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Comment by Waterluvian | Hacker News Reader

This got me wondering. Is there any native way to HTTP to get a checksum or some hash of a file at the beginning of a transfer?

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Isn't that part of the point of etag headers?

Wouldn’t that create a giant mess if someone forged the header etag to match something else?

I haven't got enough skin in this to read any RFCs but my assumption had been it's not intended to act as a point of truth or verify contents and was always only to be used in an advisory capacity. So yeah, it would cause a mess.

Yeah I think it's meant to be used in a context of mutual trust.

Thanks for the name. I looked it up and it seems close. But I think it's up to the client to provide an etag and the server to respond with the same etag and a 304.

What I'm thinking is a server blind way for the client to say: "give me th resource at address X. ... Oh you know what i think I already have this one. Let's abort the transfer."

I guess what I'm thinking is a way for th client to decide it's a 304, not the server.

The If-Match and If-None-Match headers makes the request conditional. You can also of course store the etag with the file or calculate the checksum and do a HEAD conditionally followed by a GET. But there isn’t any advantage to that over If-Match

Download the first 1mb and hash that? Or intermediate portions if the server supports resuming?

That's a clever but fatally flawed solution given how many files could share the first MB.

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