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<manualpage metafile="logs.xml.meta">
<title>Log Files</title>
<p>In order to effectively manage a web server, it is necessary
to get feedback about the activity and performance of the
server as well as any problems that may be occurring. The Apache HTTP Server
provides very comprehensive and flexible logging
capabilities. This document describes how to configure its
logging capabilities, and how to understand what the logs
<section id="overview">
The Apache HTTP Server provides a variety of different mechanisms for
logging everything that happens on your server, from the initial
request, through the URL mapping process, to the final resolution of
the connection, including any errors that may have occurred in the
process. In addition to this, third-party modules may provide logging
capabilities, or inject entries into the existing log files, and
applications such as CGI programs, or PHP scripts, or other handlers,
may send messages to the server error log.
In this document we discuss the logging modules that are a standard
part of the http server.
<section id="security">
<title>Security Warning</title>
<p>Anyone who can write to the directory where Apache httpd is
writing a log file can almost certainly gain access to the uid
that the server is started as, which is normally root. Do
<em>NOT</em> give people write access to the directory the logs
are stored in without being aware of the consequences; see the
<a href="misc/security_tips.html">security tips</a> document
for details.</p>
<p>In addition, log files may contain information supplied
directly by the client, without escaping. Therefore, it is
possible for malicious clients to insert control-characters in
the log files, so care must be taken in dealing with raw
<section id="errorlog">
<title>Error Log</title>
<directive module="core">ErrorLog</directive>
<directive module="core">ErrorLogFormat</directive>
<directive module="core">LogLevel</directive>
<p>The server error log, whose name and location is set by the
<directive module="core">ErrorLog</directive> directive, is the
most important log file. This is the place where Apache httpd
will send diagnostic information and record any errors that it
encounters in processing requests. It is the first place to
look when a problem occurs with starting the server or with the
operation of the server, since it will often contain details of
what went wrong and how to fix it.</p>
<p>The error log is usually written to a file (typically
<code>error_log</code> on Unix systems and
<code>error.log</code> on Windows and OS/2). On Unix systems it
is also possible to have the server send errors to
<code>syslog</code> or <a href="#piped">pipe them to a
<p>The format of the error log is defined by the <directive
module="core">ErrorLogFormat</directive> directive, with which you
can customize what values are logged. A default is format defined
if you don't specify one. A typical log message follows:</p>
[Fri Sep 09 10:42:29.902022 2011] [core:error] [pid 35708:tid 4328636416]
[client] File does not exist: /usr/local/apache2/htdocs/favicon.ico
<p>The first item in the log entry is the date and time of the
message. The next is the module producing the message (core, in this
case) and the severity level of that message. This is followed by
the process ID and, if appropriate, the thread ID, of the process
that experienced the condition. Next, we have the client address
that made the request. And finally is the detailed error message,
which in this case indicates a request for a file that did not
<p>A very wide variety of different messages can appear in the
error log. Most look similar to the example above. The error
log will also contain debugging output from CGI scripts. Any
information written to <code>stderr</code> by a CGI script will
be copied directly to the error log.</p>
<p>Putting a <code>%L</code> token in both the error log and the access
log will produce a log entry ID with which you can correlate the entry
in the error log with the entry in the access log. If
<module>mod_unique_id</module> is loaded, its unique request ID will be
used as the log entry ID, too.</p>
<p>During testing, it is often useful to continuously monitor
the error log for any problems. On Unix systems, you can
accomplish this using:</p>
tail -f error_log
<section id="permodule">
<title>Per-module logging</title>
<p>The <directive module="core">LogLevel</directive> directive
allows you to specify a log severity level on a per-module basis. In
this way, if you are troubleshooting a problem with just one
particular module, you can turn up its logging volume without also
getting the details of other modules that you're not interested in.
This is particularly useful for modules such as
<module>mod_proxy</module> or <module>mod_rewrite</module> where you
want to know details about what it's trying to do.</p>
<p>Do this by specifying the name of the module in your
<directive>LogLevel</directive> directive:</p>
<highlight language="config">
LogLevel info rewrite:trace5
<p>This sets the main <directive>LogLevel</directive> to info, but
turns it up to <code>trace5</code> for
<note>This replaces the per-module logging directives, such as
<code>RewriteLog</code>, that were present in earlier versions of
the server.</note>
<section id="accesslog">
<title>Access Log</title>
<directive module="mod_log_config">CustomLog</directive>
<directive module="mod_log_config">LogFormat</directive>
<directive module="mod_setenvif">SetEnvIf</directive>
<p>The server access log records all requests processed by the
server. The location and content of the access log are
controlled by the <directive module="mod_log_config">CustomLog</directive>
directive. The <directive module="mod_log_config">LogFormat</directive>
directive can be used to simplify the selection of
the contents of the logs. This section describes how to configure the server
to record information in the access log.</p>
<p>Of course, storing the information in the access log is only
the start of log management. The next step is to analyze this
information to produce useful statistics. Log analysis in
general is beyond the scope of this document, and not really
part of the job of the web server itself. For more information
about this topic, and for applications which perform log
analysis, check the <a
Open Directory</a> or <a
<p>Various versions of Apache httpd have used other modules and
directives to control access logging, including
mod_log_referer, mod_log_agent, and the
<code>TransferLog</code> directive. The <directive
module="mod_log_config">CustomLog</directive> directive now subsumes
the functionality of all the older directives.</p>
<p>The format of the access log is highly configurable. The format
is specified using a format string that looks much like a C-style
printf(1) format string. Some examples are presented in the next
sections. For a complete list of the possible contents of the
format string, see the <module>mod_log_config</module> <a
href="mod/mod_log_config.html#formats">format strings</a>.</p>
<section id="common">
<title>Common Log Format</title>
<p>A typical configuration for the access log might look as
<highlight language="config">
LogFormat "%h %l %u %t \"%r\" %&gt;s %b" common
CustomLog logs/access_log common
<p>This defines the <em>nickname</em> <code>common</code> and
associates it with a particular log format string. The format
string consists of percent directives, each of which tell the
server to log a particular piece of information. Literal
characters may also be placed in the format string and will be
copied directly into the log output. The quote character
(<code>"</code>) must be escaped by placing a backslash before
it to prevent it from being interpreted as the end of the
format string. The format string may also contain the special
control characters "<code>\n</code>" for new-line and
"<code>\t</code>" for tab.</p>
<p>The <directive module="mod_log_config">CustomLog</directive>
directive sets up a new log file using the defined
<em>nickname</em>. The filename for the access log is relative to
the <directive module="core">ServerRoot</directive> unless it
begins with a slash.</p>
<p>The above configuration will write log entries in a format
known as the Common Log Format (CLF). This standard format can
be produced by many different web servers and read by many log
analysis programs. The log file entries produced in CLF will
look something like this:</p>
<example> - frank [10/Oct/2000:13:55:36 -0700] "GET
/apache_pb.gif HTTP/1.0" 200 2326
<p>Each part of this log entry is described below.</p>
<dt><code></code> (<code>%h</code>)</dt>
<dd>This is the IP address of the client (remote host) which
made the request to the server. If <directive
module="core">HostnameLookups</directive> is
set to <code>On</code>, then the server will try to determine
the hostname and log it in place of the IP address. However,
this configuration is not recommended since it can
significantly slow the server. Instead, it is best to use a
log post-processor such as <program>logresolve</program> to determine
the hostnames. The IP address reported here is not
necessarily the address of the machine at which the user is
sitting. If a proxy server exists between the user and the
server, this address will be the address of the proxy, rather
than the originating machine.</dd>
<dt><code>-</code> (<code>%l</code>)</dt>
<dd>The "hyphen" in the output indicates that the requested
piece of information is not available. In this case, the
information that is not available is the RFC 1413 identity of
the client determined by <code>identd</code> on the clients
machine. This information is highly unreliable and should
almost never be used except on tightly controlled internal
networks. Apache httpd will not even attempt to determine
this information unless <directive
module="mod_ident">IdentityCheck</directive> is set
to <code>On</code>.</dd>
<dt><code>frank</code> (<code>%u</code>)</dt>
<dd>This is the userid of the person requesting the document
as determined by HTTP authentication. The same value is
typically provided to CGI scripts in the
<code>REMOTE_USER</code> environment variable. If the status
code for the request (see below) is 401, then this value
should not be trusted because the user is not yet
authenticated. If the document is not password protected,
this part will be "<code>-</code>" just like the previous
<dt><code>[10/Oct/2000:13:55:36 -0700]</code>
The time that the request was received.
The format is:
<p class="indent">
<code>[day/month/year:hour:minute:second zone]<br />
day = 2*digit<br />
month = 3*letter<br />
year = 4*digit<br />
hour = 2*digit<br />
minute = 2*digit<br />
second = 2*digit<br />
zone = (`+' | `-') 4*digit</code>
<p>It is possible to have the time displayed in another format
by specifying <code>%{format}t</code> in the log format
string, where <code>format</code> is either as in
<code>strftime(3)</code> from the C standard library,
or one of the supported special tokens. For details see
the <module>mod_log_config</module> <a
href="mod/mod_log_config.html#formats">format strings</a>.</p>
<dt><code>"GET /apache_pb.gif HTTP/1.0"</code>
<dd>The request line from the client is given in double
quotes. The request line contains a great deal of useful
information. First, the method used by the client is
<code>GET</code>. Second, the client requested the resource
<code>/apache_pb.gif</code>, and third, the client used the
protocol <code>HTTP/1.0</code>. It is also possible to log
one or more parts of the request line independently. For
example, the format string "<code>%m %U%q %H</code>" will log
the method, path, query-string, and protocol, resulting in
exactly the same output as "<code>%r</code>".</dd>
<dt><code>200</code> (<code>%&gt;s</code>)</dt>
<dd>This is the status code that the server sends back to the
client. This information is very valuable, because it reveals
whether the request resulted in a successful response (codes
beginning in 2), a redirection (codes beginning in 3), an
error caused by the client (codes beginning in 4), or an
error in the server (codes beginning in 5). The full list of
possible status codes can be found in the <a
specification</a> (RFC2616 section 10).</dd>
<dt><code>2326</code> (<code>%b</code>)</dt>
<dd>The last part indicates the size of the object returned
to the client, not including the response headers. If no
content was returned to the client, this value will be
"<code>-</code>". To log "<code>0</code>" for no content, use
<code>%B</code> instead.</dd>
<section id="combined">
<title>Combined Log Format</title>
<p>Another commonly used format string is called the Combined
Log Format. It can be used as follows.</p>
<highlight language="config">
LogFormat "%h %l %u %t \"%r\" %&gt;s %b \"%{Referer}i\" \"%{User-agent}i\"" combined
CustomLog log/access_log combined
<p>This format is exactly the same as the Common Log Format,
with the addition of two more fields. Each of the additional
fields uses the percent-directive
<code>%{<em>header</em>}i</code>, where <em>header</em> can be
any HTTP request header. The access log under this format will
look like:</p>
<example> - frank [10/Oct/2000:13:55:36 -0700] "GET
/apache_pb.gif HTTP/1.0" 200 2326
"" "Mozilla/4.08 [en]
(Win98; I ;Nav)"
<p>The additional fields are:</p>
<dd>The "Referer" (sic) HTTP request header. This gives the
site that the client reports having been referred from. (This
should be the page that links to or includes
<dt><code>"Mozilla/4.08 [en] (Win98; I ;Nav)"</code>
<dd>The User-Agent HTTP request header. This is the
identifying information that the client browser reports about
<section id="multiple">
<title>Multiple Access Logs</title>
<p>Multiple access logs can be created simply by specifying
multiple <directive module="mod_log_config">CustomLog</directive>
directives in the configuration
file. For example, the following directives will create three
access logs. The first contains the basic CLF information,
while the second and third contain referer and browser
information. The last two <directive
module="mod_log_config">CustomLog</directive> lines show how
to mimic the effects of the <code>ReferLog</code> and <code
>AgentLog</code> directives.</p>
<highlight language="config">
LogFormat "%h %l %u %t \"%r\" %&gt;s %b" common
CustomLog logs/access_log common
CustomLog logs/referer_log "%{Referer}i -&gt; %U"
CustomLog logs/agent_log "%{User-agent}i"
<p>This example also shows that it is not necessary to define a
nickname with the <directive
module="mod_log_config">LogFormat</directive> directive. Instead,
the log format can be specified directly in the <directive
module="mod_log_config">CustomLog</directive> directive.</p>
<section id="conditional">
<title>Conditional Logs</title>
<p>There are times when it is convenient to exclude certain
entries from the access logs based on characteristics of the
client request. This is easily accomplished with the help of <a
href="env.html">environment variables</a>. First, an
environment variable must be set to indicate that the request
meets certain conditions. This is usually accomplished with
<directive module="mod_setenvif">SetEnvIf</directive>. Then the
<code>env=</code> clause of the <directive
module="mod_log_config">CustomLog</directive> directive is used to
include or exclude requests where the environment variable is
set. Some examples:</p>
<highlight language="config">
# Mark requests from the loop-back interface
SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog
# Mark requests for the robots.txt file
SetEnvIf Request_URI "^/robots\.txt$" dontlog
# Log what remains
CustomLog logs/access_log common env=!dontlog
<p>As another example, consider logging requests from
english-speakers to one log file, and non-english speakers to a
different log file.</p>
<highlight language="config">
SetEnvIf Accept-Language "en" english<br />
CustomLog logs/english_log common env=english<br />
CustomLog logs/non_english_log common env=!english
<p>In a caching scenario one would want to know about
the efficiency of the cache. A very simple method to
find this out would be:</p>
<highlight language="config">
LogFormat "%h %l %u %t "%r " %>s %b %{CACHE_MISS}e" common-cache
CustomLog logs/access_log common-cache
<p><module>mod_cache</module> will run before
<module>mod_env</module> and when successfull will deliver the
content without it. In that case a cache hit will log
<code>-</code>, while a cache miss will log <code>1</code>.</p>
<p>In addition to the <code>env=</code> syntax, <directive
module="mod_log_config">LogFormat</directive> supports logging values
conditional upon the HTTP response code:</p>
<highlight language="config">
LogFormat "%400,501{User-agent}i" browserlog
LogFormat "%!200,304,302{Referer}i" refererlog
<p>In the first example, the <code>User-agent</code> will be
logged if the HTTP status code is 400 or 501. In other cases, a
literal "-" will be logged instead. Likewise, in the second
example, the <code>Referer</code> will be logged if the HTTP
status code is <strong>not</strong> 200, 204, or 302. (Note the
"!" before the status codes.</p>
<p>Although we have just shown that conditional logging is very
powerful and flexible, it is not the only way to control the
contents of the logs. Log files are more useful when they
contain a complete record of server activity. It is often
easier to simply post-process the log files to remove requests
that you do not want to consider.</p>
<section id="rotation">
<title>Log Rotation</title>
<p>On even a moderately busy server, the quantity of
information stored in the log files is very large. The access
log file typically grows 1 MB or more per 10,000 requests. It
will consequently be necessary to periodically rotate the log
files by moving or deleting the existing logs. This cannot be
done while the server is running, because Apache httpd will continue
writing to the old log file as long as it holds the file open.
Instead, the server must be <a
href="stopping.html">restarted</a> after the log files are
moved or deleted so that it will open new log files.</p>
<p>By using a <em>graceful</em> restart, the server can be
instructed to open new log files without losing any existing or
pending connections from clients. However, in order to
accomplish this, the server must continue to write to the old
log files while it finishes serving old requests. It is
therefore necessary to wait for some time after the restart
before doing any processing on the log files. A typical
scenario that simply rotates the logs and compresses the old
logs to save space is:</p>
mv access_log access_log.old<br />
mv error_log error_log.old<br />
apachectl graceful<br />
sleep 600<br />
gzip access_log.old error_log.old
<p>Another way to perform log rotation is using <a
href="#piped">piped logs</a> as discussed in the next
<section id="piped">
<title>Piped Logs</title>
<p>Apache httpd is capable of writing error and access log
files through a pipe to another process, rather than directly
to a file. This capability dramatically increases the
flexibility of logging, without adding code to the main server.
In order to write logs to a pipe, simply replace the filename
with the pipe character "<code>|</code>", followed by the name
of the executable which should accept log entries on its
standard input. The server will start the piped-log process when
the server starts, and will restart it if it crashes while the
server is running. (This last feature is why we can refer to
this technique as "reliable piped logging".)</p>
<p>Piped log processes are spawned by the parent Apache httpd
process, and inherit the userid of that process. This means
that piped log programs usually run as root. It is therefore
very important to keep the programs simple and secure.</p>
<p>One important use of piped logs is to allow log rotation
without having to restart the server. The Apache HTTP Server
includes a simple program called <program>rotatelogs</program>
for this purpose. For example, to rotate the logs every 24 hours, you
can use:</p>
<highlight language="config">
CustomLog "|/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common
<p>Notice that quotes are used to enclose the entire command
that will be called for the pipe. Although these examples are
for the access log, the same technique can be used for the
error log.</p>
<p>A similar but much more flexible log rotation program
called <a href="">cronolog</a>
is available at an external site.</p>
<p>As with conditional logging, piped logs are a very powerful
tool, but they should not be used where a simpler solution like
off-line post-processing is available.</p>
<p>By default the piped log process is spawned without invoking
a shell. Use "<code>|$</code>" instead of "<code>|</code>"
to spawn using a shell (usually with <code>/bin/sh -c</code>):</p>
<highlight language="config">
# Invoke "rotatelogs" using a shell
CustomLog "|$/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common
<p>This was the default behaviour for Apache 2.2.
Depending on the shell specifics this might lead to
an additional shell process for the lifetime of the logging
pipe program and signal handling problems during restart.
For compatibility reasons with Apache 2.2 the notation
"<code>||</code>" is also supported and equivalent to using
<section id="virtualhost">
<title>Virtual Hosts</title>
<p>When running a server with many <a href="vhosts/">virtual
hosts</a>, there are several options for dealing with log
files. First, it is possible to use logs exactly as in a
single-host server. Simply by placing the logging directives
outside the <directive module="core"
type="section">VirtualHost</directive> sections in the
main server context, it is possible to log all requests in the
same access log and error log. This technique does not allow
for easy collection of statistics on individual virtual
<p>If <directive module="mod_log_config">CustomLog</directive>
or <directive module="core">ErrorLog</directive>
directives are placed inside a
<directive module="core" type="section">VirtualHost</directive>
section, all requests or errors for that virtual host will be
logged only to the specified file. Any virtual host which does
not have logging directives will still have its requests sent
to the main server logs. This technique is very useful for a
small number of virtual hosts, but if the number of hosts is
very large, it can be complicated to manage. In addition, it
can often create problems with <a
href="vhosts/fd-limits.html">insufficient file
<p>For the access log, there is a very good compromise. By
adding information on the virtual host to the log format
string, it is possible to log all hosts to the same log, and
later split the log into individual files. For example,
consider the following directives.</p>
<highlight language="config">
LogFormat "%v %l %u %t \"%r\" %&gt;s %b" comonvhost
CustomLog logs/access_log comonvhost
<p>The <code>%v</code> is used to log the name of the virtual
host that is serving the request. Then a program like <a
href="programs/other.html">split-logfile</a> can be used to
post-process the access log in order to split it into one file
per virtual host.</p>
<section id="other">
<title>Other Log Files</title>
<directive module="mod_log_config">LogFormat</directive>
<directive module="mod_log_config">BufferedLogs</directive>
<directive module="mod_log_forensic">ForensicLog</directive>
<directive module="mpm_common">PidFile</directive>
<directive module="mod_cgi">ScriptLog</directive>
<directive module="mod_cgi">ScriptLogBuffer</directive>
<directive module="mod_cgi">ScriptLogLength</directive>
<title>Logging actual bytes sent and received</title>
<p><module>mod_logio</module> adds in two additional
<directive module="mod_log_config">LogFormat</directive> fields
(%I and %O) that log the actual number of bytes received and sent
on the network.</p>
<title>Forensic Logging</title>
<p><module>mod_log_forensic</module> provides for forensic logging of
client requests. Logging is done before and after processing a
request, so the forensic log contains two log lines for each
request. The forensic logger is very strict with no customizations.
It can be an invaluable debugging and security tool.</p>
<section id="pidfile">
<title>PID File</title>
<p>On startup, Apache httpd saves the process id of the parent
httpd process to the file <code>logs/</code>. This
filename can be changed with the <directive
module="mpm_common">PidFile</directive> directive. The
process-id is for use by the administrator in restarting and
terminating the daemon by sending signals to the parent
process; on Windows, use the -k command line option instead.
For more information see the <a href="stopping.html">Stopping
and Restarting</a> page.</p>
<section id="scriptlog">
<title>Script Log</title>
<p>In order to aid in debugging, the
<directive module="mod_cgi">ScriptLog</directive> directive
allows you to record the input to and output from CGI scripts.
This should only be used in testing - not for live servers.
More information is available in the <a
href="mod/mod_cgi.html">mod_cgi</a> documentation.</p>
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