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If-Match

The If-Match HTTP request header makes the request conditional. For GET and HEAD methods, the server will send back the requested resource only if it matches one of the listed ETags. For PUT and other non-safe methods, it will only upload the resource in this case.

The comparison with the stored ETag uses the strong comparison algorithm, meaning two files are considered identical byte to byte only. This is weakened when the W/ prefix is used in front of the ETag.

There are two common use cases:

  • For GET and HEAD methods, used in combination with an Range header, it can guarantee that the new ranges requested comes from the same resource than the previous one. If it doesn't match, then a 416(Range Not Satisfiable) response is returned.
  • For other methods, and in particular for PUT, If-Match can be used to prevent the lost update problem. It can check if the modification of a resource that the user wants to upload will not override another change that has been done since the original resource was fetched. If the request cannot be fulfilled, the 412 (Precondition Failed) response is returned.

Syntax

If-Match: <etag_value>
If-Match: <etag_value>, <etag_value>, …

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.
*
The asterisk is a special value representing any resource.

Examples

If-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"

If-Match: W/"67ab43", "54ed21", "7892dd"

If-Match: *

Specifications

Specification Title
RFC 7232, section 3.1: If-Match Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Browser compatibility

Feature Chrome Firefox Edge Internet Explorer Opera Safari
Basic Support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Edge mobile Firefox for Android IE mobile Opera Android iOS Safari
Basic Support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also

© 2005–2017 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Match