Go DNS trouble with TCP transport fine with UDP

Question

I've tried everything I can think of and I'm hitting a brick wall. I wonder if anyone is able to point me in the right direction or possibly spot the issue?

I'm having a go at writing my own DNS server for a couple of reasons, the main a learning experience.

A few things to mention:

• I'm using Go
• Using "gopacket" for the DNS packet de/serialisation
• The server works as expected when using the UDP protocol

So after getting responses over UDP working I decided it was time to implement TCP. Below is a snippet of the code listening for the TCP incoming connections.

// Listen TCP
    c, err := t.Accept()
    if err != nil {
        log.Println(err)
    }

    go func(c net.Conn) {
        tbuff := make([]byte, 4096)
        _, err = c.Read(tbuff)
        if err != nil {
            fmt.Println(err)
        }
        clientAddr := addrt
        packet := gopacket.NewPacket(tbuff, layers.LayerTypeDNS, gopacket.Default)
        dnsPacket := packet.Layer(layers.LayerTypeDNS)
        tcp, _ := dnsPacket.(*layers.DNS)
        if !limiter(clientAddr.String()) {
            c.Close()
        }
        tbuff = DNSHandle(tcp)
        c.Write(tbuff)
        c.Close()
    }(c)

When I issue a dig command over TCP the server accepts the connection and passed the received byte data to the DNSHandle function (same as the UDP handler). The data seems to be very different of that when using UDP.

UDP request after gopacket deserialisation:

&{{[91 174 1 32 0 1 0 0 0 0 0 1 11 116 101 115 116 101 120 97 109 112 108 101 3 99 111 109 0 0 1 0 1 0 0 41 16 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 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TCP request after gopacket deserialisation:

 <nil>

However if I change tbuff to [2:]:

packet := gopacket.NewPacket(tbuff[2:], layers.LayerTypeDNS, gopacket.Default)

It passes the request but malformed (which I suppose you'd expect):

&{{[114 11 1 32 0 1 0 0 0 0 0 1 11 116 101 115 116 101 120 97 109 112 108 101 3 99 111 109 0 0 1 0 1 0 0 41 16 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] []} 29195 false Query false false true false 2 No Error 1 0 0 1 [{[116 101 115 116 101 120 97 109 112 108 101 46 99 111 109] A IN}] [] [] [{[] OPT Unknown 0 0 [] <nil> [] [] [] [] {[] [] 0 0 0 0 0} {0 0 0 []} {0 []} [] {0 0 []} []}] [46 116 101 115 116 101 120 97 109 112 108 101 46 99 111 109]}

I appreciate this may be a long shot but any help would be very welcome.

EDIT:

Thank you to both @Steffen Ullrich, @selbie. I've managed to get my code working. It's not pretty and needs a lot more work but here is a POC snippet:

// Listen TCP
    c, err := t.Accept()
    if err != nil {
        log.Println(err)
    }

    go func(c net.Conn) {
        tbuff := make([]byte, 4096)
        msize := make([]byte, 2)
        size, err := c.Read(tbuff)
        if err != nil {
            fmt.Println(err)
        }
        DNSMsgLength := binary.BigEndian.Uint16(tbuff[:2])
        if int(DNSMsgLength) == size-2 {
            fmt.Println("Size correct")
        }
        clientAddr := addrt
        packet := gopacket.NewPacket(tbuff[2:], layers.LayerTypeDNS, gopacket.Default)
        dnsPacket := packet.Layer(layers.LayerTypeDNS)
        tcp, _ := dnsPacket.(*layers.DNS)
        if !limiter(clientAddr.String()) {
            c.Close()
        }
        tbuff = DNSHandle(tcp)
        binary.BigEndian.PutUint16(msize, uint16(len(tbuff)))
        tbuff = append(msize, tbuff...)
        c.Write(tbuff)
        c.Close()
    }(c)

Answer 1

However if I change tbuff to [2:]:

From RFC 1035 section 4.2.2:

4.2.2. TCP usage

Messages sent over TCP connections use server port 53 (decimal). The message is prefixed with a two byte length field which gives the message length, excluding the two byte length field. This length field allows the low-level processing to assemble a complete message before beginning to parse it.

Thus by treating DNS over TCP the same as DNS over UDP you are doing it wrong - in TCP a DNS message is prefixed with 2 bytes containing the length of the message.

This is needed since TCP is a byte stream and not a datagram protocol like UDP. This means that one cannot simply assume that exactly a single and full DNS message will be returned by c.Read(...). It might be possible that not the full message was received yet but also that more than one DNS message was received. Check the length prefix for this.