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):
       bash

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

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

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     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:
       bash

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       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:
     bash

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

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     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:
     bash

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     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:
       bash

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       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:
     bash

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

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     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:
     bash

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

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     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

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     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:
     bash

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     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:
      bash

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      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.