"High Availability DNS Failover for Pi-hole / AdGuard using DNSdist

dnsdist.conf

-- Bind DNS on all available interfaces on port 53
setLocal('0.0.0.0:53', {})

-- Set Access Control List to allow all IPv4 and IPv6 addresses
setACL({'0.0.0.0/0', '::/0'})

-- Bind the web server on port 8083 on all available interfaces
webserver("0.0.0.0:8083")
-- Set the web server configuration
setWebserverConfig({
    acl = "0.0.0.0/0",              -- Allow all IP addresses
    dashboardRequiresAuthentication = false  -- Disable authentication requirement for dashboard
})

-- Disable periodic security polling by setting an empty suffix
setSecurityPollSuffix("")

-- Admin control interface is currently commented out
-- controlSocket('0.0.0.0:52')

-- Define the primary and failover backend DNS servers
primaryBackend = newServer({
    address = "192.168.3.1:53",
    checkInterval = 5,   -- Frequency to check server status (in seconds)
    timeout = 1,         -- Timeout for server responses (in seconds)
    pool = "primary"     -- Identifier for the primary backend pool
})

failoverBackend = newServer({
    address = "172.0.0.2:53",
    checkInterval = 5,   -- Frequency to check server status (in seconds)
    timeout = 1,         -- Timeout for server responses (in seconds)
    pool = "failover"    -- Identifier for the failover backend pool
})

-- Function to select the backend pool based on the primary server's health.
-- Alternative names are: selectBackendPool, chooseDnsBackend, determineBackend, routeDnsRequest.
function selectBackendPool(dq)
    -- Check if the primary backend is operational
    if primaryBackend:isUp() then
        -- Log diagnostic info (if required)
        -- infolog("primary is up")
        -- Return the action to use the primary backend pool
        return DNSAction.Pool, "primary"
    else
        -- Log diagnostic info (if required)
        -- infolog("primary is down")
        -- Return the action to use the failover backend pool
        return DNSAction.Pool, "failover"
    end
end

-- Add an action rule to use our backend selection function for all DNS queries.
addAction(AllRule(), LuaAction(selectBackendPool))