Troubleshooting 'Failed to Negotiate Algorithms' Alerts in SSH Monitoring
Failed to Negotiate Algorithms – Critical Alerts in SSH Mode
The "failed to negotiate algorithms" exception occurs when an SSH client and server cannot agree on a common cryptographic algorithm for secure communication. This prevents the SSH connection from being established.
When an SSH connection is initiated, both client and server exchange supported cryptographic algorithms, including:
- Key Exchange Algorithms – Used to establish a secure connection.
- Ciphers – Used for encrypting data.
- Message Authentication Codes (MACs) – Ensure message integrity.
If there is no common algorithm between the Applications Manager SSH client and the monitored server, a failed to negotiate algorithms error occurs.
Troubleshooting Steps
- Log in to the target server (monitored via SSH).
- Open the SSH configuration file:
sudo vi /etc/ssh/sshd_config - Search for the Cipher, KexAlgorithm, and MACs directives.
- Ensure these directives are not commented out (remove # if present) and note down the values listed for each directive.
- In Applications Manager, navigate to Settings → Performance Polling → SSH Connection Settings
- Locate the Allowed Ciphers, Key Exchanges, and MACs and compare these with the values from the target server.
- If no common algorithm exists across the directives, a critical alert will be triggered.
Fixing the Issue
Option 1: Update Applications Manager SSH Settings
- Enable the missing algorithms in Applications Manager's SSH settings (under the respective directive).
- Restart Applications Manager for changes to take effect.
Option 2: Modify SSH Configuration on the Target Server
Create a backup of sshd_config before modifying it: sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
- Open the SSH configuration file:sudo vi /etc/ssh/sshd_config
- Locate and update the following directives to match the supported algorithms in Applications Manager: Ciphers aes128-ctr,aes192-ctr,aes256-ctr,chacha20-poly1305@openssh.comMACs hmac-sha2-256,hmac-sha2-512,hmac-sha1KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org
,ecdh-sha2-nistp256 - Save the file and exit.
- Restart the SSH service: sudo systemctl restart sshd
- Verify SSH is running: sudo systemctl status sshd
Final Verification
- Try reconnecting to the SSH server via Applications Manager.
- If the issue persists, check system logs for SSH errors using: sudo journalctl -u sshd --no-pager | tail -n 20
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