[ERROR] Unsupported protocol for [test]: [Plain] #3281

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Please make sure:

• You've read the documentation and this is not answered there already
• You're running the latest version of dnscrypt-proxy (precompiled binaries from this repository)
• The software is correctly installed and configured
• You can reliably reproduce the issue

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Diagnostic output

Please run these commands and paste the output:

2.1.17

• I read the documentation and this is not answered there
• Initially raised as discussion #...

What's happening?

https://dnscrypt.info/stamps/ has "Plain DNS" as an option, yet when I add generated stamp into dnscrypt-proxy, it returns

[ERROR] Unsupported protocol for [test]: [Plain]

When does this occur?

Where does it happen?

How do we replicate the issue?

1. https://dnscrypt.info/stamps/
2. Enter 192.168.34.56 (or any IP)
3. Get DNSStamp and add it as static entry to dnscrypt-proxy's toml
4. Start dnscrypt-proxy

Expected behavior

DNSCrypt-proxy test and connect to it, with or without encryption warning.

Additional context

Other DOH client have an option to use system's certstore to verify the cert, but DOH breaks frequently with dnscrypt-proxy. By adding 1 or 2 Plain DNS as a fallback (when no alternatives are online) this improves reliability.