server.rb

# https://en.wikipedia.org/wiki/List_of_IP_protocol_numbers
UDP_PROTOCOL = 0x11

loop do
  data = socket.recv(BUFFER_SIZE).bytes

  frame = EthernetFrame.new(data)

  next unless frame.ip_packet.protocol == UDP_PROTOCOL &&
    frame.ip_packet.udp_datagram.destination_port == 4321

  UDPSocket.new.send(
    frame.ip_packet.udp_datagram.body.upcase,
    0,
    frame.ip_packet.source_ip_address,
    frame.ip_packet.udp_datagram.source_port
  )
end