pdns-slave/conf/sql.json.conf

{
  "master-server":    "ns.example.com",
  "master-user":      "master-user",
  "master-password":  "master-password",
  "master-database":  "powerdns",

  "slave-server":     "127.0.0.1",
  "slave-user":       "slave-user",
  "slave-password":   "slave-password",
  "slave-database":   "powerdns"
}