Sending UDP packets with gopacket

Question

I am trying to send UDP packets to a remote host like this

conn, err := net.ListenPacket("ip4:udp", "0.0.0.0")
if err != nil {
    panic(err)
}
ip := &layers.IPv4{
    SrcIP:    saddr,
    DstIP:    dip,
    Protocol: layers.IPProtocolUDP,
}
udp := &layers.UDP{
    SrcPort: layers.UDPPort(sport),
    DstPort: layers.UDPPort(us.Port),
}
udp.SetNetworkLayerForChecksum(ip)
buf := gopacket.NewSerializeBuffer()
opts := gopacket.SerializeOptions{
    ComputeChecksums: true,
    FixLengths:       true,
}
if err := gopacket.SerializeLayers(buf, opts, udp); err != nil {
    fmt.Printf("%v", err)
}
if _, err := us.Conn.WriteTo(buf.Bytes(), &net.IPAddr{IP: dip}); err != nil {
    panic(err)
}

// reading
for {
    buf2 := make([]byte, 4096)
    n, addr, err := us.Conn.ReadFrom(buf2)
    if err != nil {
        fmt.Printf("%v", err)
    }
    if addr.String() == dip.String() {
        fmt.Printf("Got a reply")
    }
}

But this keeps erring out while reading packets with read ip4 0.0.0.0: i/o timeout. When I tcpdump, I see packets being sent out and one UDP response coming back on port 53 others are all ICMP. Why can't my code read those packets?

Answer 1

I'm not an expert in Go but it seems you have to use function net.ListenUDP and specify the port to bind the UDP socket. Then you use that connection (obtained from ListeUDP) to read from there. I assume you are specifying port 53 in 'sport'.

Here there is a simple example of a client and server: https://varshneyabhi.wordpress.com/2014/12/23/simple-udp-clientserver-in-golang/