Fix: Regenerated Host Keys In Docker Containers

Nov 26, 2025 by Alex Johnson 48 views

Have you ever faced the frustrating issue of regenerated host keys when your Docker container is recreated? It's a common problem when dealing with SSH in containerized environments, and it can lead to Jenkins master needing to re-trust the new host key every time. Let's dive into why this happens and how to tackle it effectively.

Understanding the Issue

When working with Docker containers and SSH, the host keys play a crucial role in establishing secure connections. These keys are generated using the ssh-keygen -A command, often within the container's setup process, such as in a script named setup-httpd. The problem arises because these host keys reside inside the container's file system. Consequently, every time the container is recreated, new host keys are generated, making the Jenkins master (or any other SSH client) unable to recognize the host, leading to trust issues.

To elaborate further, the ephemeral nature of Docker containers is both a strength and a challenge in this context. Containers are designed to be easily spun up and torn down, which is fantastic for scalability and resource management. However, this means that any data not persisted outside the container, like our SSH host keys, will be lost upon recreation. This leads to the dreaded “host key verification failed” errors, disrupting automated processes and requiring manual intervention to re-establish trust.

Imagine a scenario where you have an automated deployment pipeline. Your Jenkins master uses SSH to connect to a containerized environment to deploy your application. Each time the container restarts, the SSH fingerprint changes, and Jenkins refuses to connect because it sees a different host. This breaks the automation, and you need to manually update the known_hosts file or disable host key checking (which is a major security risk). This is not only inconvenient but also a significant impediment to continuous integration and continuous deployment (CI/CD) practices.

Furthermore, this issue isn't limited to Jenkins. Any system relying on SSH to connect to these containers will face the same problem. Whether it's Ansible, Chef, Puppet, or even manual SSH connections, the underlying issue remains the same: the host keys are not persistent. This can lead to a lot of wasted time and effort trying to troubleshoot connection issues that ultimately stem from this simple problem. Therefore, a robust solution is needed to ensure that host keys persist across container recreations.

The Workaround: A Temporary Fix

One common workaround involves a series of steps within the container's entry point script. This approach aims to persist the host keys by leveraging a mounted volume, which is a directory on the host machine that is shared with the container. Here’s a breakdown of the steps:

Copy host keys from the mounted volume to /etc/ssh/: This step checks if host keys already exist in the mounted volume from a previous container instance. If they do, they are copied into the container's SSH configuration directory.
Run ssh-keygen -A to generate host keys: If no keys are found in the volume, or if this is the first time the container is run, this command generates new SSH host keys.
Copy host keys from /etc/ssh/ to the mounted volume: This ensures that the newly generated or retrieved host keys are saved back to the mounted volume for future use.
Run setup-sshd: This is a placeholder for any additional SSH setup steps your container might require, such as starting the SSH daemon.

While this workaround addresses the immediate problem, it's not an ideal long-term solution. It introduces complexity into the container's entry point script and adds extra steps to the startup process. More importantly, it doesn't address the underlying issue of ephemeral storage. The container still relies on the mounted volume, and if that volume is lost or corrupted, you're back to square one. Furthermore, this approach can be a bit clunky and can slow down the container startup time, especially if the volume is on a network drive with slow access times.

Additionally, the process of copying files back and forth between the container's file system and the mounted volume adds a layer of complexity that can be prone to errors. For instance, file permissions might not be correctly preserved during the copy, leading to SSH failing to start. Debugging such issues can be time-consuming and frustrating. Therefore, while this workaround is better than nothing, it’s essential to explore more robust and maintainable solutions.

A More Robust Solution: Persistent Volumes

A more robust solution involves leveraging persistent volumes or named volumes in Docker. Unlike mounted volumes, which are tied to a specific host directory, persistent volumes are managed by Docker and can be more easily shared across multiple containers and hosts. This ensures that the host keys persist even if the container is recreated on a different host.

Persistent volumes offer several advantages over the workaround mentioned earlier. Firstly, they simplify the container's entry point script by eliminating the need for manual copying of keys. Docker handles the mounting and unmounting of the volume, so your container can focus on its primary function. Secondly, persistent volumes provide better data durability. Since they are managed by Docker, they are less likely to be accidentally deleted or corrupted compared to host-mounted volumes. This is crucial for ensuring that your SSH keys remain available and consistent over time.

To implement this, you would create a Docker volume specifically for storing SSH keys. This can be done using the docker volume create command. Then, in your Docker Compose file or Docker run command, you would mount this volume to the /etc/ssh/ directory inside the container. The first time the container runs, it will generate the host keys and store them in the volume. Subsequent container recreations will automatically use the existing keys from the volume, eliminating the need for manual intervention.

Using persistent volumes also improves the overall security posture of your containerized environment. By ensuring that SSH keys are consistently used, you reduce the risk of man-in-the-middle attacks. When a client connects to a server with a known host key, it can be confident that it is communicating with the intended server and not an imposter. This is particularly important in production environments where security is paramount.

Step-by-Step Guide to Using Persistent Volumes

Let’s walk through the steps to implement this using persistent volumes:

Create a Docker volume:
```
docker volume create ssh_keys
```
This command creates a new Docker volume named ssh_keys.
Modify your Docker Compose file (or Docker run command): If you’re using Docker Compose, add a volumes section to your service definition:
```
version: "3.8"
services:
  your_service:
    image: your_image
    volumes:
      - ssh_keys:/etc/ssh
volumes:
  ssh_keys:
```
If you’re using docker run, use the -v flag:
```
docker run -d -v ssh_keys:/etc/ssh your_image
```
This mounts the ssh_keys volume to the /etc/ssh directory inside the container.
Ensure host keys are generated on the first run: In your container's entry point script, add the following logic:
```
if [ ! -f /etc/ssh/ssh_host_rsa_key ]; then
  ssh-keygen -A
fi
# Your other setup commands here
```
This checks if the host keys exist. If they don’t, it generates them. This ensures that keys are only generated once, on the first container run.

By following these steps, you can ensure that your SSH host keys persist across container recreations, providing a more stable and secure environment.

Security Considerations

While persistent volumes solve the problem of host key regeneration, it’s crucial to consider security implications. Storing SSH keys in a persistent volume means that they are accessible to any container that mounts the volume. Therefore, you must ensure that only authorized containers have access to this volume.

One way to enhance security is to use Docker secrets to manage the SSH keys. Docker secrets allow you to securely store sensitive information, such as SSH keys, and make them available to containers at runtime. This prevents the keys from being stored in plain text in the container image or on the host file system.

Another important consideration is the permissions of the files within the persistent volume. You should ensure that the SSH key files have the correct permissions to prevent unauthorized access. Typically, the host keys should be readable only by the SSH daemon user.

Additionally, it’s a good practice to regularly rotate your SSH keys. This involves generating new keys and revoking the old ones. Key rotation reduces the risk of compromised keys being used for malicious purposes. While this adds complexity to your setup, it significantly improves the overall security of your system.

Conclusion

Dealing with regenerated host keys in Docker containers can be a headache, but with the right approach, it's a manageable problem. While workarounds exist, leveraging persistent volumes provides a more robust and maintainable solution. By ensuring that your SSH host keys persist across container recreations, you can build a more stable and secure containerized environment.

Remember to prioritize security by using Docker secrets, setting appropriate file permissions, and regularly rotating your keys. With these measures in place, you can confidently use SSH in your containers without worrying about the dreaded “host key verification failed” errors.

For further reading on Docker volumes and best practices, check out the official Docker documentation on Manage data in Docker. 💻