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<manualpage metafile="logs.xml.meta">
<title>Log Files</title>
<summary>
<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
contain.</p>
</summary>
<section id="security">
<title>Security Warning</title>
<p>Anyone who can write to the directory where Apache 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
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
logs.</p>
</section>
<section id="errorlog">
<title>Error Log</title>
<related>
<directivelist>
<directive module="core">ErrorLog</directive>
<directive module="core">LogLevel</directive>
</directivelist>
</related>
<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
is also possible to have the server send errors to
<code>syslog</code> or <a href="#piped">pipe them to a
program</a>.</p>
<p>The format of the error log is relatively free-form and
descriptive. But there is certain information that is contained
in most error log entries. For example, here is a typical
message.</p>
<example>
[Wed Oct 11 14:32:52 2000] [error] [client 127.0.0.1]
client denied by server configuration:
</example>
<p>The first item in the log entry is the date and time of the
message. The second entry lists the severity of the error being
reported. The <directive module="core">LogLevel</directive>
directive is used to control the types of errors that are sent
to the error log by restricting the severity level. The third
entry gives the IP address of the client that generated the
error. Beyond that is the message itself, which in this case
indicates that the server has been configured to deny the
client access. The server reports the file-system path (as
opposed to the web path) of the requested document.</p>
<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>It is not possible to customize the error log by adding or
removing information. However, error log entries dealing with
particular requests have corresponding entries in the <a
href="#accesslog">access log</a>. For example, the above example
entry corresponds to an access log entry with status code 403.
Since it is possible to customize the access log, you can
obtain more information about error conditions using that log
file.</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>
<example>
tail -f error_log
</example>
</section>
<section id="accesslog">
<title>Access Log</title>
<related>
<modulelist>
<module>mod_log_config</module>
<module>mod_setenvif</module>
</modulelist>
<directivelist>
<directive module="mod_log_config">CustomLog</directive>
<directive module="mod_log_config">LogFormat</directive>
<directive module="mod_setenvif">SetEnvIf</directive>
</directivelist>
</related>
<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
Yahoo</a>.</p>
<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
<section id="common">
<title>Common Log Format</title>
<p>A typical configuration for the access log might look as
follows.</p>
<example>
LogFormat "%h %l %u %t \"%r\" %>s %b" common<br />
CustomLog logs/access_log common
</example>
<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 back-slash 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>
</example>
<p>Each part of this log entry is described below.</p>
<dl>
<dt><code>127.0.0.1</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="core">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 entry will be "<code>-</code>" just like the previous
one.</dd>
(<code>%t</code>)</dt>
<dd>
The time that the server finished processing the request.
The format is:
<p class="indent">
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 as in
<code>strftime(3)</code> from the C standard library.
</dd>
(<code>\"%r\"</code>)</dt>
<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
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>%>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
href="http://www.w3.org/Protocols/rfc2616/rfc2616.txt">HTTP
specification</a> (RFC2616 section 10).</dd>
<dt><code>2326</code> (<code>%b</code>)</dt>
<dd>The last entry 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>
</dl>
</section>
<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>
<example>
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\"
\"%{User-agent}i\"" combined<br />
CustomLog log/access_log combined
</example>
<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>
(Win98; I ;Nav)"
</example>
<p>The additional fields are:</p>
<dl>
(<code>\"%{Referer}i\"</code>)</dt>
<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
(<code>\"%{User-agent}i\"</code>)</dt>
<dd>The User-Agent HTTP request header. This is the
identifying information that the client browser reports about
itself.</dd>
</dl>
</section>
<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>
<example>
LogFormat "%h %l %u %t \"%r\" %>s %b" common<br />
</example>
<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>
<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>
<example>
# Mark requests from the loop-back interface<br />
SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog<br />
# Mark requests for the robots.txt file<br />
SetEnvIf Request_URI "^/robots\.txt$" dontlog<br />
# Log what remains<br />
CustomLog logs/access_log common env=!dontlog
</example>
<p>As another example, consider logging requests from
english-speakers to one log file, and non-english speakers to a
different log file.</p>
<example>
SetEnvIf Accept-Language "en" english<br />
CustomLog logs/non_english_log common env=!english
</example>
<p>Although we have just shown that conditional logging is very
powerful and flexibly, 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>
</section>
<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 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>
<example>
mv access_log access_log.old<br />
mv error_log error_log.old<br />
apachectl graceful<br />
sleep 600<br />
</example>
<p>Another way to perform log rotation is using <a
href="#piped">piped logs</a> as discussed in the next
section.</p>
</section>
<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. Apache 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>
<example>
</example>
<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
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>
</section>
<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
hosts.</p>
<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
descriptors</a>.</p>
<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>
<example>
LogFormat "%v %l %u %t \"%r\" %>s %b"
comonvhost<br />
CustomLog logs/access_log comonvhost
</example>
<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>
<section id="other">
<title>Other Log Files</title>
<related>
<modulelist>
<module>mod_cgi</module>
<module>mod_rewrite</module>
</modulelist>
<directivelist>
<directive module="mpm_common">PidFile</directive>
<directive module="mod_rewrite">RewriteLog</directive>
<directive module="mod_rewrite">RewriteLogLevel</directive>
<directive module="mod_cgi">ScriptLog</directive>
<directive module="mod_cgi">ScriptLogBuffer</directive>
<directive module="mod_cgi">ScriptLogLength</directive>
</directivelist>
</related>
<section id="pidfile">
<title>PID File</title>
<p>On startup, Apache httpd saves the process id of the parent
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>
<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
</section>
<section id="rewritelog">
<title>Rewrite Log</title>
<p>When using the powerful and complex features of <a
href="mod/mod_rewrite.html">mod_rewrite</a>, it is almost
always necessary to use the <directive
module="mod_rewrite">RewriteLog</directive> to help
in debugging. This log file produces a detailed analysis of how
the rewriting engine transforms requests. The level of detail
is controlled by the <directive
module="mod_rewrite">RewriteLogLevel</directive> directive.</p>
</section>
</section>
</manualpage>