Understanding the “Couldn’t agree on host key algorithm” Error and How to Fix It

When you encounter the error “Couldn’t agree on host key algorithm” while trying to connect to a Linux machine using SSH, it signifies a compatibility issue between your SSH client and the server’s supported host key algorithms.

What are SSH Host Key Algorithms?

SSH (Secure Shell) is a cryptographic network protocol that enables secure communication between two systems. A critical part of this security involves verifying the identity of the server you’re connecting to. This verification happens through SSH host keys.

These host keys are like digital fingerprints unique to each server. When you connect for the first time, the SSH client receives the server’s host key and checks it against a cached or user-trusted key. If they match, the connection is considered secure. If not, it throws the “Couldn’t agree on host key algorithm” error.

Why Does This Error Occur?

This error typically arises due to a mismatch between the algorithms supported by your SSH client and the algorithms the server uses for generating and signing the host key.

There are two main reasons for this mismatch:

1. Outdated SSH Client: Older SSH clients might not support the latest host key algorithms, such as rsa-sha2-512, rsa-sha2-256, or ecdsa-sha2-nistp256, offered by the server.

2. Security Updates: Modern SSH servers are increasingly disabling support for older and less secure algorithms like sha-1 in favor of stronger algorithms like sha-2.

Fixing the “Couldn’t agree on host key algorithm” Error

The solution to this error is straightforward: ensure your SSH client is up-to-date with the latest algorithms. Here’s how you can achieve this:

1. Upgrade PuTTY (Common SSH Client):

   • If you’re using PuTTY, the popular SSH client for Windows, download and install the latest version from the official website: PuTTY Download: https://www.putty.org/
   • Running an outdated version of PuTTY is the most common cause for this error. Upgrading to the latest version ensures compatibility with the server’s host key algorithms.

2. Update mremoteng (Remote Desktop Management Software):

   • If you’re using a remote desktop management software like mremoteng that utilizes PuTTY for SSH connections, the issue might lie with an outdated PuTTY version embedded within mremoteng.
   • Check for updates to mremoteng itself. If an update is available, installing it might include an updated PuTTY version.
   • Alternatively, you can configure mremoteng to use the latest standalone PuTTY executable you downloaded from the official source.
   • Download putty software from putty.org website, navigate to mremoteng install location c:\program files(x86)\mremoteng folder. Rename existing puttyng.exe software to puttyng.exe_old and copy the latest putty.exe software to this location and rename it as puttyng.exe.
   • You should now be able to connect to your remote machine over SSH.

Additional Considerations

• By following these steps, you should be able to resolve the “Couldn’t agree on host key algorithm” error and establish a secure SSH connection to your Linux machine.
• Remember, it’s crucial to keep your SSH client software updated to benefit from the latest security improvements and ensure compatibility with modern servers.
• If you’re unsure about updating mremoteng or configuring it to use a specific PuTTY version, consult the mremoteng documentation or seek assistance from its support channels.