Server Host Key Did Not Match the Signature Supplied: A Comprehensive Guide for Devs

Greetings, Devs! In this article, we will tackle the common issue of “server host key did not match the signature supplied” and provide you a comprehensive guide to resolving it. Whether you are new to server management or a seasoned professional, this article aims to equip you with the necessary knowledge and skills to address this problem effectively. Let’s get started!

Overview of Server Host Key and Signature

Before diving into the root cause and solution of the error, it is essential to understand what server host key and signature are. A server host key is a unique identifier that allows clients to verify their connection to a specific server. It establishes a secure connection and prevents man-in-the-middle (MITM) attacks, where an attacker intercepts and modifies the data transmitted between the client and the server.

A signature, on the other hand, is used to verify the authenticity of the server host key. It is a digital fingerprint that confirms that the key belongs to a specific server and has not been tampered with. In essence, the signature acts as a certificate that guarantees the integrity of the server host key.

Common Causes of “Server Host Key Did Not Match the Signature Supplied” Error

Now that we have a basic understanding of server host key and signature let’s examine the common reasons why the error message “server host key did not match the signature supplied” occurs:

1. Host key rotation: Host key rotation, where a new host key is generated, can trigger the error as the signature on the client-side may not match the new server host key.
2. Server configuration: Incorrect configuration of the server can cause the host key and signature to mismatch. For example, if the SSH daemon is configured to use a different key or signature, the client will not be able to verify the server’s authenticity.
3. Client configuration: Similarly, if the client’s configuration is incorrect, it may not recognize the server host key or signature, resulting in the error.
4. MITM attack: In rare cases, the “server host key did not match the signature supplied” error can indicate a MITM attack. An attacker may try to intercept and modify the server’s key or signature, resulting in a mismatch.

Resolving the “Server Host Key Did Not Match the Signature Supplied” Error

Step 1: Verify Server Host Key and Signature

The first step in resolving the error is to verify the server host key and signature. You can do this by comparing the server’s host key and signature with the ones stored in the client’s known_hosts file. The known_hosts file stores the host key and signature of all servers that the client has connected to in the past. To check the server’s host key and signature, run the following command:

Command	Description
ssh-keyscan <hostname>	Retrieves the server’s host key and signature

Once you have retrieved the host key and signature, compare them with the ones stored in the known_hosts file. You can do this by opening the known_hosts file and searching for the hostname or IP address of the server. If the host key and signature do not match, you can remove the entry from the known_hosts file and try connecting to the server again.

Step 2: Update Server Configuration

If the server’s configuration is incorrect, you can update it to resolve the error. You can check the server’s configuration by examining the SSH daemon configuration file which is usually located in /etc/ssh/sshd_config. Ensure that the SSH daemon is configured to use the correct host key and signature. You can also check if the key or signature has been rotated recently and update the configuration accordingly. If you are unsure about the correct configuration, consult the server administrator or refer to the documentation.

Step 3: Update Client Configuration

If the client’s configuration is incorrect, you can update it to resolve the error. You can check the client’s configuration by examining the SSH configuration file which is usually located in ~/.ssh/config. Ensure that the client is set to verify the server’s host key and signature. You can also check if the client’s known_hosts file contains the correct host key and signature of the server. If you are unsure about the correct configuration, refer to the documentation or seek assistance from a more experienced colleague.

Step 4: Mitigate MITM Attack

If all other causes have been ruled out, and you suspect a MITM attack, you can take steps to mitigate it. A MITM attack involves an attacker intercepting the connection between the client and server, and modifying the host key and signature to match their own. To mitigate the attack, you can use SSH key fingerprints, which are unique identifiers that can help detect changes in the server’s host key and signature. You can obtain the fingerprint of the server’s key and signature by running the following command:

Command	Description
ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub	Retrieves the fingerprint of the server’s key and signature

Compare the fingerprint with the one stored in the client’s known_hosts file. If they do not match, it may indicate a MITM attack, and you should avoid connecting to the server. You can also enable strict host key checking, which will reject any connection that does not match the host key and signature in the known_hosts file.

FAQ

What is a host key?

A host key is a unique identifier that servers use to verify their identity to clients. It is used to establish a secure connection and prevent MITM attacks.

What is a signature?

A signature is a digital fingerprint that verifies the authenticity of the host key. It acts as a certificate that guarantees the integrity of the host key.

Why did I receive the “server host key did not match the signature supplied” error?

The error occurs when the client cannot verify the authenticity of the server’s host key and signature. This can be caused by host key rotation, server or client misconfiguration, or a MITM attack.

How can I resolve the error?

You can resolve the error by verifying the server’s host key and signature, updating the server or client configuration, or mitigating a MITM attack.

How can I prevent the error from occurring in the future?

You can prevent the error by regularly checking the server’s key and signature and updating the client’s known_hosts file. You can also use SSH key fingerprints and enable strict host key checking to mitigate any potential MITM attacks.

That’s it, Devs! We hope that this comprehensive guide has provided you with the knowledge and skills to resolve the “server host key did not match the signature supplied” error effectively. Remember to always practice good server management and stay vigilant against potential security threats. Happy server managing!