Hello, I'm trying to implement DNS resolver without any external C libraries for my daemon program which uses DNS lookup. I know the "hsdns" library exists. But since it calls GNU asynchronous DNS resolver library by FFI, all threads of Haskell are blocked on non-threaded RTS. If the threaded RTS is used, forkProcess kills the IO thread. With the threaded RTS of at least GHC 6.10.4, we cannot make our program as a daemon nor use pre-fork mechanism. Thus, I decided to implement DNS resolver only with non-blocking Haskell functions. I use a *connected* UDP socket like this: proto <- getProtocolNumber "udp" let hints = defaultHints { addrFlags = [AI_ADDRCONFIG, AI_NUMERICHOST, AI_PASSIVE] , addrSocketType = Datagram , addrProtocol = proto } a:_ <- getAddrInfo (Just hints) (Just "An IP address of DNS server") (Just "domain") sock <- socket (addrFamily a) (addrSocketType a) (addrProtocol a) connect sock (addrAddress a) h <- socketToHandle sock ReadWriteMode hSetBuffering h NoBuffering hSetBinaryMode h True If my understating is correct, I cannot use recvFrom since it calls recvfrom() with FFI. So, I use socketToHandle to translate a socket to a handle. Since the handle is associated with UDP, the entire data size should be known when a UDP packet arrived. But I cannot find a way to know data size. (I want a function like hFileSize.) Note that hGetContents blocks. Without data size in advance, a program to handle network protocols on UDP becomes quite dirty or sometime cannot be implemented. So, please tell me how to know data size associated with a handler of UDP? --Kazu