GET /image.gif HTTP/1.1
If-Modified-Since: Wed, 12 Jan 2005 03:32:31 GMT
HTTP/1.1 304 Not Modified
Last-Modified: Wed, 12 Jan 2005 03:32:31 GMT
YSlow tells me that ETag is misconfigured or missing.
As you can see the ETag headers matches but somehow it's ignored as the server is returning a Last-Modified extra header as it seems ignoring the check.
Is YSlow warning me about the extra header even if the cached ETag match the Response header or its something that I'm missing?
As far my knowledge goes there are probably two issues, an extra header sent by the server and YSlow probably considering this as a protocol violation but giving a wrong warning message?
YSlow runs static analysis on http headers and considers default apache and iis etags configuration as bad etags, see: https://github.com/marcelduran/yslow/blob/4b9510869483c77e8fc716b53a4e7bc4a4186dec/src/common/util.js#L384