How to enable ETag in Apache Http Server

If your using Apache HTTP Server then you can use it to generate and return ETag for static resources that are served from disk. The Apache Http Server also takes care of comparing the value of If-None-Match header with ETag of the resource and returning either 304 Not modified or 200 OK.

Support for ETag is part of the Apache Core and it is enabled by default. Ex. These are the response headers that i get when i try to access cachesample.gif, which is a static image file on disk from Apache Server



The value of ETag header in this case is combination of three things INode MTime Size

You can configure the behavior of the ETag using the FileETag directive which configures the file attributes that are used to create the ETag (entity tag) response header field when the document is based on a file. (The ETag value is used in cache management to save network bandwidth.) In Apache 1.3.22 and earlier, the ETag value was always formed from the file's inode, size, and last-modified time (mtime). The FileETag directive allows you to choose which of these -- if any -- should be used. The recognized keywords are:

1. INode : The file's i-node number will be included in the calculation. i-node is the number generated by OS to keep track of the file, it includes things like access level, creation time,... You can configure apache to use only INode by adding this line to httpd.conf
   
FileETag INode

   This is screen of how the INode only ETag looks like
   
   


2. MTime: The date and time the file was last modified will be included. You can configure apache to use only Last-Modified date of the file to generate ETag by adding this line to httpd.conf
   
FileETag MTime

   This is how my cachecontrol.gif response headers look like when i use MTime for ETag generation
   
   


3. Size: The number of bytes in the file will be included. You can configure Apache to use only size of the file for generating ETag by adding this line to httpd.conf
   
FileETag Size

   This screen shot of headers when Apache is configured to use only file Size for calculating ETag
   
   


4. All: All available fields will be used. This is equivalent to: FileETag INode MTime Size


5. None: You can disable generation of ETag by Apache Http Server by adding this line
   
FileETag None

   THis is screen shot of headers after ETag is disabled