Daniel Ford wrote:
Paul mentioned 'get the Last-Modified: header', but I'm not sure what that is and how my server would get it.
That's standardized here: https://tools.ietf.org/html/rfc7232#section-2.2 For example, on my Fedora 27 host, I can run the following two shell commands: wget -S https://www.iana.org/time-zones/repository/tzdb/tzdata.zi wget -N -S https://www.iana.org/time-zones/repository/tzdb/tzdata.zi and observe the attached transcript. The "Last-Modified:" header is one of the HTTP headers I get back from the first "wget". The second "wget" uses a conditional get, and gets a "304 Not Modified" response which means the local copy of tzdata.zi is already up-to-date, and need not be refetched. Say, it looks like the data.iana.org redirects are working! That's new. And these servers support the "Etag:" header, which is better than "Last-Modified:" in some ways. So it looks like you should be using "Etag:" instead of "Last-Modified:", as described here: https://tools.ietf.org/html/rfc7232#section-2.3