SSH account locked or expired
Authentication material may be correct, but the operating-system account is locked, expired, denied by access policy, or assigned a non-login shell.
Constrain an SSH agent key
A key in ssh-agent should be usable only for specific hosts or approved multi-hop paths.
SSH banner exchange invalid format
The TCP endpoint replies, but the bytes received before SSH identification are not valid for an SSH transport.
Change an SSH key passphrase safely
The private key is valid, but its local encryption passphrase needs to be added or changed without replacing server authorization.
SSH connection refused
The destination actively rejects the TCP connection, usually because nothing is listening at that address and port.
Verify an SSH host certificate
The server presents a CA-signed host certificate, so the client must validate its signing CA, host principals, validity interval, and critical metadata.
SSH to an IPv6 link-local address
A fe80:: link-local address is ambiguous without the interface scope that identifies which local link should carry the connection.
SSH multiplexing stale socket
A reused ControlMaster connection or control socket points to dead state, the wrong target, or credentials that should no longer be shared.
SSH post-quantum key exchange warning
A current OpenSSH client warns that the connection negotiated only classical key exchange instead of a hybrid post-quantum method.
Too many authentication failures
The client offers more identities than the server permits before it reaches the correct key.