WinSCP Server Refused Our Key: Troubleshooting Authentication Issues

Hey guys! Ever run into that super annoying “WinSCP server refused our key” error? It’s a common problem when trying to connect to a server using SSH keys, but don’t worry, we’re gonna break down how to fix it. Authentication problems can be a real headache, stopping you from securely accessing your files and servers. This error usually pops up when WinSCP, your trusty file transfer tool, tries to log in to a server using an SSH key, but the server’s all like, “Nope, not today!” There are a bunch of reasons why this might happen, ranging from simple typos to more complex permission issues. Understanding why this error occurs is the first step to resolving it. We’ll explore common causes such as incorrect key formats, mismatched usernames, and restrictive server configurations. We’ll also dive into how to verify that your private key is properly loaded and that the corresponding public key is correctly installed on the server. By the end of this guide, you’ll have the knowledge and tools to diagnose and fix the “WinSCP server refused our key” error, ensuring smooth and secure file transfers every time. So, buckle up, and let’s get started!

Common Causes for “Server Refused Our Key”

Okay, so you’re seeing that dreaded “Server Refused Our Key” message in WinSCP. Let’s play detective and figure out what’s going on. Here are the usual suspects:

1. Incorrect Key Format

Key formats matter, guys! WinSCP and the server need to be speaking the same language. This means your private key needs to be in a format that WinSCP understands, and the public key on the server needs to match. The most common formats are OpenSSH and PuTTY’s .ppk format. If you’ve converted your key from one format to another, something might have gone wrong in the process.

To ensure the key format is correct, first identify the format WinSCP is expecting. Typically, WinSCP works best with the .ppk format if you’re using PuTTY’s key generator. If you have a key in a different format (like OpenSSH), you might need to convert it using PuTTYgen. Open PuTTYgen, load your existing key, and then save it as a .ppk file. Make sure to use this .ppk file in your WinSCP connection settings. On the server side, the authorized_keys file should contain the public key in the OpenSSH format. The key should start with ssh-rsa , ssh-dss , or ssh-ed25519 , followed by the key data and a comment (usually your username or email). If there’s a mismatch between the expected and actual formats, WinSCP will be denied access. Always double-check the key formats to avoid this common pitfall. This involves ensuring that the public key is correctly placed in the ~/.ssh/authorized_keys file on the server and that the permissions on this file are correctly set to ensure the SSH server can read it without issues.

2. Mismatched Username

This is a classic facepalm moment. Make sure the username you’re using in WinSCP matches the username associated with the SSH key on the server. Servers are picky about this stuff.

The username specified in your WinSCP connection settings must exactly match the username for which the SSH key was authorized on the server. If you’ve added the public key to the .ssh/authorized_keys file under a different user’s home directory, WinSCP will fail to authenticate. For example, if your WinSCP is configured to connect as john , but the public key is in /home/jane/.ssh/authorized_keys , the server will refuse the key. To resolve this, ensure that the username in WinSCP matches the user account that owns the .ssh directory containing the authorized_keys file. You can verify the correct username by logging into the server using a different method (like password authentication) and checking the user context. Correcting this simple mismatch can often resolve the “server refused our key” error. Remember, SSH keys are user-specific, and the server enforces this strictly for security. Always double-check and confirm that the username in your WinSCP settings aligns with the user account authorized to use the SSH key on the server.

3. Incorrect Permissions

Permissions are super important. If the .ssh directory or the authorized_keys file have the wrong permissions, the server won’t trust them. Think of it like this: the server needs to be able to read the files, but not let just anyone mess with them.

Incorrect permissions on the .ssh directory and the authorized_keys file are a frequent cause of SSH key authentication failures. The SSH server is very strict about these permissions to prevent unauthorized access. The .ssh directory should have permissions set to 700 (drwx——), meaning only the owner (the user) has read, write, and execute permissions. The authorized_keys file should have permissions set to 600 (-rw——-), meaning only the owner has read and write permissions. If these permissions are too open, the SSH server will refuse to use the keys, suspecting a security risk. To correct this, you need to use the chmod command on the server. For example, to set the correct permissions for the .ssh directory, you would run chmod 700 ~/.ssh . To set the correct permissions for the authorized_keys file, you would run chmod 600 ~/.ssh/authorized_keys . After setting these permissions, restart the SSH service (e.g., sudo systemctl restart ssh ) to ensure the changes take effect. Verifying and correcting these permissions is crucial for SSH key authentication to work correctly. Always double-check that only the owner has the necessary permissions and that no one else can read or modify these files.

4. Server Configuration Issues

Sometimes, the server itself is the problem. The sshd_config file might have settings that are preventing key-based authentication. It’s like the server has a bouncer who’s not letting your key in.

Server configuration issues, particularly within the sshd_config file, can prevent key-based authentication. The sshd_config file controls various aspects of SSH server behavior, and incorrect settings can block key authentication attempts. Common problematic settings include PubkeyAuthentication no , which disables public key authentication entirely, or PasswordAuthentication no if you haven’t properly configured key-based authentication as the only allowed method. Additionally, the AuthorizedKeysFile directive specifies the location of the authorized_keys file; if this is incorrect, the server won’t be able to find your public keys. To resolve these issues, you need to edit the sshd_config file (usually located at /etc/ssh/sshd_config ) with root privileges. Ensure that PubkeyAuthentication is set to yes and that the AuthorizedKeysFile directive points to the correct path (usually .ssh/authorized_keys ). After making these changes, save the file and restart the SSH service (e.g., sudo systemctl restart ssh ). It’s also important to check for any Match blocks in the sshd_config file that might be applying different settings to specific users or groups, potentially overriding the global settings. Carefully reviewing and adjusting these configurations can resolve issues where the server refuses your key, allowing for successful SSH key authentication. Always back up the sshd_config file before making changes to avoid potential misconfigurations.

Troubleshooting Steps

Alright, now that we know the usual suspects, let’s get our hands dirty and troubleshoot this thing!

1. Verify the Private Key

First things first, make sure WinSCP is actually using the right private key . Double-check your WinSCP connection settings and ensure the correct .ppk file is selected.

To verify the private key in WinSCP, start by opening your WinSCP connection settings. Navigate to the “Authentication” section within the advanced settings. Here, you should see a field where you specify the private key file. Ensure that the path to the .ppk file is correct and that the file actually exists at that location. If the path is incorrect or the file is missing, WinSCP will obviously fail to authenticate. Next, double-check that the .ppk file is the correct private key associated with the public key you’ve installed on the server. A mismatch between the private and public keys will always result in authentication failure. If you’re unsure, try regenerating the key pair using PuTTYgen and ensure that you use the newly generated private key in WinSCP and install the corresponding public key on the server. Additionally, verify that the private key file is not corrupted. You can do this by attempting to load the key in PuTTYgen; if it fails to load, the file is likely corrupted and needs to be regenerated. By carefully checking these aspects of the private key configuration in WinSCP, you can rule out many common causes of authentication failures and ensure that WinSCP is using the correct and valid private key for authentication. This step is crucial before moving on to more complex troubleshooting steps.

2. Check the Public Key on the Server

Log into the server (maybe using a password for now) and make sure your public key is in the ~/.ssh/authorized_keys file . Also, ensure there are no typos or extra characters in the key.

Checking the public key on the server involves several steps to ensure it is correctly installed and formatted. First, log into the server using an alternative method, such as password authentication, to access the command line. Navigate to the .ssh directory in your home directory (e.g., cd ~/.ssh ). If the directory doesn’t exist, create it with mkdir ~/.ssh and set the permissions to 700 with chmod 700 ~/.ssh . Next, open the authorized_keys file using a text editor (e.g., nano ~/.ssh/authorized_keys or vi ~/.ssh/authorized_keys ). Verify that your public key is present in this file. Ensure that the entire key is on a single line and that there are no line breaks or extra spaces. The key should start with ssh-rsa , ssh-dss , or ssh-ed25519 , followed by the key data and a comment (usually your username or email). Double-check for any typos or missing characters, as even a small error can cause authentication to fail. If you’re copying and pasting the key, ensure that the copy process didn’t introduce any extra characters or line breaks. Also, make sure there are no duplicate keys in the file. Finally, verify that the permissions of the authorized_keys file are set to 600 (rw——-) using the command chmod 600 ~/.ssh/authorized_keys . Incorrect permissions can prevent the SSH server from reading the key. By carefully checking these aspects, you can ensure that the public key is correctly installed and formatted on the server, resolving many common authentication issues.

3. Review Server Logs

The server logs can give you clues about why the key is being refused. Check the /var/log/auth.log file (or wherever your server stores its SSH logs) for any error messages related to authentication.

Reviewing server logs is crucial for diagnosing SSH key authentication failures. The primary log file to check is usually /var/log/auth.log on Debian and Ubuntu systems, or /var/log/secure on CentOS and RHEL systems. These logs contain detailed information about authentication attempts, including the reasons for failures. To effectively review these logs, use commands like grep to filter for relevant entries. For example, you can use grep sshd /var/log/auth.log to see all SSH-related log entries, or `grep