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

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