High-Availability Pi-hole Synchronization & Docker Networking

Last Updated: March 2026 | Target Hardware: Micro PC Cluster (Intel i5 7th-gen)

Overview: This article documents the validated procedure for deploying a high-availability DNS sinkhole architecture using two micro PCs running CasaOS. [cite: 1, 3, 7] By leveraging Gravity Sync, the Secondary Pi-hole (192.168.x.x) mirrors the DNS payload of the Primary Pi-hole (192.168.x.x).

Prerequisites

• Primary Server (LinuxServer): 192.168.x.x
• Secondary Server (LinuxServer2): 192.168.x.x
• Pi-hole installed via Docker (CasaOS) on both nodes.
• Active SSH credentials for both servers.

Step 1: Install Gravity Sync on Both Servers

The synchronization tool must be installed on the host OS of both micro PCs. [cite: 16] To bypass retired domain links, execute the following command on both nodes:

curl -sSL https://raw.githubusercontent.com/vmstan/gs-install/main/gs-install.sh | bash

Step 2: Configure the Secondary Node

Because the Secondary Server "pulls" data from the Primary, execute the configuration wizard from LinuxServer2 (192.168.0.152): [cite: 21]

1. Initiate Wizard: Run gravity-sync config.
2. Define Remote Host: Enter the IP of the Primary Server (192.168.x.x
3. Authenticate: Enter the Primary Server's SSH username and password to register RSA keys.
4. Define Container Names: Manually confirm the container names  pihole for both Local and Remote prompts.

Step 3: Perform Initial Manual Sync

On the Secondary Server (192.168.x.x), run the following to establish the baseline mirror: 

gravity-sync pull

Step 4: Automate the Synchronization

To ensure future changes to blocklists or local DNS are synced automatically, enable the background service on the Secondary Server:

gravity-sync auto

Note: This registers a systemd timer that compares database hashes every 5 minutes with zero noticeable overhead on NVMe drives. [cite: 38, 39]

Step 5: Resolve Docker Network Restrictions (Pi-hole v6 Fix)

By default, CasaOS Docker deployments may cause Pi-hole to drop incoming LAN requests. [cite: 42] You must manually configure the secondary node to trust LAN traffic:.

• Log into the Secondary Web UI: http://192.168.x.x:81/admin 
• Navigate to Settings > DNS.
• Toggle the top-right corner from Basic to Advanced.
• In Interface settings, select Permit all origins.
• Click Save & Apply.

Step 6: Validate Failover

Perform a direct query test from a Windows client on the same network: [cite: 51]

nslookup google.com 192.168.x.x

Expected Result: The terminal should return a list of IP addresses. [cite: 54] If it returns "Timeout," re-verify the settings in Step 5.