If-None-Match header

The HTTP If-None-Match request header makes a request conditional. The server returns the requested resource in GET and HEAD methods with a 200 status, only if it doesn't have an ETag matching the ones in the If-None-Match header. For other methods, the request will be processed only if the eventually existing resource's ETag doesn't match any of the values listed.

When the condition fails for GET and HEAD methods, the server must return a 304 Not Modified and any of the following header fields that would have been sent in a 200 response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary. For methods that apply server-side changes, the 412 Precondition Failed is used when the condition fails.

The comparison with the stored ETag uses the weak comparison algorithm, meaning two files are considered identical if the content is equivalent — they don't have to be identical byte-by-byte. For example, two pages that differ by their creation date in the footer would still be considered identical.

When used in combination with If-Modified-Since, If-None-Match has precedence if the server supports it.

There are two common cases for using If-None-Match in requests:

• For GET and HEAD methods, to update a cached entity that has an associated ETag.
• For other methods, and in particular for PUT, If-None-Match used with the * value can be used to save a file only if it does not already exist, guaranteeing that the upload won't accidentally overwrite another upload and lose the data of the previous PUT; this problem is a variation of the lost update problem.