HTTP: Generating ETag Header

Question

How do I generate an ETag HTTP header for a resource file?

Answer 1

From http://developer.yahoo.com/performance/rules.html#etags:

By default, both Apache and IIS embed data in the ETag that dramatically reduces the odds of the validity test succeeding on web sites with multiple servers.

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If you're not taking advantage of the flexible validation model that ETags provide, it's better to just remove the ETag altogether.