How If-None-Match works
Browsers add it automatically when revalidating a cached response that had an ETag. A match gets 304 Not Modified and the browser reuses its copy. On a PUT, If-None-Match: * means “only create this if it doesn’t exist yet”, which prevents overwriting.
Examples
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"If-None-Match: *Common mistakes
- When both are sent, If-None-Match wins over If-Modified-Since.
Check it
# Show the request headers curl sends (lines starting with >)
curl -sv -o /dev/null https://example.com 2>&1 | grep '^>'Paste a full set of response headers into the HTTP header checker to get every header explained and common mistakes flagged.
Related headers
- ETag: gives a version identifier to a response, so caches can ask “has it changed?” cheaply.
- If-Modified-Since: asks for the resource only if it changed after the given date.
- Cache-Control: tells browsers, CDNs and proxies whether a response may be stored, by whom, and for how long.
More caching headers
Last-Modified , Expires , Age , Vary .
Sources
Spotted a mistake? Email hello@devtoolsmith.com and we’ll fix it.
Frequently asked questions
What does the If-None-Match header do?
The If-None-Match request header asks the server to send the resource only if its ETag differs from the ones listed. Browsers add it automatically when revalidating a cached response that had an ETag. A match gets 304 Not Modified and the browser reuses its copy. On a PUT, If-None-Match: * means “only create this if it doesn’t exist yet”, which prevents overwriting.
How do I see the If-None-Match header?
Open your browser’s dev tools, select a request in the Network tab and look under Request Headers. On the command line, "curl -v https://example.com" prints the request headers it sends, marked with ">".