I have written a first draft of a new document which discusses logfiles in Apache. I did not directly use Rich Bowen's great logging tutorial, but I borrowed ideas from it extensively (and the same for the older tutorial from apacheweek).
I'd appreciate feedback, including, but not limited to: 1. Is this appropriate for the docs? 2. Is it at the right level? 3. What is missing? 4. What is overdone? 5. Should I add sections for pidfile, rewritelog, scriptlog (I'm leaning towards yes) 6. Should I try to add more general background stuff like "How do I tell how many PEOPLE visited my website?" (I'm leaning towards no) Included inline below, and available in rendered form at http://garibaldi.commerce.ubc.ca:8080/ap13/htdocs/manual/logs.html <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <HTML> <HEAD> <TITLE>Log Files and Status Reporting in Apache</TITLE> </HEAD> <!-- Background white, links blue (unvisited), navy (visited), red (active) --> <BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#000080" ALINK="#FF0000" > <!--#include virtual="header.html" --> <h1 align="center">Log Files and Status Reporting</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> <ul> <li><a href="#security">Security Warning</a></li> <li><a href="#errorlog">Error Log</a></li> <li><a href="#accesslog">Access Log</a> <ul> <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> </ul></li> <li><a href="#rotation">Log Rotation</a></li> <li><a href="#piped">Piped Logs</a></li> <li><a href="#virtualhosts">VirtualHosts</a> </ul> <hr> <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> </td></tr></table> <p>The server error log, the location of which 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 encouters 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). However, on unix systems it is also possible to have the server send errors to the <code>syslog</code> or pipe them through a program (see <a href="#rotation">Log Rotation</a> below).</p> <p>The format of the error log is relatively free-form and descriptive. However, there is certain information that is contained in most error log entries. For example, here is a typical message.</p> <blockquote><code> [Wed Oct 11 14:32:52 2000] [error] [client 127.0.0.1] client denied by server configuration: /export/home/live/ap/htdocs/test </code></blockquote> <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 which 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 and gives the file-system 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. However, 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. So it is often possible to customize the access log in order to get more information about error conditions.</p> <p>During testing, it is often useful to continuously monitor the error log for any problems. On unix systems, this is easily accomplished using:</p> <blockquote><code> tail -f error_log </code></blockquote> <p>Other operating systems may have similar commands.</p> <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> </td></tr></table> <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 webserver itself. For more information about this topic, and for applications which perform log analysis, check the <a href="http://dmoz.org/Computers/Software/Internet/Site_Management/Log_analysis/"; >Open Directory</a> or <a href="http://dir.yahoo.com/Computers_and_Internet/Software/Internet/World_Wide_Web/Servers/Log_Analysis_Tools/"; >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. However, 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. The sections below explain some of the commonly used formats, as examples.</p> <h3><a name="common">Common Log Format</a></h3> <p>A typical configuration for the access log might look as follows.</p> <blockquote><code> LogFormat "%h %l %u %t \"%r\" %>s %b" common<br> CustomLog logs/access_log common </code></blockquote> <p>This defines the <em>nickname</em> <code>common</code> and associates it with a particular log format string. Notice that the format string consists of percent-directives, each of which tell the server to log a particular piece of information. In addition, literal characters may be placed in the format string. The percent (<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 <code>CustomLog</code> directive sets up a new log file using the defined <em>nickname</em>.</p> <p>This 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 by this configuration will look something like this:</p> <blockquote><code> 127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET /apache_pb.gif HTTP/1.0" 200 2326 </code></blockquote> <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 <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 "identity" of the remote user as 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. This is the same value that is typically provided to CGI scripts in the <code>REMOTE_USER</code> environment variable. 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 specified in CLF as: <BLOCKQUOTE><CODE> date = [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> </dd> <dt><code>"GET /apache_pb.gif HTTP/1.0"</code> (<code>\"%r\"</code>)</dt> <dd>The request line from the client is given in double quotes. The request line itself 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> <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). Some of the common status codes are <dl> <dt>200 OK</dt> <dd>The request has succeeded.</dd> <dt>206 Partial Content</dt> <dd>The client requested a part of a resource and the server responded in kind.</dd> <dt>301 Moved Permanently</dt> <dd>The requested resource has been permanently relocated at a new URI.</dd> <dt>302 Moved Temporarily</dt> <dd>The requested resource habe been temporarily relocated at a new URI.</dd> <dt>304 Not Modified</dt> <dd>The document has not been modified since the last time it was requested by the client.</dd> <dt>401 Unauthorized</dt> <dd>The resource requires authentication, but the client has not yet supplied the correct credentials.</dd> <dt>403 Forbidden</dt> <dd>The client is not allowed to access the requested resource.</dd> <dt>404 Not Found</dt> <dd>The server does not have a resource matching the requested URI.</dt> <dt>500 Internal Server Error</dt> <dd>The server encountered an unexpected condition which prevented it from fulfilling the request.</dd> </dl> The full list of possible status codes can be found in the HTTP specification.</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.</dd> </dl> <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> <blockquote><code> logformat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\"" combined<br> CustomLog log/acces_log combined </code></blockquote> <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> 127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET /apache_pb.gif HTTP/1.0" 200 2326 "http://www.example.com/start.html"; "Mozilla/4.08 [en] (Win98; I ;Nav)" </code></blockquote> <p>The additional fields are:</p> <dl> <dt><code>"http://www.example.com/start.html";</code> (<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>/apache_pb.gif</code>). <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> </dl> <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 information, while the second and third contain referer and browser information. The last two <code>CustomLog</code> lines show how to mimick the effects of the <code>ReferLog</code> and <code>AgentLog</code> directives.</p> <blockquote><code> 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" </code></blockquote> <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> directive.</p> <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 is used to include or exclude requests where the environment variable is set. Some useful examples:</p> <blockquote><code> # Exclude requests from the loop-back interfact<br> SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog<br> # Exclude 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 </code></blockquote> <p>As another example, consider logging requests from english-speakers to one log file, and non-english speakers to a different log file.</p> <blockquote><code> SetEnvIf Accept-Language "en" english<br> CustomLog logs/english_log common env=english<br> CustomLog logs/non_english_log common env=!english </code></blockquote> <p>Although we have just shown that conditional logging is very powerful and flexibly, we do not recommend using it in general. Log files are more useful when they contain a complete record of server activity. It is usually best 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 logs 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> <blockquote><code> 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 </code></blockquote> <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 in Apache, 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>Some simple examples using piped logs:</p> <blockquote><code> # 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 </code></blockquote> <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. A simple program called <a href="programs/rotatelogs.html">rotatelogs</a> that is included with the server can be used for this purpose. For example, to rotate the logs every 24 hours, you can use:</p> <blockquote><code> CustomLog "|/usr/local/apache/bin/rotatelogs /var/log/access_log 86400" common </code></blockquote> <p>A similar, but much more flexible log rotation program called <a href="http://www.ford-mason.co.uk/resources/cronolog/";>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 over used. When it is possible to get the same results by post-processing the logs off-line (after they are rotated), it is usually wise to use that simpler technique.</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><VirtualHost></code> sections in the main server context, it is possible to log all requests in the same access log and error log. However, this technique does not allow for easy collection of statistics on individual virtual hosts.</p> <p>If, instead, <code>CustomLog</code> or <code>ErrorLog</code> directives are placed inside a <code><VirtualHost></code> section, all requests or errors for that virtual host can be logged to a separate file. Any virtual host which does not have logging directives will 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> <blockquote><code> LogFormat "%v %l %u %t \"%r\" %>s %b" comonvhost<br> CustomLog logs/access_log comonvhost </code></blockquote> <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> <!--#include virtual="footer.html" --> </BODY> </HTML> --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
