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
andHEAD
methods, used in combination with anRange
header, it can guarantee that the new ranges requested comes from the same resource than the previous one. If it doesn't match, then a416
- 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, the412
(Precondition Failed) response is returned.
Header type | Request header |
---|---|
Forbidden header name | no |
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 byW/
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
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
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
ETag
If-Unmodified-Since
If-Modified-Since
If-None-Match
416
Range Not Satisfiable
412
Precondition Failed