An Entity tag (ETag) is a unique identifier assigned to a specific version of a given resource on a web server. ETags are used as a cache control mechanism to allow client browsers to make conditional requests. This lets caches work more efficiently by reusing unchanged resources on the client, and avoiding full server responses if the content has not changed. Efficient caching saves bandwidth and improves performance by delivering only what the client needs, not what it already has.
Anatomy of an ETag
In Apache, ETags are made out of three components: the INode, MTime, and Size.
FileETag INode MTime Size
An ETag looks like this:
When clients cache a resource they also save its ETag. If the server resource changes, its ETag is updated. When a client revisits a page, it checks to see if the resource has changed by sending a conditional header If-None-Match and the value of the object’s ETag:
On the subsequent request, the server compares the ETag sent with the current Etag of the resource. If the Etags match the resource has not changed so the server sends the short response of HTTP 304 Not Modified status. The “not modified” status tells the client that the resource in the cache is good, and it can be reused. If the ETags do not match the server sends a complete response including the modified resource.
The Problem with ETags
The problem with ETags is that by default they are intended to be used on a single server (the inode portion above). Apache and Microsoft IIS default to the full ETag configuration, including the server’s iNode. For websites served from multiple servers, ETags can cause an unnecessary load by serving the same resource from different servers, even though the resource is identical and unchanged.
The Solution: Configure ETags
The solution for sites with multiple servers is to remove the iNode portion of Etags. To do this in Apache add the following lines to your configuration file.
<Directory /usr/local/httpd/htdocs> FileETag MTime Size </Directory>
An example from a site using iNode-less ETag follows:
Server Apache Last-Modified Fri, 28 Jan 2011 16:30:52 GMT Etag "15c3-4d42ef3c" Accept-Ranges bytes Content-Length 5571 Content-Type image/jpeg Date Mon, 31 Jan 2011 18:56:41 GMT
This server-independent ETag avoids the problem of the default ETag configuration, and allows more efficient caching of the same object across different servers.
Remove ETags Entirely
Another option is to remove ETags entirely, and rely on other cache control headers like Last-Modified timestamps. To remove ETags in Apache, add the following to your server configuration file.
Header unset Etag FileETag none
This has the added benefit of reducing the size of your headers, which we’ll talk about in a future tweak.
Used correctly ETags can improve cache efficiency and improve the performance of your website. However, by default Apache and Microsoft IIS servers send fully specified ETags with a server component that can cause unnecessary requests for an identical resource served from multiple servers. One solution is to configure your ETags to omit the server information (iNode). Another is to eliminate ETags entirely.
- Clausen. L. “Concerning Etags and datestamps,”
- In Proc. Web Archiving Workshop, Bath, United Kingdom, September 2004. – Tested the optimal combination of ETag and datestamp caching.
- Code: Configure or Eliminate ETags
- An excerpt from Website Optimization showing how to configure ETags.
- ETag from HTTP 1.1 Specification
- Describes the use of ETags.