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SSH Checklist

SSH Key Setup Checklist

  1. Generate SSH Key Pair
    • Ensure that you have generated an SSH key pair on the client machine (system a).
      • Run the following command and check for the existence of ~/.ssh/id_rsa (private key) and ~/.ssh/id_rsa.pub (public key):

        ls -l ~/.ssh/id_rsa ~/.ssh/id_rsa.pub
    • If the keys do not exist, generate them using:

      ssh-keygen -t rsa -b 4096
    • Ensure the keys have appropriate permissions (600 for private and 644 for public keys):

      chmod 600 ~/.ssh/id_rsa chmod 644 ~/.ssh/id_rsa.pub

  2. Copy Public Key to Remote Server

    • Ensure that the public key has been copied to system b and appended to the ~/.ssh/authorized_keys file.
      • Use the ssh-copy-id command to copy the key:

        ssh-copy-id username@system_b
      • Alternatively, you can manually copy the contents of ~/.ssh/id_rsa.pub to ~/.ssh/authorized_keys on system b.
    • Verify that the authorized_keys file has the correct permissions:

      chmod 600 ~/.ssh/authorized_keys
    • Make sure the .ssh directory has the correct permissions:

      chmod 700 ~/.ssh
  3. Check SSH Daemon Configuration on Remote Server
    • Ensure the SSH daemon is configured to allow key-based authentication. Check /etc/ssh/sshd_config on system b for the following settings:
      • PubkeyAuthentication yes
      • AuthorizedKeysFile .ssh/authorized_keys
      • PasswordAuthentication no (optional, for enforcing key-based auth only)
    • Restart the SSH daemon to apply changes:

      sudo systemctl restart sshd
  4. Verify Ownership and Permissions
    • Check ownership and permissions of the user's home directory on system b:
      • Ensure that the home directory and .ssh directory are owned by the user and have appropriate permissions:

        chown -R username:username /home/username chmod 700 /home/username/.ssh
  5. Ensure Correct SSH Command Usage
    • Use the correct username and hostname when attempting to SSH into system b:

      ssh username@system_b
    • If using a non-standard SSH port, specify it using the -p option:

      ssh -p <port_number> username@system_b
  6. Check SSH Agent (Optional)
    • Ensure that the SSH agent is running and the key is added to it, especially if using ssh-agent for key management:

      eval "$(ssh-agent -s)" ssh-add ~/.ssh/id_rsa
    • Verify the keys added to the agent:

      ssh-add -l
  7. Review SSH Debugging Output
    • Use the -v option with the SSH command to enable verbose mode, which can provide detailed debugging information:
      bash
      ssh -v username@system_b
    • Look for lines indicating whether the key is being offered and any errors that may indicate why the key is being rejected.
  8. Network Issues
    • Ensure there are no network issues preventing the connection. Check that system b is reachable via the network.
    • Verify that the firewall on system b is allowing incoming SSH connections on the specified port.
  9. SELinux and AppArmor (Linux Specific)
    • If system b uses SELinux, ensure it is not blocking SSH connections:

      sudo setenforce 0 # Temporarily disable for testing
    • If using AppArmor, check that it allows SSHD to read the authorized_keys file.
  10. Check for Multiple Keys (Optional)
    • If multiple keys are present, specify the exact key to use:

      ssh -i ~/.ssh/id_rsa username@system_b

By following this checklist, you should be able to identify and resolve most issues preventing SSH key-based authentication. If problems persist, examining the verbose output from the SSH command can provide further clues.

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