$sent_http_last_modified

Description

The $sent_http_last_modified variable is automatically set by NGINX during the processing of a request that involves fetching a resource. When NGINX responds to a request and includes a Last-Modified header in the HTTP response, this variable captures the value of that header. The Last-Modified header is used to indicate the date and time at which the resource was last modified, allowing clients to make decisions about caching or re-fetching the resource based on its freshness. This variable is useful in scenarios where caching behavior needs to be finely tuned or when implementing conditional GET requests using the If-Modified-Since header on the client side. By examining the value of $sent_http_last_modified, server-side logic can be implemented to alter headers or the content of responses based on the freshness of served resources. Typical values for this variable align with common HTTP date formats, such as 'Wed, 21 Oct 2015 07:28:00 GMT'. Note that if the Last-Modified header is not sent, the variable will be empty.

Config Example

location /example {
    add_header Last-Modified $sent_http_last_modified;
}