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Fix SSH “No Matching Host Key Type Found” Error Easily

Posted on by William Zheng

If you’ve tried connecting to a server using SSH and suddenly see the dreaded “no matching host key type found” message, you’re not alone. This error tends to pop up when new security settings or server updates disrupt your usual workflow, leaving you locked out and frustrated.

Understanding why this happens is vital for anyone who relies on secure connections. In this article, we’ll explain what causes this error and guide you through straightforward steps to fix it—so you can get back to work quickly and securely.

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Understanding the “No Matching Host Key Type Found” SSH Error

When you try to connect to a server using SSH, encountering the error “No matching host key type found. Their offer: ssh-rsa (or ssh-dss)” can be frustrating and confusing, especially if SSH connections have always just worked in the past. This article explains what this error means, why it happens, and how you can quickly and safely resolve it.

What Does “No Matching Host Key Type Found” Mean?

SSH (Secure Shell) relies on cryptographic keys to authenticate servers and clients securely. When you connect to a server, your SSH client must agree on a host key type with the server. If the server only offers key types that your client does not support (or has disabled), you’ll see this error.

In simple terms, your SSH tool and the server can’t agree on a common “language” (key type) to trust each other. For example:
– “Their offer: ssh-dss” means the server only supports SSH-DSS (DSA) keys.
– “Their offer: ssh-rsa” means it only supports SSH-RSA keys.


Fixing SSH

Why Does This Error Occur?

Over time, the security standards for cryptography have changed. Older host key algorithms like ssh-dss (DSA) and old variants of ssh-rsa are now considered weak and have been disabled in many SSH clients for security reasons.

Some recent updates in popular SSH clients (such as OpenSSH version 8.8 and later) aggressively remove support for these weaker key types. When you attempt to connect to an older server that hasn’t been updated or reconfigured, you’re likely to run into this error.

Main causes include:
– The SSH server is using outdated or weak key types (like DSA or SHA-1-based RSA).
– The SSH client no longer accepts these key types for security.
– Security policies updated on either side blocking older algorithms.

How to Fix “No Matching Host Key Type Found” Error

1. Identify the Offered Key Type and Algorithm

First, read the error message carefully to see which key type the server offers. Common types mentioned are ssh-dss and ssh-rsa. This information tells you what the server supports — and what your client is refusing.

2. Decide on an Approach

You have two broad choices:
– Enable support for the legacy algorithm on your client side (temporarily or permanently).
– Update the server to use modern, secure key types.

A) Enable the Key Type on Your SSH Client (Quick Fix)

You can instruct your SSH client to allow use of older key algorithms, usually with a -o (option) flag.

Example: Enabling SSH-RSA for a Specific Connection

ssh -o HostKeyAlgorithms=+ssh-rsa user@host

Example: Enabling SSH-DSS (DSA keys)

ssh -o HostKeyAlgorithms=+ssh-dss user@host

Use Case: This is helpful as a temporary workaround, especially if you’re unable to update the server immediately.

Security warning: This makes your client accept less secure keys, increasing the risk of compromise. Use only with trusted hosts.

B) Update Your SSH Client Configuration (Permanent on Specific Host)

If you frequently connect to this server, you can add settings to your ~/.ssh/config file:

Host example-host
    HostName server_address
    User your_username
    HostKeyAlgorithms +ssh-rsa
    PubkeyAcceptedAlgorithms +ssh-rsa

Replace example-host, server_address, and your_username as appropriate.

C) Update the SSH Server (Recommended, Permanent Fix)

The best solution is to update or reconfigure the server to use secure, modern host key algorithms:

  1. Log in directly (if still possible) or via local console.
  2. List current host keys.
    Check /etc/ssh/ for files like ssh_host_ecdsa_key (ECDSA), ssh_host_ed25519_key (ED25519), and ssh_host_rsa_key.
  3. Regenerate Host Keys (if needed).
  4. To add a newer ED25519 key:
    bash
    sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
  5. To add a stronger RSA key:
    bash
    sudo ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
  6. Edit sshd_config to Offer Modern Algorithms.
  7. Open /etc/ssh/sshd_config and ensure it includes lines like:
    HostKey /etc/ssh/ssh_host_ed25519_key
    HostKey /etc/ssh/ssh_host_rsa_key
    Consider commenting out DSA keys.

  8. Restart the SSH Service.
    bash
    sudo systemctl restart sshd
    (On some systems, the service name may be sshd.)

  9. Test your SSH connection.

Advantages:
– Future-proofs your server.
– Maintains strong security practices.

Challenges:
– Requires server access and administrative privileges.
– May require coordination if multiple users or automation tools connect.

Practical Tips and Best Practices

  • Prioritize Security:
    Always prefer updating the server to use strong and supported host key algorithms. Older keys (DSA, MD5-based RSA) are deprecated for good reasons.

  • Use Per-Host Workarounds, Not Global:
    Avoid setting insecure algorithms globally in your SSH client (/etc/ssh/ssh_config). Instead, use the ~/.ssh/config file for specific hosts.

  • Stay Updated:
    Update both SSH servers and clients regularly. This ensures compatibility and security.

  • Test Changes First:
    Before applying permanent changes, especially on production servers, test in a controlled environment.

  • Backup Host Keys:
    Before making changes to host keys, back up existing key files.

  • Inform Users:
    If you’re an admin, notify users before making changes that could impact their SSH access.

Additional Aspects and Common Scenarios

Connecting with Automation Tools (e.g., Git)

When using Git (over SSH) or automated tools, you may also encounter this error:
– Often, the underlying cause and solution are the same: your SSH client has disabled the offered key type.
– Update .ssh/config accordingly for the automation user, or reconfigure the server if possible.

Legacy Devices (e.g., Network Appliances)

Many network devices (firewalls, routers, old NAS units) still use older SSH software that only offers insecure key types.
– Some devices can’t be updated. In these cases, limit access, segregate networks, and consider using the SSH client-side workarounds only from dedicated, well-controlled systems.

Windows SSH Clients

Modern Windows 10/11 include OpenSSH, subject to the same security rules. The same options and fixes apply.

Cost Considerations (If Shipping/Remote Access Is Involved)

While fixing this error generally doesn’t have a direct “shipping” cost, there can be indirect expenses:

  • Device Replacement:
    If a device is too old to update or reconfigure, you might need to replace it.

  • Downtime:
    Updating SSH servers might temporarily prevent remote access; schedule maintenance windows accordingly.

  • Consulting or Support Costs:
    You may incur costs if you need external help managing legacy systems.

  • Risk Costs:
    Accepting weaker SSH algorithms for remote access can increase the risk and potential future costs due to security incidents. Always weigh quick fixes against your security policies.

Summary

The “No matching host key type found” SSH error is a result of your SSH client refusing to use insecure or outdated server key types. While you can often connect using a workaround option, the most secure fix is to update your SSH server to support modern algorithms like ED25519 or new RSA keys. Always aim for the most secure, future-proof approach, and use temporary fixes with caution and understanding of the risks.

Frequently Asked Questions (FAQs)

1. What is a host key type in SSH?

A host key type refers to the cryptographic algorithm used by an SSH server for identifying itself securely to clients (like RSA, DSA, ECDSA, or ED25519). Clients and servers must agree on an algorithm for a secure connection.

2. Is it safe to enable old host key types like ssh-rsa or ssh-dss?

Allowing old algorithms (like DSA or old RSA) reduces the security of your SSH connections. It’s okay as a temporary workaround—especially with trusted, isolated systems—but always switch back to secure defaults and update the server as soon as possible.

3. How do I know which key types my SSH client supports?

You can check supported key types by viewing your SSH client manual (man ssh_config) or with command-line tools. Newer clients support ED25519 and newer RSA variants but may disable old types for security.

4. I can’t update the server. What’s the best workaround?

If server updates aren’t possible, use SSH client options (-o HostKeyAlgorithms=+ssh-rsa or similar) for only the affected host. Never make global changes—apply workarounds only where strictly necessary, and minimize exposure.

5. After updating keys or settings, my connection is still failing. What should I do?

Ensure you’ve restarted the SSH service after making changes to keys or configuration files. Double-check file permissions, your SSH client’s settings, and verify you’re connecting to the correct IP or hostname. If problems persist, consult server logs (/var/log/auth.log or equivalent) for more details.

This comprehensive guide should help you not only understand the meaning of the SSH error but also take confident and secure steps to resolve it, now and in the future.

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