Earlier quoted context omitted.
> How does Tailscale make money? They spy on your network behavior by default, so free users are still paying with their behavioral data. See https://tailscale.com/docs/features/logging “Each Tailscale agent in your distributed network streams its logs to a central log server (at log.tailscale.com). This includes real-time events for open and close events for every inter-machine connection (TCP or UDP) on your networ…
Pretty much this. DNS, SNI, and otherwise plaintext traffic sniffing. That together with user/device 'fingerprinting' (a much more amorphous concept), and that's why such-and-such thing you were just talking about with so-and-so pops up on your screen/feed/whatever, sometimes only minutes later. I highly doubt any of this can actually be opted-out of. How else would they stay in business?
The traffic that does go through their servers is encrypted, and bandwidth limited on the free plan. Any snooping on client behavior would have to be done client side, and the clients are all open source. To some extent the coordination server might be able to deduce some metadata about connections; but definitely not snoop all plaintext traffic.
I think they do have some “service detection” which can basically port-scan your devices to make services visible in the web UI. But that is easy to disable. And premium/enterprise tiers can intentionally log traffic statistics.