Resolving SSH Copy ID Permission Denied: A Guide to Public Key Authentication Issues

When you encounter the “Permission denied (publickey)” error while using ssh-copy-id, it typically means that the SSH server is unable to authenticate your public key. This issue can arise due to several reasons, such as incorrect permissions on the .ssh directory or the authorized_keys file, or the public key not being correctly added to the server.

Resolving this error is crucial for maintaining secure SSH connections, as it ensures that only authorized users can access the server, protecting sensitive data and preventing unauthorized access.

Common Causes

Here are the common causes for the “SSH copy ID permission denied (publickey)” error:

1. Incorrect SSH Key Permissions:

   • The private key file should have permissions set to 600 (readable only by the user).
   • The .ssh directory should have permissions set to 700 (accessible only by the user).
   • The authorized_keys file on the server should have permissions set to 600.

2. Missing Public Key in the authorized_keys File:

   • Ensure that the public key is correctly copied into the ~/.ssh/authorized_keys file on the server.
   • Verify that there are no extra spaces or newlines in the authorized_keys file that might cause issues.

3. SSH Configuration Errors:

   • The sshd_config file on the server might not be set up to allow key-based authentication. Ensure PubkeyAuthentication yes is set.
   • The path to the authorized_keys file might be incorrect. Ensure AuthorizedKeysFile .ssh/authorized_keys is correctly specified.
   • The SSH client configuration might be pointing to the wrong key file. Check the IdentityFile setting in the ssh_config file.

4. Key Mismatch:

   • Ensure that the private key on the client matches the public key on the server. Use ssh-keygen -lf /path/to/key to verify the fingerprints.

5. Outdated SSH Software:

   • Ensure both the client and server are running compatible and up-to-date versions of SSH software.

6. Firewall or Security Software:

   • Firewalls or security software might be blocking SSH connections. Ensure that port 22 (or the configured SSH port) is open.

These steps should help you diagnose and resolve the “permission denied (publickey)” error.

Checking SSH Key Permissions

Here are the steps to check and correct SSH key permissions:

1. Check the permissions of the .ssh directory:

   ls -ld ~/.ssh
   

   Expected output:

   drwx------ 2 user user 4096 date time .ssh
   

2. Set the correct permissions for the .ssh directory:

   chmod 700 ~/.ssh
   

3. Check the permissions of the private key file (id_rsa):

   ls -l ~/.ssh/id_rsa
   

   Expected output:

   -rw------- 1 user user 1675 date time id_rsa
   

4. Set the correct permissions for the private key file:

   chmod 600 ~/.ssh/id_rsa
   

5. Check the permissions of the public key file (id_rsa.pub):

   ls -l ~/.ssh/id_rsa.pub
   

   Expected output:

   -rw-r--r-- 1 user user 400 date time id_rsa.pub
   

6. Set the correct permissions for the public key file:

   chmod 644 ~/.ssh/id_rsa.pub
   

7. Check the permissions of the authorized_keys file:

   ls -l ~/.ssh/authorized_keys
   

   Expected output:

   -rw------- 1 user user 400 date time authorized_keys
   

8. Set the correct permissions for the authorized_keys file:

   chmod 600 ~/.ssh/authorized_keys
   

These steps should help resolve the “permission denied (publickey)” error when using ssh-copy-id.

Verifying Public Key in authorized_keys

To verify that the public key is correctly added to the authorized_keys file on the server, follow these steps:

1. Log in to the server: Use a different method (e.g., password) to access the server.
2. Navigate to the .ssh directory:

   cd ~/.ssh
   

3. Check the authorized_keys file:

   cat authorized_keys
   

   Ensure your public key is listed in this file.

If you keep getting ssh-copy-id permission denied (publickey) errors, consider these points:

1. Permissions: Ensure the .ssh directory and authorized_keys file have the correct permissions:

   chmod 700 ~/.ssh
   chmod 600 ~/.ssh/authorized_keys
   

2. Correct User: Verify you are copying the key to the correct user and server:

   ssh-copy-id user@server
   

3. SSH Configuration: Check the server’s SSH configuration (/etc/ssh/sshd_config) to ensure PubkeyAuthentication is enabled.

These steps should help resolve the permission denied (publickey) issue and confirm your public key is correctly added.

SSH Configuration Errors

Potential SSH Configuration Errors

1. Misconfigured SSH Configuration File (sshd_config):

   • Issue: Public key authentication might be disabled, or the path to the authorized_keys file is incorrect.
   • Fix: Ensure PubkeyAuthentication yes is set and the AuthorizedKeysFile path is correct in /etc/ssh/sshd_config. Restart the SSH service after making changes.

2. Incorrect Permissions on .ssh Directory and authorized_keys File:

   • Issue: Permissions that are too open or restrictive can prevent SSH from using the keys.
   • Fix: Set the correct permissions:

     chmod 700 ~/.ssh
     chmod 600 ~/.ssh/authorized_keys
     

3. Key Mismatch:

   • Issue: The public key on the server does not match the private key on the client.
   • Fix: Verify the keys match using:

     ssh-keygen -lf /path/to/key
     

     If they don’t match, generate a new key pair and copy the new public key to the server.

4. Incorrect Username:

   • Issue: Using the wrong username can lead to permission denied errors.
   • Fix: Ensure you are using the correct username in your SSH command:

     ssh user@hostname
     

5. Verbose Output for Debugging:

   • Issue: Lack of detailed error messages can make troubleshooting difficult.
   • Fix: Use verbose mode to get more information:

     ssh -vvvv user@hostname
     

By checking these common issues, you can identify and fix the errors causing the “Permission denied (publickey)” message.

Using Verbose Mode for Debugging

To debug the “permission denied (publickey)” error when using ssh-copy-id, you can use the verbose mode with the -v flag. This provides detailed output of the SSH connection process, helping you identify where the issue lies.

Here’s how to use it:

ssh -v [email protected]

Interpreting the Output

1. Key Exchange:

   • Look for lines starting with debug1:, debug2:, or debug3:. These indicate the stages of the SSH connection.
   • Example: debug1: SSH2_MSG_KEXINIT sent shows the key exchange initialization.

2. Authentication Methods:

   • The output will list the authentication methods attempted.
   • Example: debug1: Authentications that can continue: publickey indicates that public key authentication is being tried.

3. Public Key Authentication:

   • Look for lines like debug1: Offering public key: /path/to/key.
   • If you see debug1: Authentications that can continue: publickey, it means the key was not accepted.

4. Permission Issues:

   • Check for messages about permissions, such as debug1: Authentication succeeded (publickey).
   • If you see Permission denied (publickey), it indicates a problem with the key or its permissions.

By carefully reading these debug messages, you can pinpoint whether the issue is with the key itself, its permissions, or the SSH configuration.

To Troubleshoot the ‘Permission Denied (publickey)’ Error

When using ssh-copy-id, it’s essential to understand the common issues that can cause this problem.

• Permissions: Ensure that the SSH keys and their directories have the correct permissions. Run chmod 700 ~/.ssh and chmod 600 ~/.ssh/authorized_keys to set them correctly.
• Key Mismatch: Verify that the public key on the server matches the private key on the client using ssh-keygen -lf /path/to/key. If they don’t match, generate a new key pair and copy the new public key to the server.
• Incorrect Username: Use the correct username in your SSH command. Ensure you are using the correct username in your SSH command with ssh user@hostname.
• Verbose Output for Debugging: Use verbose mode to get more information about the SSH connection process by running ssh -vvvv user@hostname.

When debugging the issue, pay attention to the output of the SSH connection. Look for lines starting with debug1:, debug2:, or debug3: which indicate the stages of the SSH connection. Check for authentication methods attempted and public key authentication specifically.

If you see a message about permissions, such as ‘Permission denied (publickey)’, it indicates a problem with the key or its permissions. By carefully reading these debug messages, you can pinpoint whether the issue is with the key itself, its permissions, or the SSH configuration.

Correct SSH Key Management and Configuration are crucial to resolving this error. Ensure that your keys are properly generated, stored, and configured on both the client and server sides.