Home Assistant kept wasting power on my home lab, and this tiny change fixed it

At a glance:

• Home Assistant's built-in DNS tool caused CPU spikes and noise in the author's home lab setup.
• The issue stems from a loop in the hassio_dns plugin when it can't connect to Cloudflare's DNS servers.
• The fix is a simple command: ha dns options --fallback=false to disable problematic fallback requests.

The Problem with Home Assistant

The author, João Carrasqueira, has relied on Home Assistant for two years as a core component of their smart home. Recently, their home lab—hosted on a mini PC running Proxmox—started exhibiting erratic behavior. The fan would constantly spin up and down, indicating sudden bursts of activity despite no apparent workload. Initially, the author suspected their TrueNAS VM, which handles heavier workloads, but monitoring revealed no anomalies there. Upon closer inspection of Proxmox, Home Assistant's CPU usage was found to spike to around 40% repeatedly, even though the VM was resource-constrained to just 2 of the host's 22 CPU cores and 2GB of RAM.

This unexpected behavior was particularly puzzling because Home Assistant is designed to be lightweight. The author had never encountered such issues in the past year of running it in a Proxmox VM. The sporadic CPU spikes suggested a background process was malfunctioning, prompting an investigation into integrations and system logs. After ruling out common culprits, the author turned to online forums and discovered a recurring but unresolved issue tied to Home Assistant's DNS configuration.

The Root Cause: DNS Loop

The problem lies in Home Assistant's hassio_dns plugin, a CoreDNS integration that ensures reliable DNS resolution by using Cloudflare's servers as a fallback. When a DNS query fails to return a response, the plugin enters a loop, repeatedly attempting to connect to Cloudflare's DNS servers. This loop generates constant CPU activity, especially if the network blocks these requests. The author's setup included AdGuard Home on their router, which wasn't configured to override client DNS settings. This meant Home Assistant should still have reached Cloudflare, yet the issue emerged recently.

According to a 2022 Home Assistant forum post, this DNS loop has been documented for years. The plugin's behavior conflicts with custom DNS environments, leading to resource exhaustion. The author's network configuration hadn't changed, but the issue surfaced abruptly—possibly due to a botched Home Assistant update or an undocumented change in Cloudflare's response handling. Despite its long history, the problem remains unfixed in the software, leaving users to discover workarounds independently.

The Fix and Why It Worked

The solution, discovered via Reddit, involves disabling the fallback mechanism via SSH. The command ha dns options --fallback=false stops CoreDNS from making incessant requests to Cloudflare. After executing this command and restarting the Home Assistant VM, the fan noise ceased immediately, and CPU usage stabilized. This fix eliminates the loop by preventing fallback attempts, forcing Home Assistant to rely solely on the network's primary DNS server.

The author remains uncertain why the issue manifested suddenly, as their network setup had been stable for months. AdGuard Home was enabled months prior, but the problem began only recently. This suggests a possible interaction with a Home Assistant update or an external factor like a transient network glitch. The simplicity of the fix highlights how a minor configuration oversight can cause significant operational disruptions, underscoring the importance of monitoring even in resource-efficient systems.

Preventing Future Issues

To maintain a quiet and energy-efficient home lab, the author recommends proactive monitoring and resource management. In their case, removing unused voice-related integrations reduced background processes. Both Proxmox and TrueNAS allow capping VM/container resources—limiting Home Assistant to 2 cores prevented the issue from escalating. Additionally, enabling power-saving features in Proxmox, such as setting the CPU governor to 'powersave' and enabling BIOS-level C-States and Eco mode, minimizes idle power draw.

Regular audits of integrations and DNS configurations are crucial. Home Assistant users should verify that their network's DNS settings align with the platform's expectations, especially when using custom DNS services like AdGuard Home. Proxmox's monitoring tools can pinpoint resource-hungry containers, while TrueNAS offers similar oversight for its services. By combining these practices, users can avoid unexpected noise and power waste, ensuring their home labs remain efficient and reliable.