You can try to use a partial GET (byte range) to seek around the file to find its end by trial (and avoid downloading 20GB), but I'm pretty sure 99% of the resources supporting a partial GET will also report a Content-Length in the header.
They're definitely not required to be present together.

Furthermore, the server doesn't necessarily have to tell you that it doesn't support range requests at all (let alone in a useful manner).
The RFC says:

Note: clients cannot depend on servers to send a 416 (Requested range not satisfiable) response instead of a 200 (OK) response for an unsatisfiable Range request-header, since not all servers implement this request-header.
The definitive answer is no.

Now, for hacker...

Alternatively, why not use the ':content_cb' callback of LWP::UserAgent. With that callback, you can implement your own semantics for max-content-length; if you decide you no-longer want to fetch the file when you reach say 2k, just abort the request by die()ing.

-David

In reply to Re^2: Determining Content-Length when there is no Content-Length header by erroneousBollock
in thread Determining Content-Length when there is no Content-Length header by hacker

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