Troubleshooting SSH Authentication: Understanding ‘Permission Denied (publickey,gssapi-keyex,gssapi-with-mic)’

In the realm of remote access, there exists a guardian far superior to the rest known as SSH (Secure Shell). This cryptographic network protocol serves as the vigilant gatekeeper, granting you the privilege of securely connecting to remote servers, issuing commands, and transferring files. Yet, even the most steadfast sentinels have their moments of uncertainty, and SSH is no exception. Picture this: You’re poised to access a remote server but instead, you’re met with an enigmatic declaration – “permission denied (publickey,gssapi-keyex,gssapi-with-mic).” Your journey to server access grinds to a halt, and bewilderment takes hold. Fear not, as this guide will walk you through understanding potential solutions.

Introduction

In this comprehensive guide, we embark on a quest to unveil the secrets behind the “permission denied (publickey,gssapi-keyex,gssapi-with-mic)” SSH error. We’ll delve into the intricacies of SSH authentication, dissect the components of this cryptic message, and equip you with the knowledge and tools necessary to triumph over it. Whether you’re a seasoned system administrator, an inquisitive developer, or new to the domain of SSH, this guide will serve as your trusted navigational chart.

Understanding The Error Message

When it comes to troubleshooting the SSH error, the first step is to decipher the cryptic message itself. Let’s break down each component of this error message to gain a clearer understanding.

• Permission Denied: At its core, this part of the error message signifies one thing: access denied. In the world of SSH, it means that the remote server has rejected the connection request. The server has determined that the client attempting to establish a connection does not have the necessary permissions or credentials.
• publickey: This segment of the error message indicates that public key authentication was attempted. SSH supports various authentication methods, and public key authentication is one of the most secure options. It relies on a key pair consisting of a public key (stored on the server) and a private key (kept securely on the client). When this part of the error message appears, it suggests that the client attempted to authenticate using a public key.
• gssapi-keyex and gssapi-with-mic: These terms refer to GSSAPI-based authentication methods. GSSAPI, or Generic Security Services Application Program Interface, is a framework that allows for the integration of external security mechanisms into SSH authentication. “gssapi-keyex” and “gssapi-with-mic” represent variations of GSSAPI-based authentication methods. When these elements are present in the error message, it implies that GSSAPI authentication methods were also attempted during the authentication process.

Why This Error Occurs

The “permission denied (publickey,gssapi-keyex,gssapi-with-mic)” error can occur for several reasons:

1. Incorrect Key: If public key authentication is used, it’s possible the public key doesn’t match the corresponding private key stored on the server.
2. Key Permissions: Improper permissions on the key files or directories can lead to authentication failures. The key files should be accessible only to the user or system they belong to.
3. Server Configuration: Configuration settings on the server, such as SSHd configuration, might not align with the chosen authentication methods.

Understanding these components and potential causes of the error message is crucial for troubleshooting. In the following sections, we’ll explore steps to resolve this error and regain access to your remote server.

Verifying And Correcting Key Mismatch

You can think of the key as a physical key for a lock, where you need to have the matching key and lock to be able to unlock the door. For the most part, this idea also applies to public and private key pairs.

When working with many SSH keys it is rather easy to use the wrong key on occasion. Let’s go ahead and verify that the correct private and public keys are being used by generating a hash of them both.

Note: To do this we need terminal access to a system or systems that contains the private key and public key respectively.

For each key on their respective machines run the following command:

ssh-keygen -lf /path/to/key

Steps After Matching Keys Confirmed

If they do match we know this isn’t the issue so you can move on to the permissions section.

Steps After Mismatching Keys Confirmed

Discovering a mismatch between your SSH keys can be the root cause of your connection issues. When the public and private keys don’t match, SSH authentication will fail. Here’s what to do if you find yourself in this situation:

1. Regenerate the Key Pair

If the keys do not match, the simplest and most secure course of action is often to generate a new key pair.

Use the ssh-keygen command to create a new key pair. For example:

ssh-keygen -t rsa -b 4096

• This command creates a new RSA key pair with a key length of 4096 bits, which is considered secure.

2. Deploy the New Public Key to the Server

After generating the new key pair, you need to place the new public key in the authorized_keys file on the server.

You can use ssh-copy-id for simplicity:

ssh-copy-id -i ~/.ssh/id_rsa.pub user@hostname

• Replace user@hostname with your actual username and server address.

3. Verify Key Deployment

Ensure that the new public key is correctly added to the authorized_keys file on the server.

You can do this by logging into the server and checking the contents of ~/.ssh/authorized_keys.

Adjusting Key Permissions

Proper key permissions are essential for secure SSH authentication. This section not only covers the permissions for the private and public keys but also emphasizes the importance of correctly setting permissions for the authorized_keys file on the server.

Understanding Ideal Permissions

• Private Key Permissions: The private key on the client should have 600 (-rw——-) permissions, meaning it is only readable and writable by the user.
• authorized_keys Permissions: This file, located in the .ssh directory on the server, contains the public keys that are allowed to authenticate. It should have 600 or 640 permissions to ensure that it is not writable by other users.
• .ssh Permissions: The .ssh directory on both the client and server should have 700 (drwx——) permissions, meaning it is only accessible by the user.

Step-by-Step Guide to Adjust Permissions

Open Terminal: On the client for the private key, and on the server for the public key and authorized_keys file.

Navigate to the SSH Directory: Use cd ~/.ssh/.

Set Private Key Permissions (Client):

chmod 600 id_rsa

Replace id_rsa with your private key file name.

Set authorized_keys File Permissions (Server):

chmod 600 authorized_keys

This ensures that only the user (and root) can modify this file.

Set .ssh Directory Permissions (Both Client and Server):

chmod 700 ~/.ssh

Verify Permissions:
Use ls -l to confirm the permissions.

Note: keep an eye on file ownership, the key files and .ssh directory should be owned by the correct user. Use chown to change ownership if needed.

Using SSH in Verbose Mode

When troubleshooting SSH connectivity issues, one of the most effective tools at your disposal is SSH’s verbose mode. This mode provides detailed information about the connection process, which can be invaluable for diagnosing problems. Running SSH in verbose mode (-vvv) outputs detailed debug information, shedding light on what’s happening behind the scenes during the authentication process.

Understanding Verbose Mode

Levels of Verbosity: SSH has multiple levels of verbosity (-v, -vv, -vvv), with -vvv providing the most detailed output.

Diagnostic Information: Verbose mode displays each step in the SSH connection process, including client-server communication, configuration parsing, and authentication attempts.

How to Use Verbose Mode

Open Terminal: Access your terminal on the client machine.

Initiate SSH with Verbose Mode:

ssh -vvv user@hostname

• Replace the user with your username and hostname with the server’s address.
• The command triggers SSH connection attempts while displaying detailed debug information.

Analyze the Output:

• Look for messages immediately preceding failure points.
• Common indicators to look for include authentication method attempts, rejected public keys, and error messages related to specific authentication stages.

Inspecting Server Configuration

SSH issues can often be traced back to the server’s configuration settings. The SSH daemon (sshd) on the server has a configuration file, usually located at /etc/ssh/sshd_config, which dictates how SSH operates. Misconfigurations here can lead to various issues, including the permission denied error. This section will guide you through checking and adjusting the server’s SSH configuration.

Understanding Key Configuration Settings

• Authentication Methods: Ensure the desired authentication methods (like public key, password, or GSSAPI) are enabled.
• Key File Locations: Check if the server is looking for key files in the correct locations.
• Root Login: Decide if root login should be permitted or denied.
• Port Number: Confirm if SSH runs on the default port (22) or a custom port.

Steps to Inspect And Modify SSH Server Configuration

1. Access the Server: Log into the server where you have administrative or sudo privileges.
2. Open the SSH Configuration File:

sudo nano /etc/ssh/sshd_config

• You can use any text editor (like nano, vi, etc.) to open the file.

3. Review Configuration Settings:
   • Look for lines concerning authentication methods, such as PubkeyAuthentication, PasswordAuthentication, and GSSAPIAuthentication.
   • Check AuthorizedKeysFile to ensure it points to the correct location of the authorized_keys file.
   • Verify the PermitRootLogin setting according to your security policies.
   • If SSH runs on a non-standard port, note the Port setting.
4. Make Necessary Changes:
   • If changes are needed, edit the configuration file. Be cautious and understand each change’s implications.
   • Save the file after making changes.
5. Restart SSH Service:

sudo systemctl restart sshd

• This step is crucial for changes to take effect.

6. Test SSH Connection: After restarting, test the SSH connection to ensure the changes resolve the issue.

Common Pitfalls and Tips

• Backup Configuration: Always backup the sshd_config file before making changes.
• Syntax and Typos: Be vigilant about syntax and avoid typos; even small mistakes can prevent SSH from starting.
• Incremental Changes: Make changes one at a time and test after each change to isolate their effects.

Conclusion

Navigating the complexities of SSH authentication can be daunting, but it’s a manageable and rewarding task with the right approach and understanding. In this guide, we’ve explored various aspects of SSH authentication issues, particularly the ‘permission denied (publickey,gssapi-keyex,gssapi-with-mic)’ error. From understanding the error message and verifying key pairs to adjusting key permissions and inspecting server configurations, each step is designed to bring you closer to a solution.