Chrome doesn't send “If-Modified-Since”

Question

I want browsers to always add (except first time) \"If-Modified-Since\" request header to avoid unnecessary traffic.

Response headers are:

Accept-Ranges:         


        

Answer 1

I just now found this question, and after puzzling over Chrome's If_Modified_Since behavior, I've found the answer.

Chrome's decision to cache files is based on the Expires header that it receives. The Expires header has two main requirements:

1. It must be in Greenwich Mean Time (GMT), and
2. It must be formatted according to RFC 1123 (which is basically RFC 822 with a four-digit year).

The format is as follows:

Expires: Sat, 07 Sep 2013 05:21:03 GMT

For example, in PHP, the following outputs a properly formatted header.

$duration = time() + 3600 // Expires in one hour.
header("Expires: " . gmdate("D, d M Y H:i:s", $duration) . " GMT");

("GMT" is appended to the string instead of the "e" timezone flag because, when used with gmdate(), the flag will output "UTC," which RFC 1123 considers invalid. Also note that the PHP constants DateTime::RFC1123 and DATE_RFC1123 will not provide the proper formatting, since they output the difference to GMT in hours [i.e. +02:00] rather than "GMT".)

See the W3C's date/time format specifications for more info.

In short, Chrome will only recognize the header if it follows this exact format. This, combined with the Cache-Control header...

header("Cache-Control: private, must-revalidate, max-age=" . $duration);

...allowed me to implement proper cache control. Once Chrome recognized those headers, it began caching the pages I sent it (even with query strings!), and it also began sending the If_Modified_Since header. I compared it to a stored "last-modified" date, sent back HTTP/1.1 304 Not Modified, and everything worked perfectly.

Hope this helps anyone else who stumbles along!