If-None-Match

The If-None-Match HTTP request header makes the request conditional. For GET and HEAD methods, the server will send back the requested resource, with a 200 status, only if it doesn't have an ETag matching the given ones. 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, then the server must return HTTP status code 304 (Not Modified). For methods that apply server-side changes, the status code 412 (Precondition Failed) is used. Note that the server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary.

The comparison with the stored ETag uses the weak comparison algorithm, meaning two files are considered identical not only if they are identical byte to byte, but if the content is equivalent. For example, two pages that would differ only by the date of generation in the footer would be considered as identical.

When used in combination with If-Modified-Since, it has precedence (if the server supports it).

There are two common use cases:

• 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 not known to exist, guaranteeing that another upload didn't happen before, losing the data of the previous put; this problem is a variation of the lost update problem.

Syntax

If-None-Match: "<etag_value>"
If-None-Match: "<etag_value>", "<etag_value>", …
If-None-Match: *

Directives

<etag_value>

Entity tags uniquely representing the requested resources. They are a string of ASCII characters placed between double quotes (Like "675af34563dc-tr34") and may be prefixed by W/ to indicate that the weak comparison algorithm should be used (This is useless with If-None-Match as it only uses that algorithm).

*

The asterisk is a special value representing any resource. They are only useful when uploading a resource, usually with PUT, to check if another resource with the identity has already been uploaded before.

Examples

If-None-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"

If-None-Match: W/"67ab43", "54ed21", "7892dd"

If-None-Match: *

Specifications

Browser compatibility

See also

• ETag
• If-Unmodified-Since
• If-Modified-Since
• If-Match
• 304 Not Modified
• 412 Precondition Failed