what value should cache-control header have to enable ETag\Last-Modified

Question

What value should have cache-control header to enable ETag\Last-Modified? I want my resources files to be cached but never used without validation from server, i.e. browser should send If-none-match or If-modified-since header and receive 304 HTTP status code to use file from cache.

Answer 1

The short answer is Cache-control: no-cache. Browser/caching proxy will have to always validate data before serving. For success validation ETag and Last-Modified headers must be present. Otherwise resource will be downloaded always fully from the server.