使用 UDP 使用 TCP 传输可以解决 DNS 问题

Question

我已经尝试了所有我能想到的方法，但我正在碰壁。我想知道是否有人能够指出正确的方向或可能发现问题？

出于几个原因，我正在尝试编写自己的 DNS 服务器，主要是学习经验。

有几点要提一下：

我正在使用 Go
对 DNS 数据包使用“gopacket”de/serialisation
服务器在使用 UDP 协议时正常工作

因此，在通过 UDP 工作获得响应后，我决定是时候实施 TCP 了。下面是侦听 TCP 传入连接的代码片段。

// 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)

当我通过 TCP 发出挖掘命令时，服务器接受连接并将接收到的字节数据传递给 DNSHandle 函数（与 UDP 处理程序相同）。使用UDP时的数据似乎有很大不同。

gopacket反序列化后的UDP请求：

&{{[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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gopacket反序列化后的TCP请求：

 <nil>

但是，如果我将 tbuff 更改为 [2:]:

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

它通过了请求但格式错误（我想你会预料到）：

&{{[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]}

我很感激这可能是一个远景，但我们非常欢迎任何帮助。

编辑：

感谢@Steffen Ullrich 和@selbie。我设法让我的代码工作。它不是很漂亮，需要做更多的工作，但这是一个 POC 片段：

// 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:]:

来自 RFC 1035 第 4.2.2 节：

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.

因此，将基于 TCP 的 DNS 视为基于 UDP 的 DNS 是错误的 - 在 TCP 中，DNS 消息的前缀是包含消息长度的 2 个字节。

这是必需的，因为 TCP 是字节流，而不是像 UDP 那样的数据报协议。这意味着不能简单地假设 c.Read(...) 将返回一个完整的 DNS 消息。可能尚未收到完整的消息，但也可能收到了不止一条 DNS 消息。为此检查长度前缀。

使用 UDP 使用 TCP 传输可以解决 DNS 问题

Go DNS trouble with TCP transport fine with UDP

go

tcp

dns

buffer

gopacket