Troubleshooting Docker Startup Hangs Caused By Firewalld Reloads

by StackCamp Team 65 views

Introduction

Hey guys! Ever faced the frustrating issue where Docker just hangs on startup? It's like you're ready to deploy your containers, but Docker decides to take a coffee break without telling you. This article dives deep into a specific scenario causing this issue: Docker hangs due to Firewalld reloads. We'll break down the problem, explore the technical details, and provide insights on how to troubleshoot it. So, if you've been scratching your head over this, you're in the right place!

Understanding the Problem

The core issue revolves around Docker's interaction with Firewalld, the dynamic firewall manager prevalent in many Linux distributions. Firewalld is essential for managing network traffic, but sometimes, its reloads can interfere with Docker's operations, especially during the startup phase. Imagine Docker, eagerly setting up networks and containers, suddenly getting interrupted by Firewalld doing its own thing. This interruption can lead to a deadlock, causing Docker to hang indefinitely. The problem seems to surface particularly when Firewalld reloads its rules while Docker is in the middle of creating networks. This timing-sensitive situation isn't always easy to reproduce, making it a tricky bug to nail down. This usually happens with Docker versions 28.3.0 and may not occur on older versions like v25.0.3. Let's get into the specifics and see how to tackle this beast.

Reproducing the Issue

To truly understand and fix this Docker hang issue, it's essential to replicate it. Here’s how you can try reproducing the problem:

  1. Start Docker: Initiate Docker, either through systemd or manually. This sets the stage for the potential conflict.
  2. Trigger a Firewalld Reload: This is the crucial step. You need to make a Firewalld reload signal occur at a specific moment during Docker's startup. This can be a bit like trying to catch lightning in a bottle, but timing is everything. One way to achieve this is by manually triggering a Firewalld reload using the firewall-cmd --reload command while Docker is starting up.

This process aims to simulate the scenario where Firewalld interrupts Docker during a critical network setup phase. If you can consistently reproduce this, you're one step closer to finding a solution. Keep in mind that the timing has to be precise, which makes it a bit challenging. If you manage to get Docker to hang, you’ll know you’re on the right track. Now, let's dive into what the expected behavior should be and what happens when things go south.

Expected Behavior vs. Reality

Ideally, a Firewalld reload should not cause Docker to deadlock. Docker should be resilient enough to handle these signals without getting stuck. The expected behavior is that Firewalld reloads its rules in the background, and Docker continues its startup process smoothly. However, in reality, a Firewalld signal at the wrong moment can bring Docker to a standstill, which is far from ideal. This unexpected Docker hang disrupts workflows, delays deployments, and generally makes life difficult. It's like trying to build a house during an earthquake – not fun! Understanding the discrepancy between the expected and actual behavior is crucial for diagnosing and fixing the issue. So, what's the next step? Let's take a look at the technical details, including Docker versions, configurations, and logs, to get a clearer picture.

Docker Version and Environment Details

When troubleshooting, the Docker version and environment details provide crucial context. In this case, the issue was observed on Docker version 28.3.0, with the API version being 1.51 and Go version go1.24.4. The Git commit hash is 7cbee73f19, and the build date is Wed Jun 25 15:21:12 2025. The OS/Arch is linux/arm64. This information helps pinpoint the specific version where the problem arises, which is essential for developers to replicate and fix the bug.

The server-side details are equally important. The Engine version is 28.3.0, with API version 1.51 (minimum version 1.24). The Go version matches the client, and the Git commit is e0183475e03cd05b6a560d8b22fe0a83cd1cba14, built on the same date. The OS/Arch is linux/arm64, matching the client. Experimental features are disabled. This uniformity between client and server versions is vital for consistent behavior, but even with this consistency, the hang issue persists. Next, let's examine the Docker info to understand the broader environment.

Docker Info: A Deeper Dive

The docker info command provides a comprehensive snapshot of the Docker environment. Key details include:

  • Containers: The system has 5 containers, with 1 running and 4 stopped. This indicates an active Docker environment, making the hang issue even more disruptive.
  • Images: There are 3 images, suggesting a typical development or production setup.
  • Server Version: As mentioned earlier, the server version is 28.3.0.
  • Storage Driver: The storage driver is overlay2, which is a common choice for its performance and efficiency. It uses extfs as the backing filesystem and supports d_type, with metacopy disabled and native overlay diff enabled.
  • Logging Driver: The logging driver is local, which is suitable for basic logging needs.
  • Cgroup Driver: The cgroup driver is cgroupfs, and the Cgroup Version is 1. Cgroups are crucial for resource management in containers.
  • Plugins: The system supports various volume, network, and log plugins, showing a versatile Docker setup.
  • Swarm: Swarm is inactive, meaning the system isn't running in a Swarm cluster mode.
  • Runtimes: The runtimes include io.containerd.runc.v2 and runc, with the default runtime being runc.
  • Security Options: Security options include seccomp with the builtin profile, enhancing container isolation.
  • Kernel Version: The kernel version is 5.10, indicating a modern Linux kernel.
  • Operating System: The OS is Linux, with OSType as linux.
  • Architecture: The architecture is aarch64, which is an ARM64 architecture.
  • CPUs: The system has 4 CPUs.
  • Total Memory: The total memory is 3.808GiB.
  • Insecure Registries: There are insecure registries configured, which might be used for local development but should be carefully managed in production.
  • Live Restore Enabled: Live restore is disabled.

This detailed info paints a picture of a standard Docker environment, but the hang issue suggests there's a deeper problem lurking. Let's move on to the clues hidden in the logs.

Log Analysis: Deciphering the Clues

Log files are like the detective's notes in a mystery novel – they hold vital clues. Examining the log output when Docker hangs can reveal the sequence of events leading up to the deadlock. The provided logs show a typical startup sequence:

  • Docker starts up, as indicated by the Starting Docker Application Container Engine... message.
  • Docker initializes, with messages like Starting up, OTEL tracing is not configured, and CDI directory does not exist appearing.
  • A containerd client is created, and the storage driver (overlay2) is initialized.
  • Containers are loaded, and the Firewalld: docker zone already exists, returning message appears, suggesting Docker is interacting with Firewalld.
  • Docker creates a docker-forwarding policy.

Then, things start to get interesting:

  • There are warnings about ip6tables setup failing, which might indicate networking issues.
  • xtables contention detected messages suggest conflicts while manipulating iptables rules.
  • Docker reports issues with sandbox IDs not being found, which could be related to container networking.
  • More xtables contention detected messages appear, reinforcing the idea of networking conflicts.
  • Finally, there are ignoring event messages, which could be Docker trying to recover from the issues.

The logs clearly show Docker grappling with networking, particularly Firewalld and iptables. The contention and errors suggest that the Firewalld reloads are indeed interfering with Docker's network setup, leading to the hang. The next step is to look at the stack traces for a more granular view of what's happening internally.

Stack Traces: A Granular View

Stack traces provide a snapshot of what the Docker daemon is doing at a given moment. They're like looking under the hood of a car while the engine is running. The provided goroutine stacks show the internal operations of Docker when it hangs. Analyzing these stack traces can pinpoint the exact functions and processes that are stuck.

Without diving into the nitty-gritty of Go code, we can look for patterns and clues. For instance, if multiple goroutines are waiting on the same resource or lock, it suggests a deadlock. If certain functions related to networking or Firewalld appear repeatedly in the stack traces, it reinforces the idea that these are the areas causing trouble. The stack traces, combined with the log analysis, paint a detailed picture of Docker getting stuck during network operations due to Firewalld interference.

Potential Solutions and Workarounds

So, we've identified the problem: Docker hangs on startup due to Firewalld reloads. What can we do about it? Here are some potential solutions and workarounds:

  1. Delay Firewalld Reloads: The most direct approach is to avoid Firewalld reloads during Docker startup. This might involve scheduling reloads at off-peak times or implementing a mechanism to prevent reloads while Docker is starting.
  2. Configure Firewalld Policies: Fine-tuning Firewalld policies can help minimize conflicts. Ensuring that Firewalld rules don't interfere with Docker's network configurations can prevent deadlocks.
  3. Update Docker: Since this issue was observed in version 28.3.0, checking for newer versions or patches might provide a fix. Sometimes, bug fixes are released to address specific issues like this.
  4. Downgrade Docker: If updating isn't an option, downgrading to a stable version (like v25.0.3, where this issue wasn't observed) could be a temporary workaround.
  5. Adjust Docker Startup: Modifying Docker's startup process to be more resilient to Firewalld reloads could help. This might involve adding retries or handling Firewalld signals more gracefully.
  6. Investigate iptables: Since iptables contention is part of the problem, ensuring iptables is correctly configured and doesn't conflict with Firewalld is crucial.

These solutions range from simple workarounds to more complex configurations. The best approach depends on your specific environment and constraints. Let’s wrap up with a summary of key takeaways and future steps.

Conclusion: Key Takeaways and Future Steps

Alright guys, we've journeyed through the murky waters of Docker hangs caused by Firewalld reloads. We've seen how a seemingly simple firewall operation can bring a container deployment to its knees. The key takeaways are:

  • The Problem: Docker can hang on startup due to Firewalld reloads interfering with network setup.
  • The Culprit: Timing-sensitive conflicts between Firewalld and Docker's network operations.
  • The Clues: Log messages about xtables contention and stack traces showing deadlocks related to networking.
  • The Solutions: Delaying Firewalld reloads, configuring policies, updating/downgrading Docker, adjusting startup processes, and investigating iptables.

Where do we go from here? If you're facing this issue, try implementing the solutions discussed. Keep an eye on Docker's release notes for potential fixes. And if you're feeling adventurous, dive deeper into the stack traces and code to understand the root cause. Troubleshooting Docker hangs can be a challenge, but with the right approach, you can get your containers running smoothly again. Happy Dockering!