Redirect checker

What this will not do

It only fetches public addresses. Anything resolving to a private network is refused, on every hop, because a public URL is free to redirect somewhere internal and this runs on our server rather than yours. It reads the status and the location and never the page body, it gives up after ten hops, and it is rate limited. It is a diagnostic, not a scanner: it cannot tell you whether the destination is safe, only where it is.