Can't read UDP reply (golang)

Question

I'm working on a Go program that sends out a UDP broadcast to query existence of devices on the local network and then reads the replies. Using Wireshark I confirm that the packet is broadcast and that the single device on (my) network replies (ten times, in fact) but my application blocks on the read as if it does not see the incoming packet. Here is the code:

func Discover(timeout int) ([]string, error) {
    inBuf := make([]byte, 1024)
    devices := make([]string, 0)
    var readLen int
    var fromAddr *net.UDPAddr

    // get server connection
    server := fmt.Sprintf("%s:%d", bcastIP, udpDiscoverPort) // "255.255.255.255", 10000
    serverAddr, err = net.ResolveUDPAddr("udp", server)
    checkErr(err)
    ourAddr, err = net.ResolveUDPAddr("udp", "192.168.1.132:10000")
    checkErr(err)
    conn, err = net.DialUDP("udp", ourAddr, serverAddr)
    checkErr(err)
    defer conn.Close()

    // send the Discover message
    discoverMsg := []byte(magic)
    discoverMsg = append(discoverMsg, discovery...)
    sendLen, err := conn.Write(discoverMsg)
    checkErr(err)
    fmt.Println("Sent", sendLen, "bytes")

    // read one reply
    readLen, fromAddr, err = conn.ReadFromUDP(inBuf)
    fmt.Println("Read ", readLen, "bytesfrom ", fromAddr)
    txtutil.Dump(string(inBuf[:readLen]))
    return devices, nil
}

checkErr(err) prints a diagnostic and exits if err is not nil, BTW.

The information in the replies looks like:

Internet Protocol Version 4, Src: 192.168.1.126 (192.168.1.126), Dst: 192.168.1.132 (192.168.1.132)
User Datagram Protocol, Src Port: ndmp (10000), Dst Port: ndmp (10000)

I have tried "0.0.0.0:10000", ":10000" and "127.0.0.1:10000" in place of "192.168.1.132:10000" and none seem to make any difference.

Any suggestions as to what I'm doing wrong are welcome!

Answer 1

You need to use ListenUDP instead of DialUDP. When you use DialUDP, it creates a "connected" UDP port, and only packets originating from the remote address are returned on read.

conn, err = net.ListenUDP("udp", ourAddr)

Since the connection doesn't have a default destination, you will also need to use WriteTo* methods to send packets:

sendLen, err := conn.WriteToUDP(discoverMsg, serverAddr)