All Errors

This warning appears when SSH detects that a server's host key has changed since your last connection. While it could indicate a real attack, it's often caused by legitimate server maintenance, reinstalls, or key rotations. Learn how to verify the change is legitimate and safely reconnect.

The "WARNING: UNPROTECTED PRIVATE KEY FILE!" warning appears when your SSH private key file has overly permissive permissions. OpenSSH requires private keys to be readable only by you (mode 600 or 400) as a security measure. When permissions are too open, SSH refuses to use the key and falls back to password authentication.

The 'Load key invalid format' error occurs when OpenSSH cannot parse your private key file because it's in an incompatible format. This commonly happens when the key is in the newer OpenSSH format (introduced in OpenSSH 7.8) but your SSH client or tool expects the traditional PEM format.

This error occurs when an SSH client and server cannot agree on a mutually supported signature algorithm for public key authentication. It typically happens when using deprecated RSA keys with newer OpenSSH versions (8.8+) that disabled SHA-1 signatures, or when connecting to legacy servers that don't support modern signature algorithms.

SSH refuses to connect when the ~/.ssh/known_hosts file has incorrect permissions or is owned by the wrong user. This is a security check SSH performs to prevent unauthorized file modifications from compromising your connections.

SSH enforces strict file permissions to prevent unauthorized access to your private keys. This error occurs when SSH detects that your private key file has permissions that are too permissive (world-readable or group-readable), which could expose your credentials to other users on the system.

This error occurs when SSH refuses to read your configuration file because it has incorrect file permissions or ownership. SSH strictly requires the .ssh directory and its contents to be readable only by your user account, not by group or others.

This error occurs when the SSH agent refuses to sign a key operation, typically due to incorrect file permissions on your private key or agent authentication issues. It prevents SSH authentication even when the key exists locally.

This error occurs when SSH cannot communicate with the ssh-agent service, typically because the agent isn't running. Restart the agent using eval "$(ssh-agent -s)" to resolve the issue.

This error occurs when SSH has exhausted all available authentication methods on the remote server without successfully authenticating. It means the client tried every configured authentication method but all were rejected, usually because the required credentials weren't provided or don't match what's configured on the server.

The "Permission denied (publickey,password)" error means SSH authentication failed using both public key and password methods. Both authentication methods were either disabled on the server or the credentials provided don't match what the server expects. This is one of the most common SSH connection problems.

The 'Connection closed by UNKNOWN port 65535' error occurs when an SSH connection is terminated before completing the key exchange and authentication process. This typically indicates a proxy/firewall issue, incompatible key exchange algorithms, authentication failures, or network connectivity problems between your client and the SSH server.

This warning appears when SSH detects that a remote server's host key has changed. It prevents connections as a security measure to detect potential man-in-the-middle attacks or legitimate server reinstallations. The fix involves removing the old key from your local known_hosts file.

This error occurs when SSH (client or daemon) tries to bind to a port that is already in use by another process or service. Fix it by finding the conflicting process and either terminating it, changing the SSH port, or waiting for socket cleanup.

The "Host is down" error occurs when SSH cannot reach the remote server at the network level. This indicates a network connectivity issue—the target host is unreachable, offline, or has network problems preventing any communication.

This error occurs when SSH authentication succeeds but the session is immediately terminated by the server after the user authenticates. It typically indicates file permission issues, configuration problems, or incompatible authentication settings between client and server.

The SSH "Access denied" error occurs when the SSH server rejects your authentication attempt. This is usually due to incorrect credentials, misconfigured keys, disabled authentication methods, or account/group restrictions on the server.

This error occurs when the ssh-add command cannot communicate with the SSH agent process. The agent may not be running, the socket file may be missing, or environment variables may be misconfigured. This is a common issue on new terminal sessions or after system restarts.

This error occurs when SSH port forwarding or remote command execution fails because the destination host is unreachable or the connection times out before establishing a channel. Common causes include firewall rules, network timeouts, incorrect host/port, or disabled TCP forwarding on the SSH server.

The "Permission denied (publickey)" error occurs when SSH cannot verify your identity using public key authentication. This typically results from missing SSH keys, incorrect file permissions, or the public key not being registered on the server.