Tuesday, September 19, 2023

Troubleshooting the "No Supported Authentication Methods Available" Error in SSH


Encountering the "No supported authentication methods available server sent public key" error when connecting to an EC2 instance via SSH is a common frustration. This error can prevent you from accessing your remote server. In this article, we'll explore the causes of this error and provide solutions to resolve it.

Understanding the Error: The "no supported authentication methods available (server sent public key)" error message occurs when your SSH client cannot successfully authenticate with the remote EC2 instance. Several factors can contribute to this issue:

  • Incorrect Login Credentials: Entering an incorrect username or password during the SSH connection attempt will result in failed authentication.
  • Incorrect SSH Key: If you're using SSH keys for authentication, an invalid or incorrect key can prevent successful connection to the remote server.
  • Server Configuration: If the remote server is not properly configured to allow the chosen authentication method, you won't be able to authenticate.

Solving the Problem: Let's explore steps to resolve the "no supported authentication methods available server sent public key" error:

  1. Address SSH Public Key Issues:

    • Edit the /etc/ssh/sshd_config file.
    • Set PasswordAuthentication and ChallengeResponseAuthentication to 'yes'.
    • Restart SSH:
      • Option 1: sudo /etc/init.d/ssh restart

      • Option 2: sudo service sshd restart

  2. Refer to AWS Documentation:

    • AWS provides comprehensive documentation on connecting to EC2 instances using various SSH clients. You can find detailed instructions here: AWS EC2 SSH Documentation.

  3. Verify Correct Logins for Specific AMIs:

    • Depending on the Amazon Machine Image (AMI) you're using, the login usernames may vary. Use the following logins based on your AMI:
      • Ubuntu or root for Ubuntu AMIs
        • ec2-user for Amazon Linux AMIs
          • centos for CentOS AMIs
            • debian or root for Debian AMIs
              • ec2-user or fedora for Fedora AMIs
                • ec2-user or root for RHEL AMIs, SUSE AMIs, and others.

            • Using SSH on Different Operating Systems:

              • For Windows:
                • Obtain the PEM key from the AWS website and generate a PPK file using PuttyGen. Then, use Putty for SSH, selecting the PPK file under "Connection -> SSH -> Auth" for authorization.
              • For Linux:
                • Run the following command: ssh -i your-ssh-key.pem login@IP-or-DNS.

            • Accessing Your EC2 Instance:

              • Open an SSH client or refer to PuTTY for Windows users.
              • Locate your private key file (e.g., test_key.pem), ensuring it has the appropriate permissions (use chmod 400 if needed).
              • Connect to your EC2 instance using its Public DNS, e.g., ssh -i "test_key.pem" ubuntu@xxx-yyy-100-10-100.us-east-2.compute.amazonaws.com.

            Conclusion: While the "no supported authentication methods available server sent public key" error can be frustrating, it is often resolvable. By double-checking your login credentials, SSH key, and trying different authentication methods, you can usually overcome this issue. If problems persist, it's advisable to investigate the server's configuration and consult server logs or contact the server administrator for assistance.

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