I picked up this snippet when netsurfing.

Establish_server_using_Lwt.ml

let () =
  let sockaddr = Unix.ADDR_INET (Unix.inet_addr_of_string "0.0.0.0", 3333) in
  let _server =
    Lwt_io.establish_server sockaddr (fun (in_, out) ->
      Lwt.catch (fun () ->
        in_
        |> Lwt_io.read_lines
        |> Lwt_stream.iter_s (fun line -> Lwt_io.write_line out line)
      ) (fun _exn -> Lwt.return_unit (* callback shouldn't raise *))
      |> Lwt.ignore_result
    ) in
  let (t, _) = Lwt.wait () in
  Lwt_main.run t