All Errors

SSH public-key auth fails because client and server cannot agree on a signature algorithm, usually after OpenSSH 8.8+ disabled the SHA-1-based ssh-rsa algorithm for RSA keys.

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 SSH agent refused to sign an authentication challenge with your key. This is usually caused by an ssh-add -c confirmation requirement, a hardware-key touch prompt, or gpg-agent/keyring acting as the agent.

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.

SSH refuses your config because the file or .ssh dir has loose permissions. Fix with chmod 600 ~/.ssh/config and chmod 700 ~/.ssh.

SSH exhausted every authentication method the server would accept without a successful login. Usually the right key isn't offered, isn't registered server-side, or the wrong user/key is being used.

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.

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.

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.

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.

This error occurs when your SSH client and server cannot agree on a compatible key exchange algorithm. Older servers may only support deprecated algorithms that modern SSH clients disable by default for security.

The remote server's SSH host key no longer matches what's stored in your known_hosts file. This warning protects against man-in-the-middle attacks but usually indicates a legitimate server change. You can safely resolve it by removing the old key.

This error occurs when your SSH configuration file (~/.ssh/config) contains an unrecognized or unsupported option. It's typically caused by typos, unsupported options for your SSH version, or platform-specific settings used on an incompatible system.

X11 forwarding fails when SSH is misconfigured or missing dependencies. The SSH server cannot establish an X11 tunnel to display graphical applications on your local machine.