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Subject cvs commit: httpd-docs-1.3/htdocs/manual logs.html index.html.en
Date Fri, 10 Aug 2001 15:46:44 GMT
slive       01/08/10 08:46:44

  Modified:    htdocs/manual index.html.en
  Added:       htdocs/manual logs.html
  Add a document that provides a unifying discussion of log
  files in Apache.
  Many ideas for this document originated from Rich Bowen's ApacheToday
  tutorial and and older Apacheweek article.
  Revision  Changes    Path
  1.51      +2 -0      httpd-docs-1.3/htdocs/manual/index.html.en
  Index: index.html.en
  RCS file: /home/cvs/httpd-docs-1.3/htdocs/manual/index.html.en,v
  retrieving revision 1.50
  retrieving revision 1.51
  diff -u -d -b -u -r1.50 -r1.51
  --- index.html.en	2001/07/11 14:41:41	1.50
  +++ index.html.en	2001/08/10 15:46:44	1.51
  @@ -136,6 +136,8 @@
   <a href="server-wide.html">Server-Wide Configuration</a>
  +<a href="logs.html">Log Files</a>
   <a href="urlmapping.html">Mapping URLs to the Filesystem</a>
   <a href="vhosts/index.html">Virtual Hosts</a>
  1.1                  httpd-docs-1.3/htdocs/manual/logs.html
  Index: logs.html
  <TITLE>Log Files - Apache HTTP Server</TITLE>
  <!-- Background white, links blue (unvisited), navy (visited), red (active) -->
  <!--#include virtual="header.html" -->
  <h1 align="center">Log Files</h1>
  <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 occuring.  The Apache HTTP Server provides
  very comprehensive and flexible logging capabilities.  This document
  describes how to configure the various log files, and how to
  understand what the logs contain.</p>
  <li><a href="#security">Security Warning</a></li>
  <li><a href="#errorlog">Error Log</a></li>
  <li><a href="#accesslog">Access Log</a>
      <li><a href="#common">Common Log Format</a></li>
      <li><a href="#combined">Combined Log Format</a></li>
      <li><a href="#multiple">Multiple Access Logs</a></li>
      <li><a href="#conditional">Conditional Logging</a></li>
  <li><a href="#rotation">Log Rotation</a></li>
  <li><a href="#piped">Piped Logs</a></li>
  <li><a href="#virtualhosts">VirtualHosts</a>
  <li><a href="#other">Other Log Files</a>
      <li><a href="#pidfile">PID File</a></li>
      <li><a href="#scriptlog">Script Log</a></li>
      <li><a href="#rewritelog">Rewrite Log</a></li>
  <h2><a name="security">Security Warning</a></h2>
  <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 <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 logs.</p>
  <h2><a name="errorlog">Error Log</a></h2>
  <table border="1">
  <tr><td valign="top">
  <strong>Related Directives</strong><br><br>
  <a href="mod/core.html#errorlog">ErrorLog</a><br>
  <a href="mod/core.html#loglevel">LogLevel</a>
  <p>The server error log, whose name and location is set by the <a
  href="mod/core.html#errorlog">ErrorLog</a> 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
  <p>The error log is usually written to a file (typically
  <code>error_log</code> on unix systems and <code>error.log</code>
  Windows and OS/2).  On unix systems it is also possible to have the
  server send errors to the <code>syslog</code> or <a href="#pipe">pipe
  them through 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>
  [Wed Oct 11 14:32:52 2000] [error] [client] client denied by server configuration:
  <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 <a href="mod/core.html#loglevel">LogLevel</a> 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
  <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 
  tail -f error_log
  <h2><a name="accesslog">Access Log</a></h2>
  <table border=1><tr><td valign="top">
  <strong>Related Modules</strong><br><br>
  <a href="mod/mod_log_config.html">mod_log_config</a><br>
  </td><td valign="top">
  <strong>Related Directives</strong><br><br>
  <a href="mod/mod_log_config.html#customlog">CustomLog</a><br>
  <a href="mod/mod_log_config.html#logformat">LogFormat</a><br>
  <a href="mod/mod_setenvif.html#setenvif">SetEnvIf</a>
  <p>The server access log records all requests processed by the server.
  The location of the access log as well as its contents are controlled
  by the <a href="mod/mod_log_config.html#customlog">CustomLog</a>
  directive.  The <a
  href="mod/mod_log_config.html#logformat">LogFormat</a> 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
  <code>CustomLog</code> 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 <a href="mod/mod_log_config.html#format">format
  string</a> 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 <a
  href="mod/mod_log_config.html">mod_log_config documentation</a>.</p>
  <h3><a name="common">Common Log Format</a></h3>
  <p>A typical configuration for the access log might look
  as follows.</p>
  LogFormat "%h %l %u %t \"%r\" %>s %b" common<br>
  CustomLog logs/access_log common
  <p>This defines the <em>nickname</em> <code>common</code>
  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 <code>CustomLog</code> directive sets up a new log file using
  the defined <em>nickname</em>.  The filename for the access log is
  relative to the <a href="mod/core.html#serverroot">ServerRoot</a>
  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
  <blockquote><code> - 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 <a
  href="mod/core.html#hostnamelookups">HostNameLookups</a> 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 <a
  href="programs/logresolve.html">logresolve</a> 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 <a href="mod/core.html#identitycheck">IdentityCheck</a> 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>
  <dt><code>[10/Oct/2000:13:55:36 -0700]</code> (<code>%t</code>)</dt>
  <dd>The time that the server finished processing the request.  The
  format is:
  <BLOCKQUOTE><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></BLOCKQUOTE>
  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.
  <dt><code>"GET /apache_pb.gif HTTP/1.0"</code>
  (<code>\"%r\"</code>)</dt> <dd>The request line from the client
  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>.</dd> 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
  <dt><code>200</code></dt> (<code>%>s</code>) <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 specification</a> (RFC2616 section 10).</dd>
  <dt><code>2326</code> (<code>%b</code>)
  <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>
  <h4><a name="combined">Combined Log Format</a></h4>
  <p>Another commonly used format string is called the
  Combined Log Format.  It can be used as follows.</p>
  LogFormat "%h %l %u %t \"%r\" %&gt;s %b \"%{Referer}i\" \"%{User-agent}i\"" combined<br>
  CustomLog log/acces_log combined
  <p>This format is exactly the same as the Common Log Format,
  with the addition of two more fields.  The access log under this
  format will look like:</p>
  <blockquote><code> - 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>
  (<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
  <dt><code>"Mozilla/4.08 [en] (Win98; I ;Nav)"</code>
  (<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>
  <h3><a name="multiple">Multiple Access Logs</a></h3>
  <p>Multiple access logs can be created simply by specifying multiple
  <code>CustomLog</code> 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
  <code>CustomLog</code> lines show how to mimic the effects of the
  <code>ReferLog</code> and <code>AgentLog</code> directives.</p>
  LogFormat "%h %l %u %t \"%r\" %>s %b" common<br>
  CustomLog logs/access_log common<br>
  CustomLog logs/referer_log "%{Referer}i -> %U"<br>
  CustomLog logs/agent_log "%{User-agent}i"
  <p>This example also shows that it is not necessary to define a
  nickname with the <code>LogFormat</code> directive.  Instead, the log
  format can be specified directly in the <code>CustomLog</code>
  <h3><a name="conditional">Conditional Logging</a></h3>
  <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 <a
  href="mod/mod_setenvif.html#setenvif">SetEnvIf</a>.  Then the
  <code>env=</code> clause of the <code>CustomLog</code> directive
  used to include or exclude requests where the environment variable is
  set.  Some examples:</p>
  # 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
  <p>As another example, consider logging requests from english-speakers
  to one log file, and non-english speakers to a different log file.</p>
  SetEnvIf Accept-Language "en" english<br>
  CustomLog logs/english_log common env=english<br>
  CustomLog logs/non_english_log common env=!english
  <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>
  <h2><a name="rotation">Log Rotation</a></h2>
  <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>
  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.</p>
  <h2><a name="piped">Piped Logs</a></h2>
  <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>Some simple examples using piped logs:</p>
  # compressed logs<br>
  CustomLog "|/usr/bin/gzip -c >> /var/log/access_log.gz" common<br>
  # almost-real-time name resolution<br>
  CustomLog "|/usr/local/apache/bin/logresolve >> /var/log/access_log" 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>One important use of piped logs is to allow log rotation without
  having to restart the server.  Apache HTTPD includes a simple program
  called <a href="programs/rotatelogs.html">rotatelogs</a> for this
  purpose.  For example, to rotate the logs every 24 hours, you can
  CustomLog "|/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common
  <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>
  <h2><a name="virtualhosts">Virtual Hosts</a></h2>
  <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
  <code>&lt;VirtualHost&gt;</code> 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 <code>CustomLog</code> or <code>ErrorLog</code> directives
  placed inside a <code>&lt;VirtualHost&gt;</code> 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>
  LogFormat "%v %l %u %t \"%r\" %>s %b" comonvhost<br>
  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>
  <p>Unfortunately, no similar technique is available for the error log,
  so you must choose between mixing all virtual hosts in the same error
  log and using one error log per virtual host.</p>
  <h2><a name="other">Other Log Files</a></h2>
  <table border=1><tr><td valign="top">
  <strong>Related Modules</strong><br><br>
  <a href="mod/mod_cgi.html">mod_cgi</a><br>
  <a href="mod/mod_rewrite.html">mod_rewrite</a>
  </td><td valign="top">
  <strong>Related Directives</strong><br><br>
  <a href="mod/core.html#pidfile">PidFile</a><br>
  <a href="mod/mod_rewrite.html#RewriteLog">RewriteLog</a></br>
  <a href="mod/mod_rewrite.html#RewriteLogLevel">RewriteLogLevel</a></br>
  <a href="mod/mod_cgi.html#scriptlog">ScriptLog</a><br>
  <a href="mod/mod_cgi.html#scriptloglength">ScriptLogLength</a><br>
  <a href="mod/mod_cgi.html#scriptlogbuffer">ScriptLogBuffer</a>
  <h3><a name="pidfile">PID File</a></h3>
  <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 <A HREF="mod/core.html#pidfile">PidFile</A>
  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.
  <h3><a name="scriptlog">Script Log</a></h3>
  <p>In order to aid in debugging, the
  <a href="mod/mod_cgi.html#scriptlog">ScriptLog</a>
  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 documentation</a>.
  <h3><a name="rewritelog">Rewrite Log</a></h3>
  <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 <a
  href="mod/mod_rewrite.html#RewriteLog">RewriteLog</a> 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 <a
  <!--#include virtual="footer.html" -->

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