When does a webbrowser send If-Modified-Since?

Question

I'm serving a dynamically created recent.rss via python flask and naturally that way it is always sent via 200 OK. I was going to fix this by setting the header Expires a week into the future and checking if the browser sent If-Modified-Since.

If so (after comparing dates) I would just do return nothing, 304 instead of return response, but according to Firebug and request.environ my browser just never sends If-Modified-Since for that ressource.

I know I could just save the data to a file and serve it as a real static file using a webserver, but I might end up really needing to serve static files via WSGI and even if not I'd like to know why this doesn't work.

Thanks for any help.

Answer 1

RFC 2616 HTTP/1.1 has this note in 14.25 If-Modified-Since:

When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header field whenever possible.

This indicates that you should send a Last-Modified header when you expect/want the client to send If-Modified-Since.