Why browser does not send “If-None-Match” header?

Question

I\'m trying to download (and hopefully cache) a dynamically loaded image in PHP. Here are the headers sent and received:

Request:

GET /url:resource/P         


        

Answer 1

Posting this for future me...

I was having a similar problem, I was sending ETag in the response, but the HTTP client wasn't sending a If-None-Match header in subsequent requests (which was strange because it was the day before).

Turns out I was using http://localhost:9000 for development (which didn't use If-None-Match) - by switching to http://127.0.0.1:9000 Chrome¹ automatically started sending the If-None-Match header in requests again.

Additionally - ensure Devtools > Network > Disable Cache [ ] is unchecked.

Chrome: Version 71.0.3578.98 (Official Build) (64-bit)

¹ I can't find anywhere this is documented - I'm assuming Chrome was responsible for this logic.