recvmsg.rb

require 'socket'

server = Socket.new(:INET, :DGRAM)
client = Socket.new(:INET, :DGRAM)

server.bind(Socket.sockaddr_in(0, '127.0.0.1'))

client.connect(Socket.sockaddr_in(server.connect_address.ip_port, '127.0.0.1'))

client.write('hello world how are you doing')

msg, addr, flags, *controls = server.recvmsg

found = Socket.constants.grep(/^MSG_/).map do |name|
  flags & Socket.const_get(name)
end

p flags # => 1073741824
p found # => [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]