mirror

mirror sends a copy of every datagram to shadow servers and continues the chain. Replies of the shadow servers are discarded, so they never reach the client.

server:
  udp:
    middlewares:
      shadow:
        mirror:
          servers:
            - address: 10.0.0.50:53
              percent: 10
      backend:
        load_balancer:
          servers:
            - address: 10.0.0.1:53
    routers:
      dns:
        entrypoints: [dns]
        middlewares: [shadow, backend]

Field	Default	Description
servers[].address		Shadow server host:port.
servers[].percent	100	Percent of datagrams to mirror.
network	udp	udp, udp4 or udp6.

Place mirror before the terminal middleware. Mirroring is best effort; send errors are only logged at debug level.