If-Unmodified-Since

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

A requisição If-Unmodified-Since do cabeçalho HTTP realiza uma requisição condicional: o servidor enviará de volta o recurso solicitado ou o aceitará no caso de um POST ou non-safe método, somente se não tiver sido modificado pela última vez após a data especificada. Se a solicitação tiver sido modificada após a data já especificada, a resposta será um erro 412 (Precondition Failed).

A requisição

There are two common use cases:

  • In conjunction with non-safe methods, like POST, it can be used to implement an optimistic concurrency control, like done by some wikis: editions are rejected if the stored document has been modified since the original has been retrieved.
  • In conjunction with a range request with a If-Range header, it can be used to ensure that the new fragment requested comes from an unmodified document.
Header type Request header
Forbidden header name no

Sintaxe

If-Unmodified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT

Diretivas

<day-name>

One of "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", or "Sun" (case-sensitive).

<day>

2 digit day number, e.g. "04" or "23".

<month>

One of "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" (case sensitive).

<year>

4 digit year number, e.g. "1990" or "2016".

<hour>

2 digit hour number, e.g. "09" or "23".

<minute>

2 digit minute number, e.g. "04" or "59".

<second>

2 digit second number, e.g. "04" or "59".

GMT

Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local time.

Exemplos

If-Unmodified-Since: Wed, 21 Oct 2015 07:28:00 GMT

Especificações

Especificação Título
RFC 7232, sessão 3.4: If-Unmodified-Since Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Compatibilidade com navegadores

Report problems with this compatibility data on GitHub
desktopmobile
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
If-Unmodified-Since

Legend

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Full support
Full support

Veja também