SSH from Windows system picking up wrong public key from known hosts file

When using SSH (Secure Shell) to connect to a remote server from your Windows system, you may encounter a situation where the wrong public key is picked up from the known hosts file. This can lead to authentication errors and prevent you from accessing the server. In this article, we will discuss the possible causes of this issue and provide solutions to resolve it.

Understanding SSH and Known Hosts

SSH is a cryptographic network protocol that allows secure communication between two computers. It is commonly used for remote administration and secure file transfers. When you connect to a remote server using SSH for the first time, the server's public key is stored in a file called the "known hosts" file on your local system.

The known hosts file contains a list of trusted servers and their public keys. When you attempt to connect to a remote server, SSH checks the known hosts file to verify the server's identity. If the server's public key matches the one stored in the known hosts file, the connection is established. However, if the public key does not match, SSH raises a warning or error to prevent potential security risks.

The Problem: Wrong Public Key

In some cases, when you try to connect to a remote server from your Windows system using SSH, the wrong public key is picked up from the known hosts file. This can happen due to various reasons, including:

• Multiple entries for the same server in the known hosts file.
• Changes in the server's public key.
• Incorrect formatting of the known hosts file.
• Issues with the SSH client or configuration.

When the wrong public key is used, SSH raises an error similar to:

This error indicates that the server's public key has changed or does not match the one stored in the known hosts file.

Solutions

To resolve the issue of SSH picking up the wrong public key from the known hosts file, you can try the following solutions:

1. Remove Duplicate Entries

If there are multiple entries for the same server in the known hosts file, SSH may pick up the wrong public key. To fix this, open the known hosts file (usually located at C:\Users\your_username\.ssh\known_hosts) in a text editor and remove any duplicate entries for the affected server. Save the file and try connecting again.

2. Update the Known Hosts File

If the server's public key has changed, you need to update the known hosts file. To do this, follow these steps:

1. Open the known hosts file in a text editor.
2. Locate the entry for the affected server.
3. Delete the corresponding line containing the server's old public key.
4. Save the file.

When you try to connect to the server again, SSH will prompt you to verify the new public key. Once you confirm the new key, it will be added to the known hosts file, and future connections will be established without any issues.

3. Verify Known Hosts File Format

Ensure that the known hosts file is correctly formatted. Each entry in the file should be on a separate line and follow the format:

If the formatting is incorrect, SSH may not be able to match the server's public key, resulting in authentication errors. Make sure there are no extra characters or spaces in the file, and each entry is properly structured.

4. Clear SSH Cache

If the above solutions do not resolve the issue, you can try clearing the SSH cache on your Windows system. To do this, open a command prompt and run the following command:

Replace hostname_or_IP_address with the actual hostname or IP address of the affected server. This command removes the cached public key for the specified server, allowing SSH to fetch and store the correct key from the known hosts file during the next connection attempt.

Encountering an issue where SSH picks up the wrong public key from the known hosts file can be frustrating, but it can be resolved by following the solutions mentioned above. By removing duplicate entries, updating the known hosts file, verifying the file format, or clearing the SSH cache, you can ensure that SSH establishes secure connections to remote servers without any authentication errors.