Windows SSH-Server is only reading first line from "authorized_keys" file

3 min read 26-10-2024

the ifix

Windows SSH-Server is only reading first line from "authorized_keys" file

If you're facing an issue with your Windows SSH-Server only reading the first line of the authorized_keys file, you're not alone. This problem can be frustrating, especially when you're trying to set up secure key-based authentication for SSH. In this article, we will explore the original problem, analyze potential causes, and offer practical solutions to resolve the issue.

Understanding the Problem

When configuring the SSH server on a Windows machine, many users have encountered the issue where the server only recognizes the first entry in the authorized_keys file. This means that if you have multiple public keys listed in this file, only the first one will be accepted for authentication, potentially limiting access for users.

Original Code/Configuration Issue

Here’s a simplified representation of how the authorized_keys file may be structured:

ssh-rsa AAAAB3... [email protected]
ssh-rsa AAAAB3... [email protected]
ssh-rsa AAAAB3... [email protected]

In this example, only the first ssh-rsa key will be read and accepted for authentication.

Analyzing the Issue

Common Causes

File Permissions: SSH servers are sensitive to the permissions of the authorized_keys file. If the permissions are too lenient, the server may refuse to read the file entirely.
Formatting Issues: The file must be properly formatted. Extra spaces, newlines, or incorrect line endings can lead to only the first key being recognized.
Line Breaks: Windows and Unix-like systems handle line breaks differently. Ensure that the file uses the correct format for your SSH server.
SSH Server Configuration: Sometimes the SSH server configuration itself may restrict the reading of multiple keys.

Solutions

To resolve the issue where your Windows SSH-Server only reads the first line from the authorized_keys file, consider the following steps:

Check File Permissions:
- Make sure that the permissions for the authorized_keys file are set appropriately. Typically, the file should have read and write permissions for the user and no access for others. You can set it using PowerShell:
```
$acl = Get-Acl "C:\path\to\.ssh\authorized_keys"
$acl.SetAccessRuleProtection($true, $false)
Set-Acl "C:\path\to\.ssh\authorized_keys" $acl
```
Format the File Correctly:
- Open the authorized_keys file in a plain text editor that supports Unix line endings (like Notepad++ or Visual Studio Code). Ensure that each key is on its own line without any extra spaces or characters.
- Save the file with Unix (LF) line endings to avoid issues.
Validate Server Configuration:
- Check your sshd_config file (usually found in the C:\ProgramData\ssh directory). Ensure that there are no settings limiting the number of keys that can be read. Look for any unusual configurations related to AuthorizedKeysFile.
Testing:
- After making changes, restart the SSH server to apply the changes. Use the command:
```
Restart-Service sshd
```

Practical Example

Let’s say you want to add three different public keys for three users. Ensure the authorized_keys file looks like this:

ssh-rsa AAAAB3... [email protected]
ssh-rsa AAAAB3... [email protected]
ssh-rsa AAAAB3... [email protected]

After confirming proper permissions, formatting, and configuration, you should be able to allow SSH access for all three users. Each key should authenticate properly when the respective user connects.

Additional Resources

Conclusion

Troubleshooting the Windows SSH-Server reading only the first line of the authorized_keys file can be resolved by checking file permissions, formatting the file correctly, and validating the server’s configuration. By following these steps, you can ensure a smooth key-based authentication setup, allowing multiple users to access your server securely.

Remember to keep your SSH configuration and keys well-organized and regularly audit them for any security vulnerabilities. If you continue to face issues, consulting the community forums or official documentation can provide further insight.