a>
601 is available at an external site.</p>
603 <p>As with conditional logging, piped logs are a very powerful
604 tool, but they should not be used where a simpler solution like
605 off-line post-processing is available.</p>
607 <p>By default the piped log process is spawned without invoking
608 a shell. Use "<code>|$</code>" instead of "<code>|</code>"
609 to spawn using a shell (usually with <code>/bin/sh -c</code>):</p>
612 # Invoke "rotatelogs" using a shell<br />
613 CustomLog "|$/usr/local/apache/bin/rotatelogs
614 /var/log/access_log 86400" common
617 <p>This was the default behaviour for Apache 2.2.
618 Depending on the shell specifics this might lead to
619 an additional shell process for the lifetime of the logging
620 pipe program and signal handling problems during restart.
621 For compatibility reasons with Apache 2.2 the notation
622 "<code>||</code>" is also supported and equivalent to using
623 "<code>|</code>".</p>
626 <section id="virtualhost">
627 <title>Virtual Hosts</title>
629 <p>When running a server with many <a href="vhosts/">virtual
630 hosts</a>, there are several options for dealing with log
631 files. First, it is possible to use logs exactly as in a
632 single-host server. Simply by placing the logging directives
633 outside the <directive module="core"
634 type="section">VirtualHost</directive> sections in the
635 main server context, it is possible to log all requests in the
636 same access log and error log. This technique does not allow
637 for easy collection of statistics on individual virtual
640 <p>If <directive module="mod_log_config">CustomLog</directive>
641 or <directive module="core">ErrorLog</directive>
642 directives are placed inside a
643 <directive module="core" type="section">VirtualHost</directive>
644 section, all requests or errors for that virtual host will be
645 logged only to the specified file. Any virtual host which does
646 not have logging directives will still have its requests sent
647 to the main server logs. This technique is very useful for a
648 small number of virtual hosts, but if the number of hosts is
649 very large, it can be complicated to manage. In addition, it
650 can often create problems with <a
651 href="vhosts/fd-limits.html">insufficient file
654 <p>For the access log, there is a very good compromise. By
655 adding information on the virtual host to the log format
656 string, it is possible to log all hosts to the same log, and
657 later split the log into individual files. For example,
658 consider the following directives.</p>
661 LogFormat "%v %l %u %t \"%r\" %>s %b"
663 CustomLog logs/access_log comonvhost
666 <p>The <code>%v</code> is used to log the name of the virtual
667 host that is serving the request. Then a program like <a
668 href="programs/other.html">split-logfile</a> can be used to
669 post-process the access log in order to split it into one file
670 per virtual host.</p>
674 <title>Other Log Files</title>
678 <module>mod_logio</module>
679 <module>mod_log_config</module>
680 <module>mod_log_forensic</module>
681 <module>mod_cgi</module>
685 <directive module="mod_log_config">LogFormat</directive>
686 <directive module="mod_log_config">BufferedLogs</directive>
687 <directive module="mod_log_forensic">ForensicLog</directive>
688 <directive module="mpm_common">PidFile</directive>
689 <directive module="mod_cgi">ScriptLog</directive>
690 <directive module="mod_cgi">ScriptLogBuffer</directive>
691 <directive module="mod_cgi">ScriptLogLength</directive>
696 <title>Logging actual bytes sent and received</title>
698 <p><module>mod_logio</module> adds in two additional
699 <directive module="mod_log_config">LogFormat</directive> fields
700 (%I and %O) that log the actual number of bytes received and sent
705 <title>Forensic Logging</title>
707 <p><module>mod_log_forensic</module> provides for forensic logging of
708 client requests. Logging is done before and after processing a
709 request, so the forensic log contains two log lines for each
710 request. The forensic logger is very strict with no customizations.
711 It can be an invaluable debugging and security tool.</p>
714 <section id="pidfile">
715 <title>PID File</title>
717 <p>On startup, Apache httpd saves the process id of the parent
718 httpd process to the file <code>logs/httpd.pid</code>. This
719 filename can be changed with the <directive
720 module="mpm_common">PidFile</directive> directive. The
721 process-id is for use by the administrator in restarting and
722 terminating the daemon by sending signals to the parent
723 process; on Windows, use the -k command line option instead.
724 For more information see the <a href="stopping.html">Stopping
725 and Restarting</a> page.</p>
728 <section id="scriptlog">
729 <title>Script Log</title>
731 <p>In order to aid in debugging, the
732 <directive module="mod_cgi">ScriptLog</directive> directive
733 allows you to record the input to and output from CGI scripts.
734 This should only be used in testing - not for live servers.
735 More information is available in the <a
736 href="mod/mod_cgi.html">mod_cgi</a> documentation.</p>