What's in a Log File?

There are a number of different log file formats but they are all fairly similar. The most common format is called CLF (Common Logfile Format). There are also different types of log files - access, referer, error, and agent are the primary ones.

Access Log
Analyzing the access log will give you information about who visited your site, which pages they visited, and how long they stayed on the site. This is useful information in determining whether or not your site is working as you intend.

The record below shows the visitor's IP number or hostname, date and time of the request, the command received from the client, the status code returned, the size of the document transferred, and the browser and operating system the visitor was using.

nas-112-52.slc.speedynet.net - - [15/Jan/2001:18:15:12
-0500] "GET page.html HTTP/1.1" 200 23443
"http://www.somedomain.com/page.html" "Mozilla/4.0
(compatible; MSIE 5.01; Windows 95)"

Referer Log
The referer log contains referral information - the source that referred the visitor to your site. If the referrer was a search engine, you will also find the keywords that were entered to find your site - very useful information. Here are some example records.

The record below shows that the visitor followed a link from somedomain.com to the index page of your site.

http://www.somedomain.com/page.html -> /

This record shows that the visitor came to your site from a search engine link. Notice the keyword data is included in the record.

http://search.yahoo.com/bin/search?p=web+design+tips -> /

Agent Log
This log provides information on which browser and operating system was used to access your site.

Mozilla/4.0 (compatible; MSIE 5.01; Windows 95)

Error Log
The error log obviously provides a record of errors generated by the server and sent back to the client.

The record below shows the type of server, date and time of the error, client identification, explanation of the error code generated by the server, and the path to the file that caused the error.

apache: [Sun Jan 15 11:27:57 2001] [error] [client] File does not exist:

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