Safari not sending "If-Modified-Since" and "If-None-Match" headers

Question

I'm generating dynamic content with PHP.

I'm sending the following HTTP-header:

HTTP/1.1 304 Not Modified
Date: Sun, 09 Dec 2012 17:24:41 GMT
Server: Apache
Connection: keep-alive, Keep-Alive
Keep-Alive: timeout=1, max=100
Etag: "237f43b800e655dbe6567f7d32d34c99"
Expires: Sun, 16 Dec 2012 17:24:41 GMT
Cache-Control: max-age=604800
Vary: Accept-Encoding

I later check for the Etag to send a header('HTTP/1.1 304 Not Modified') if it matches. This works perfectly with Chrome and Firefox. However, Safari (Version 6.0.2) does not send "If-Modified-Since" and "If-None-Match" headers. This is the Request-Header sent by Safari on the second page hit:

User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_2) AppleWebKit/536.26.17 (KHTML, like Gecko) Version/6.0.2 Safari/536.26.17
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Cache-Control: max-age=0

All other files linked on that page receive the right Request-Headers and Safari used cached files or 304 where appropriate.

Why doesn't send Safari the correct Request-Header? What could I change?

Thank you!

Toomer

Are you forcing a refresh via f5/refresh button? Or loading the page more normally(submitting url field w/ enter key, or clicking a link)? I ask because it sent Cache-Control: max-age=0, which smells of a refresh. – Hasp

Are these the headers for the actual page being requested listed in the nav bar or a linked page such as a resource within the page? It seems that Safari doesn't check the page cache if it's the page being requested in the nav bar, but if it's a resource within another page (ajax, js, css, img), it will check the cache first. – Venetian

Those are indeeed the headers generated by a "reload" (CMD+r). However accessing the site by entering it's URL lets Safari generate the same Request-Header, just without the Cache-Control-Line. What I am irritated about is that Safari NEVER sends the Date or Etag in the Request-Header but Chrome and Firefox do. – Toomer

Similar issue: #5616515 – Chronic

The problem is too old by now to verify this but are you really sending these headers with a 304 Not Modified response or is it just a copy & paste mistake and you are sending these headers in a 200 OK? – Chiao

@Chiao – I can't check now anymore. But I remember to have tried Chrome and Firefox (as stated) which worked just fine. – Toomer

@Toomer Your Expires header is 2 years in the past. Im not using safari, but I would not be suprised if they disregard all other caching header you send if that header explicitly state that the image should not be cached. – Chema

@Chema the first thread is two years old – the date was seven days in the future back then. – Toomer

@Chiao resending these headers on a 304 response isn't bad practise, since clients have been known to 'forget' some otherwise. – Warmblooded

Answer 1

If-Modified-Since

The notes of If-Modified-Since warn against using something different than the contents of the Last-Modified response header:

Note: If a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time.

Your response doesn't contain a Last-Modified, but even if it did, nothing in the spec says that user agents MUST or SHOULD send If-Modified-Since, they just MAY.

If-None-Match

As to why Safari doesn't send an If-None-Match beats me; your ETag looks valid.

Enforcing Caching in general

Again in general nothing in the spec says you MUST cache, just that when you do, you MUST obey Cache-Control. So it's a bit of an asymmetric relation, you can enforce (transparent) caches to recheck with the origin server, but as an origin server, you can't enforce useragents to cache.

What can I do about it?

Safari is only partly free software. So nothing much, if adding a Last-Modified doesn't help.

Warmblooded

I ran into the same issue concerning the If-Modified-Since header and was able to get Safari to provide it after initially returning both the Last-Modified header but also the Expires header with the same value. For more, I posted a solution on this related question, #5616515 – Careycarfare

Answer 2

I ended up solving this one by looking for what headers are sent by various big sites, and have ended up with

Last-Modified: {some date}
Cache-Control: max-age=0, must-revalidate
Expires: -1

This is working on Safari for me.

Ebeneser

This works in 2020 and forces Safari to return HTTP 304 correctly. – Barracoon

I could not make this work for the initial page load (details in another answer). – Howdoyoudo

That's weird @J – Ebeneser

Answer 3

Safari 16.3 (March 2023) never sends If-Modified-Since or If-None-Match for the initial document GET request initiated by the browser.

Compare the request and response headers between Safari and Chrome for example.com, which uses both ETags and Last-Modified.

Safari

When reloading example.com, Safari does not send the If-Modified-Since or If-None-Matches headers.

The server must return HTTP code 200 with all content since Safari does not include caching information in the request.

Chrome

When reloading example.com (don't reload too quickly--Chrome interprets rapid reloads as force-reload), Chrome sends both the If-Modified-Since and If-None-Match headers populated from the first load ETag and Last-Modified headers.

The server returns HTTP code 304 since the ETag hasn't changed.

Howdoyoudo

That's weird @Howdoyoudo (and unfortunately not that surprising). Is that a documented change? I don't have a Mac to play with, but I wonder if it's odd behaviour with devtools enabled? Sorry if I'm asking the obvious, but these things sometimes happen. – Ebeneser

I had caching enabled for dev tools and I’m able to observe the behavior without dev tools open. I wasn’t able to find a bug report. There are many references to Safari’s behavior—looks like this behavior has been around since at least Safari 12: gertjans.home.xs4all.nl/javascript/cache-control.html – Howdoyoudo

Thanks. It's surprising anything works! – Ebeneser