Decoding the SSH Key Authentication Freeze on Debian Systems


“I’m encountering an issue with SSH key authentication on a headless Debian server. Despite having root SSH enabled and using RSA key authentication for two accounts, the login process hangs indefinitely at the ‘Authenticating with public key’ stage. This occurs for multiple accounts, not just the root. Attempts to reboot the VPS, remove and recreate the `authorized_keys` file, and verify the keys have not resolved the issue. Interestingly, another account logs in successfully with its key. Could you provide insight into what might cause SSH key authentication to stall at the authentication phase?”


When it comes to managing headless servers, SSH (Secure Shell) is the lifeline for administrators, providing a secure channel over an unsecured network. However, SSH key authentication issues can be a significant roadblock. Let’s delve into a scenario where the SSH login process hangs indefinitely at the ‘Authenticating with public key’ stage for multiple accounts, including the root.

The issue described involves a headless Debian server where RSA key authentication is enabled. Despite correct configurations, the server does not progress past the authentication phase. This problem persists even after rebooting the Virtual Private Server (VPS), recreating the `authorized_keys` file, and verifying the integrity of the keys. Curiously, another account can access the server without any hiccups using both password and private key authentication.

Potential Causes and Solutions


Verbose Mode for Diagnostics

: The first step in troubleshooting is to initiate the SSH connection in verbose mode using `ssh -vvv user@host`. This will provide detailed debug information and potentially pinpoint where the process stalls.


Permissions Check

: Incorrect permissions on the `.ssh` directory and the `authorized_keys` file can prevent successful authentication. Ensure that the `.ssh` directory has `700` permissions and the `authorized_keys` file has `600` permissions.


SSH Daemon Configuration

: The SSH daemon itself might be misconfigured. Check the `/etc/ssh/sshd_config` file for any anomalies, especially with the `AuthenticationMethods` and `UsePAM` settings, which can affect key-based authentication.


Server Resources and Performance

: If the server is under heavy load or experiencing performance issues, it might not respond to authentication requests promptly. Monitoring the server’s resources during the login attempt could reveal resource-related problems.


Corrupted SSH Keys

: Although the keys are verified, they might be corrupted on the server. Regenerating the keys and updating the `authorized_keys` file could resolve the issue.


Network Issues

: Network latency or instability can cause timeouts during the authentication process. Checking the network connectivity between the client and the server might uncover network-related causes.


SELinux Contexts

: If SELinux is enforcing, it could be interfering with the SSH service. Temporarily setting SELinux to permissive mode can help determine if it’s the cause.


Home Directory Encryption

: If the home directories are encrypted, the `authorized_keys` file may not be accessible until after login, which creates a catch-22 situation. Moving the `authorized_keys` file outside the encrypted home directory could solve this.


SSH key authentication issues can stem from a variety of sources, ranging from permissions and configurations to server performance and network stability. By methodically checking each potential cause, one can usually pinpoint the issue. If all else fails, consulting the server logs and seeking assistance from the community or a professional may provide the necessary insight to resolve the problem.

Remember, maintaining regular backups of critical configuration files and keys can prevent prolonged downtime and ensure a swift recovery from such issues.

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