OpenSSH sshd.exe does not start with KexAlgorithms parameter

2 min read 25-10-2024

the ifix

OpenSSH sshd.exe does not start with KexAlgorithms parameter

When configuring OpenSSH's sshd.exe, a common issue that users encounter is the failure of the SSH daemon to start when the KexAlgorithms parameter is specified. This article explores the problem, provides an easy-to-understand explanation, and offers solutions to ensure sshd.exe runs smoothly with the desired configurations.

Understanding the Problem

The original problem can be summarized as follows: OpenSSH's sshd.exe fails to start when the KexAlgorithms parameter is set.

Here’s an example of the code snippet where the problem might occur:

# sshd_config
KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1

When this configuration is added to the sshd_config file, users may notice that the SSH daemon does not start as expected.

Analyzing the Issue

The KexAlgorithms parameter in the sshd_config file specifies the key exchange algorithms that the SSH server will use to establish secure connections. If the SSH daemon does not start, it may be due to several reasons:

Unsupported Algorithms: The specified algorithms might not be supported in the version of OpenSSH you are using. It's essential to confirm that the algorithms listed are valid for your OpenSSH version.
Incorrect Configuration: A syntax error or misplaced configuration can lead to startup failures. Ensure that your sshd_config file follows the proper syntax and structure.
Permissions Issues: The configuration file may not have the correct permissions set, preventing sshd.exe from reading it properly.

Practical Example

Let’s say you added the above KexAlgorithms configuration, but when attempting to start the sshd.exe, you encounter errors in the logs. Here's how to troubleshoot:

Check for Errors: Open the Windows Event Viewer or check the logs of OpenSSH to see if there are specific error messages that indicate what's wrong.
Validate Algorithms: Run the following command to list all supported algorithms in your version:
```
ssh -Q kex
```
Compare the output with your KexAlgorithms setting to ensure all specified algorithms are supported.
Test Configuration: You can test the configuration file syntax using:
```
sshd.exe -t
```
This command checks the configuration for any potential syntax errors without starting the service.

Additional Solutions

Use Default Settings: If troubleshooting does not work, consider reverting to the default KexAlgorithms configuration by commenting out the line or removing it entirely:
```
# KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
```
Update OpenSSH: Ensure you are running the latest version of OpenSSH. Newer releases may have fixed bugs or added support for additional algorithms.
Community Resources: For additional help, consider checking forums such as Stack Overflow or the OpenSSH mailing list, where developers and users discuss similar issues.

Conclusion

Troubleshooting issues with OpenSSH sshd.exe not starting due to the KexAlgorithms parameter can be straightforward if approached methodically. By understanding the implications of the KexAlgorithms configuration and using the provided troubleshooting steps, users can resolve these issues effectively.

For more information on configuring OpenSSH and the available parameters, you can visit the official OpenSSH documentation.

Useful Resources

By following these guidelines and utilizing the resources provided, users can enhance their experience with OpenSSH and ensure reliable SSH connections in their environments.