HTTP headers
If-None-Match HTTP Header Meaning
If-None-Match asks the server to respond only if the ETag no longer matches.
Meaning
The client is validating cached content using one or more ETag values.
Where it appears
Browsers and API clients send it on conditional GET or HEAD requests.
What to check
Check the current ETag and whether the correct response is 304 Not Modified or a fresh representation.
Example
If-None-Match: "33a64df551"