PiHole

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] 
 
 Initiate Wizard: Run gravity-sync config . 
 Define Remote Host: Enter the IP of the Primary Server (192.168.x.x 
 Authenticate: Enter the Primary Server's SSH username and password to register RSA keys. 
 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.