How to set up DNS failover in Pi-hole for high availability

Why local DNS needs redundancy in the homelab

Anyone managing a home lab or a robust home network knows the weight a centralized name server carries. When we use Pi-hole to block ads and organize internal hostnames, it becomes the brain of all web navigation. In practice, this means if the machine running it reboots or encounters issues, the entire internet seems to go down at home, even if the ISP connection is working perfectly. To avoid this kind of headache, creating a high-availability strategy with two instances working together shifts from a luxury to an operational necessity.

In a standard architecture, we typically point the Pi-hole address directly in the router settings or via DHCP (the protocol that assigns IP addresses to devices on the network). If that single server fails, devices lose track of where to send address translation queries, halting access to any website. Before moving forward, keeping network redundancy in mind shares similarities with what we explore when learning how to set up automatic WAN failover on OpenWrt, where traffic must find an alternative path without human intervention.

The ideal architecture with synchronized instances

The basic idea behind high availability is simple in theory but demands careful execution: we need at least two independent Pi-hole servers running on separate hardware, such as two different mini PCs or a primary server and a spare Raspberry Pi. If one stops responding, the second seamlessly takes over the workload. For this to happen without unpleasant surprises, both instances must maintain identical blocklists, local DNS records (the nicknames we give to our machines), and forwarding rules.

Keeping these data sets identical manually is an invitation to chaos, since any minor change would require double the work. This is where synchronization tools like Gravity-Sync come in, connecting both servers behind the scenes and keeping everything aligned automatically. In practice, you make a change on the primary server, and the tool replicates the configuration files to the secondary server within seconds, ensuring endpoint consistency.

Configuring dynamic routing and IP addresses

The major challenge in home DNS is that most consumer devices accept only two name server addresses (primary and secondary). If we put the IP of the first Pi-hole and the IP of the second Pi-hole, operating systems will alternate between them unpredictably. Some applications might send queries to the secondary server and end up receiving different responses if the servers are not perfectly synchronized at that exact moment.

To bypass this erratic behavior, an advanced engineering approach utilizes dynamic routing or shared virtual IP addresses (using protocols like VRRP, which allows two machines to intelligently share a single IP address). When the primary machine stops responding to network heartbeats, the secondary immediately takes over the virtual IP. Thus, to network devices, absolutely nothing has changed and the service continues flowing without noticeable interruptions.

Step-by-step guide to deploying the second instance

Deploying the second server requires prior planning of the network address and installed packages. The procedure below summarizes the initial preparation of the secondary machine before activating automated synchronization with the primary.

1. Install the base operating system (such as Debian or Raspberry Pi OS) on the secondary hardware and assign a static IP address on your local network.
2. Run the official Pi-hole installation script using the standard terminal command to ensure the exact same base version:

   curl -sSL https://install.pi-hole.net | bash

3. Configure the same upstream DNS forwarding options (such as Cloudflare or Google) that you use on your first instance, ensuring standardized responses.

After completing this initial step, both servers will be ready to receive the synchronization layer and begin operating together within your infrastructure. To complement this implementation in production environments, see our related technical analyses on Short-circuited motherboard? How to find the bad component and How to Inspect IoT MQTT Traffic with Wireshark.

Validation and operational best practices

With both instances running and synchronized, it is time to test system resilience before relaxing. Physically unplug your primary server and observe the behavior of computers and phones connected to the network. If the virtual IP or multi-server configuration is correct, browsing should experience no perceptible interruptions, and DNS queries will continue being resolved by the backup server transparently.

Monitor resource usage as well to ensure chosen hardware supports the load and avoids unexpected bottlenecks. A well-planned homelab infrastructure prevents mundane network hiccups from turning a relaxing evening into an endless troubleshooting session.