I was running into "PLINK closed unexpectedly" errors when I tried to connect through a SSH tunnel using keyboard-interactive authentication.
Trying to invoke PLINK in a shell revealed the underlying problem:
Using username "foo".
FATAL ERROR: Server sent disconnect message
type 2 (protocol error):
"Too many authentication failures for foo"
This is caused by inadvertently offering multiple ssh keys to the server. The server will reject any key after too many keys have been offered. (source)
PAGEANT was running and served too many keys.
I'd propose to add the '-noagent' option to PLINK's arguments whenever the connection is about to use username/password authentication to avoid similar issues.
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