Using ETag Headers with Cloudflare

ETag headers identify whether the version of a resource cached in the browser is the same as the resource at the origin web server. A visitor's browser stores ETags. When a visitor revisits a site, the browser compares each ETag to the one it stored. Matching values cause a 304 Not-Modified HTTP response that indicates the cached resource version is current. Cloudflare supports both strong and weak ETags configured at your origin web server.

Weak ETag headers indicate a cached resource is semantically equivalent to the version on the web server but not necessarily byte-for-byte identical.

Strong ETag headers ensure the resource in browser cache and on the web server are byte-for-byte identical. Use Cache Rules to enable strong ETag headers.

When you enable Respect Strong ETags in a cache rule, Cloudflare will use strong ETag header validation to ensure that resources in the Cloudflare cache and on the origin server are byte-for-byte identical.

However, in some situations Cloudflare will convert strong ETags to weak ETags. For example, given the following conditions:

• Respect Strong ETags is enabled
• Brotli compression is enabled
• The origin server's response includes an etag: "foobar" strong ETag header

The Cloudflare network will take the following actions, depending on the visitor's accept-encoding header and the compression used in the origin server's response: