How to resolve Ubuntu 20.04 Container Signature Errors on Raspberry Pi ARM Devices

When working with Docker containers on Raspberry Pi devices, you might encounter frustrating signature verification errors during routine operations like apt update.

These errors can bring your development workflow to a grinding halt, especially when building ARM-based images for IoT projects. After spending countless hours troubleshooting this issue across multiple Raspberry Pi boards, I’ve compiled a comprehensive guide to help you understand and resolve these persistent signature verification problems.

Understanding the Problem: Invalid Signatures in Ubuntu 20.04 ARM Containers

The signature verification error typically manifests when running apt update within an Ubuntu 20.04 container on ARM architecture. The error message looks something like this:

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W: GPG error: http://ports.ubuntu.com/ubuntu-ports focal InRelease: At least one invalid signature was encountered.
E: The repository 'http://ports.ubuntu.com/ubuntu-ports focal InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.

This error occurs specifically when:

• You’re using ARM-based hardware like Raspberry Pi 4B
• You’re running Ubuntu 20.04 (focal) Docker containers
• You attempt to update package repositories inside the container

Read: How to install and setup Docker on Ubuntu 22.04

Root Cause: The libseccomp Library Issue

After extensive investigation, I discovered that the root cause lies in the libseccomp library. This library handles system call filtering in Linux, which is crucial for container security. In older versions of this library, there’s a compatibility issue with newer Ubuntu releases on ARM architecture that prevents proper signature verification.

The problem specifically affects:

• Raspberry Pi OS (formerly Raspbian) Buster and similar Debian-based distributions
• Ubuntu 20.04 containers running on ARM32 or ARM64 architectures
• Docker deployments where the host system has an outdated libseccomp version

Solution 1: Upgrading libseccomp on the Host System

The most reliable solution is to upgrade the libseccomp library on the host system. This addresses the root cause without compromising container security.

Here’s how to implement this solution:

1. First, check your current libseccomp version:

dpkg -l | grep libseccomp

2. Download a newer version of libseccomp from Debian’s repositories. For Raspberry Pi (ARM32):

cd /tmp
wget http://ftp.us.debian.org/debian/pool/main/libs/libseccomp/libseccomp2_2.5.1-1_armhf.deb

For ARM64 systems:

cd /tmp
wget http://ftp.us.debian.org/debian/pool/main/libs/libseccomp/libseccomp2_2.5.1-1_arm64.deb

3. Install the new version:

sudo dpkg -i libseccomp2_2.5.1-1_armhf.deb  # or libseccomp2_2.5.1-1_arm64.deb for ARM64

4. No reboot is necessary! The change takes effect immediately, and you can now run Ubuntu 20.04 containers without signature verification errors.

When I implemented this solution on my Raspberry Pi 4 cluster, I was able to immediately start working with Ubuntu 20.04 containers without any additional configuration.

Read: The Complete Ubuntu Container Guide: From Basics to Production for System Administrators

Solution 2: Using Security Options to Bypass Restrictions

If upgrading the host system isn’t feasible, you can use Docker’s security options to work around the issue. This approach is less secure but provides a quick fix when necessary.

Run your container with the --security-opt seccomp:unconfined flag:

docker run -it --security-opt seccomp:unconfined arm32v7/ubuntu:20.04

This disables the seccomp profile, allowing the container to bypass the signature verification issues. However, this solution comes with security implications since it removes an important container isolation mechanism.

I recommend using this approach only temporarily, as it reduces the security boundaries between the container and the host system.

Solution 3: Upgrading Docker to the Latest Version

Some users have reported success by upgrading Docker to version 19.03.12 or newer. This isn’t consistently effective, but it’s worth trying if other solutions aren’t viable.

To upgrade Docker on Raspberry Pi or other ARM devices:

# Remove existing Docker installations
sudo apt-get remove docker docker-engine docker.io containerd runc

# Install Docker using the convenience script
curl -fsSL https://get.docker.com -o get-docker.sh
sudo sh get-docker.sh

# Add your user to the docker group
sudo usermod -aG docker $USER

After running these commands, you’ll need to log out and back in for the group changes to take effect.

Read: How to Pass Environment Variables to Docker Containers

Solution 4: Debugging and Manually Fixing Repository Files

If you’re still encountering issues, you can dig deeper into the problem by running apt update with debug flags:

apt-get -oDebug::pkgAcquire::Worker=1 update

This will help identify which specific files have corrupted signatures. Once identified, you can manually remove the problematic files from /var/lib/apt/lists/ inside the container, allowing them to be freshly downloaded on the next update.

Prevention: Best Practices for ARM-based Docker Deployments

To avoid similar issues in future deployments:

1. Keep host systems updated: Regularly update your Raspberry Pi OS and system libraries to ensure compatibility with newer containers.
2. Use version pinning: Pin your Docker images to specific versions known to work with your system configuration.
3. Consider alternative base images: For some projects, Alpine Linux or Debian-based images might be more reliable on ARM devices than Ubuntu.
4. Create a testing pipeline: Implement a small test script that runs apt update to verify container functionality before deploying to production.

Verification: Testing Your Solution

After implementing any of the solutions above, verify that the issue is resolved by:

1. Starting a fresh Ubuntu 20.04 container:

docker run -it arm32v7/ubuntu:20.04

2. Running apt update inside the container:

apt update

3. Confirming that all repositories are properly signed and updated without errors.

When the solution is working correctly, you should see output indicating successful updates of all repositories without any signature verification errors.

FAQ: Common Questions About Ubuntu ARM Container Signature Issues

Why does this issue only affect Ubuntu 20.04 and not earlier versions?

Ubuntu 20.04 introduced changes to the repository security infrastructure that requires newer versions of libseccomp to properly handle signature verification on ARM architectures. Earlier versions like 18.04 use a different security model that doesn’t encounter the same issues.

Will upgrading libseccomp affect other containers or applications?

Upgrading libseccomp is generally safe and shouldn’t negatively impact other containers or applications. In fact, it often improves system security and compatibility across all containerized workloads.

Is there a permanent fix coming from Docker or Ubuntu?

Yes, newer versions of both Docker and Ubuntu are addressing these compatibility issues. However, many existing deployments, especially on Raspberry Pi OS, still require manual intervention to resolve the problem.

What if I’m using Docker Compose?

If you’re using Docker Compose, you can add security options to your service definitions:

services:
  my_ubuntu_service:
    image: arm32v7/ubuntu:20.04
    security_opt:
      - seccomp:unconfined
    # other configuration...

However, I still recommend upgrading libseccomp for a more secure solution.

Can I use a different container runtime like Podman to avoid this issue?

Different container runtimes might handle security differently, but they generally rely on the same underlying libraries. If the issue is with libseccomp on the host system, changing the container runtime is unlikely to completely resolve the problem.

By understanding the root cause and applying the appropriate solution, you can quickly overcome these signature verification errors and get back to productive work with Ubuntu 20.04 containers on your ARM devices. The libseccomp upgrade approach provides the most comprehensive and secure solution for most deployments.

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