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What a pairing QR carries.
A phone that has just been installed knows neither where the server is nor how to prove itself to it, and typing a tailnet hostname and a 64-character hex token on a soft keyboard is the worst part of setting ecr up. One code carries both.
The format is a URI so that it is unambiguous when something other than ecr scans it, and so a future field can be added without the reader having to guess what it is looking at:
ecr://pair?url=http%3A%2F%2Fbox%3A8383&token=8f1c…A bare token with no scheme is still accepted, because that is exactly what
ecr token new --qr produced before this existed and codes already printed
or photographed have to keep working.