Apache Server for Windows Little Black Book:Tracking Your Server's Activity
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Apache’s Log Files
Virtually all servers, including Apache, provide their Webmasters with information contained in log files. Typically, the following types of log files are provided:


•  Access file—Records any requests made to the server
•  Error log file—Records requests that encounter errors.
•  Server log file—Provides a record of the server’s activity.
•  Referrer file—Stores URLs that contain links to the documents that are actually requested on the server.
•  Agent file—Lists the type of browsers that visitors used to access the server.

Apache provides you with an access log (called access.log) and an error log (error.log) in the logs directory. Referrer information and user agent data are provided by the mod_log_config module. Server status information is provided by ApacheModuleStatus.dll. In addition to mod_log_config, there are two modules—mod_log_agent and mod_log_referer—that you can convert to DLLs and load into Apache: mod_log_agent lets you record information about specific browsers, and mod_log_referer lets you record information about referring URLs used by your visitors. Both modules enable you to store the data in separate log files.

The mod_log_config Module
The mod_log_config module, which is compiled into Apache by default, is the module that produces Apache’s log files. By default, it produces data in the standard common log format (CLF), although you can customize the format via the LogFormat directive.
Common Log Format
Common log format is a standardized way of producing log files that was originally defined by NCSA (National Center for Supercomputing Applications) and CERN, the European Laboratory for Particle Physics. It includes seven data fields (called tokens) for each record:

•  Host—Provides either the IP address of the visitor or its fully qualified domain name.
•  Ident—Provides identity information from the client, provided that the client’s computer runs identd and the IdentityCheck directive has been enabled.
•  Authuser—Provides the username for the visitor if the URL that was accessed required Basic HTTP authentication.
•  Date—Lists the date and time of the request.
•  Request—Reports the request line that the visitor sent, enclosed in double-primes ('').
•  Bytes—Records the size, in bytes, of the file or object sent to the client.
•  Status—Reports the three-digit HTTP status code sent to the client.

In the actual log file, each token is separated by a blank space, in this format:



host ident authuser date request status bytes


You can use the CustomLog or LogFormat directives to choose other formats for log files. The httpd.conf uses the LogFormat directive to provide definitions of four different log file formats: common, agent, referer, and combined. See “Customizing Log Files” in the Immediate Solutions section of this chapter for further explanation.
The Access Log File
Apache records all client requests in an access log file that, by default, is called access.log and is located in the logs directory. Some typical lines from the access log might look like this:



127.0.0.1 - - [04/Oct/1998:09:12:29 +0100]
"GET /server-status HTTP/1.0" 404 192
www.acmeinc.com - - [05/Oct/1998:09:13:04 +0100]
"Get /icons/logo.gif HTTP/1.0" 20 370
www.acmeinc.com - - [05/Oct/1998:09:13:04 +0100]
"Get /icons/banner.gif HTTP/1.0" 200 460
www.acmeinc.com - - [05/Oct/1998:09:13:04 +0100]
"Get /icons/advert.gif HTTP/1.0" 200 689


The date field, which is contained within the [] brackets in the preceding lines, can take the following format:


[day/month/year:hour:minute:second zone]



TIP:  An interesting article on how to use Perl to interpret Apache’s access log file is at www.devdaily.com/perl/edu/articles/pl020003/pl020003.shtml.

You can either change the location of the access log or create specific access logs for any virtual hosts that you serve by using the TransferLog directive. (See “Specifying Log Files” in the Immediate Solutions section of this chapter.)
The Error Log
Apache’s error log is specified by default with the ErrorLog directive in httpd.conf and is located in C:\Apache\logs\error.log. Using the LogLevel directive, you can tell Apache what kinds of error messages should be recorded in the log. (See “Recording Server Errors” in the Immediate Solutions section of this chapter.)
Rotating Logs
It’s important to remember that log files are always open while Apache is running. They can accumulate data and grow at an alarming rate. If you have multiple log files on your server, they can easily begin to consume a substantial amount of hard disk space.

You should maintain your log files by periodically rotating them. This means that you stop the server, copy the files to another disk or another computer, and restart the server. When the server restarts, it will create new log files that you have specified with CustomLog and TransferLog.
The Windows version of Apache does not have utilities such as rotatelog that handle log file rotation automatically without requiring that you restart the server. Currently, you have to stop the server and make your Web site(s) unavailable for a short time until you restart.



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