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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 ETags

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 ETags

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.

Behavior with Respect Strong ETags enabled

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:

`accept-encoding` header from visitor	Compression used in origin server response	Cloudflare actions
`gzip, br`	GZIP	Return GZIP-compressed response to visitor with strong ETag header: `etag: "foobar"`.
`gzip, br`	Brotli	Return Brotli-compressed response to visitor with strong ETag header: `etag: "foobar"`.
`br`	GZIP	Decompress GZIP and return uncompressed response to visitor with weak ETag header: `etag: W/"foobar"`.
`gzip`	Brotli	Decompress Brotli and return uncompressed response to visitor with weak ETag header: `etag: W/"foobar"`.
`gzip`	(none)	Return uncompressed response to visitor with strong ETag header: `etag: "foobar"`.
`gzip, br, zstd`	Zstandard	Return zstd-compressed response to visitor with strong ETag header: `etag: "foobar"`.
`gzip, br`	Zstandard	Decompress zstd and return br response to visitor with weak ETag header: `etag: W/"foobar"`.
`zstd`	Brotli/GZIP	Decompress zstd and return zstd response to visitor with weak ETag header: `etag: W/"foobar"`.

Enabling Respect Strong ETags in Cloudflare automatically disables Rocket Loader, Email Obfuscation, and Automatic HTTPS Rewrites.

Behavior with Respect Strong ETags disabled

When Respect Strong ETags is disabled, Cloudflare will preserve strong ETag headers set by the origin web server if all the following conditions apply:

The origin server sends a response compressed using GZIP or Brotli, or an uncompressed response.
If the origin server sends a compressed response, the visitor accepts the same compression (GZIP, Brotli), according to the accept-encoding header.
Rocket Loader and Email Obfuscation features are disabled.

In all other situations, Cloudflare will either convert strong ETag headers to weak ETag headers or remove the strong ETag. For example, given the following conditions:

Respect Strong ETags is disabled
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:

`accept-encoding` header from visitor	Compression used in origin server response	Cloudflare actions
`gzip, br`	GZIP	Decompress GZIP and return Brotli-compressed response to visitor (since Brotli compression is enabled) with weak ETag header: `etag: W/"foobar"`.
`gzip, br`	Brotli	Return Brotli-compressed response to visitor with strong ETag header: `etag: "foobar"`.
`br`	GZIP	Decompress GZIP and return Brotli-compressed response to visitor with weak ETag header: `etag: W/"foobar"`.
`gzip`	Brotli	Decompress Brotli and return GZIP-compressed response to visitor with weak ETag header: `etag: W/"foobar"`.
`gzip`	(none)	Compress origin response using GZIP and return it to visitor with weak ETag header: `etag: W/"foobar"`.
`gzip, br, zstd`	Zstandard	Return zstd-compressed response to visitor with strong ETag header: `etag: "foobar"`.
`gzip, br`	Zstandard	Decompress zstd and return uncompressed response to visitor with weak ETag header: `etag: W/"foobar"`.
`zstd`	Brotli	Decompress zstd and return uncompressed response to visitor with weak ETag header: `etag: W/"foobar"`.

Refer to Content compression for more information.

Important remarks

You must set the value in a strong ETag header using double quotes (for example, etag: "foobar"). If you use an incorrect format, Cloudflare will remove the ETag header instead of converting it to a weak ETag.
If a resource is cacheable and there is a cache miss, Cloudflare does not send ETag headers to the origin server. This is because Cloudflare requires the full response body to fill its cache.
If your origin (or R2) applies compression based on accept-encoding, the first compression type will be cached. Consider whether strong ETags fit your use case, or use cache key rules to handle different compression types.