How to implement DNS forwarding service using Go

From the experience of implementing a DNS server, the ease of use and simplicity of the Go language make it a good choice. In this article, we will discuss how to implement a DNS forwarding service using Go and explore the details of the process.

What is DNS?

DNS (Domain Name System) is a system on the Internet used to resolve domain names into corresponding IP addresses. The main function is to convert human-readable domain names into computer-recognizable IP addresses.

The DNS server is a computer responsible for processing DNS query services, accepting DNS query requests from clients and returning corresponding answers. DNS query requests are initiated by the client, usually communicating via UDP protocol using port 53.

DNS forwarding

DNS forwarding, also known as DNS redirection, refers to sending DNS query requests from the local DNS server to other DNS servers for resolution. The reason for DNS forwarding may be that the local DNS server is unable to provide an answer, or the relevant query results are not cached.

There are two mechanisms for DNS forwarding. The first is recursive query. If the local DNS server cannot resolve the DNS query request, it will send a request to the root DNS server and continue to query downward until the answer is found, and then return the result to the client.

The second is iterative query, where the local DNS server sends queries to other DNS servers and gets answers from other servers. This method requires that the local DNS server be better able to resolve DNS query requests because all responses require resolution by the local DNS server.

Use Go to implement DNS forwarding service

Using Go to implement DNS forwarding service is very simple. We use the third-party library Miekg/dns to handle the parsing and forwarding of DNS requests. The method to install the Miekg/dns library is as follows:

go get github.com/miekg/dns

We will use the following components in the code:

• net: used to receive and send packets, Used as a UDP server.
• strconv: Used to convert strings to other data types.
• time: Used to handle expired answers and caching.

First, let us define a DNS client and server so that we can listen and process DNS requests and responses:

type DNSClient struct {
    net.Conn
}

func (c DNSClient) writeMsg(msg []byte) {
    c.Write(msg)
    c.SetReadDeadline(time.Now().Add(time.Second * 5))
}

func (c DNSClient) readMsg() []byte {
    buf := make([]byte, 2048)
    c.Read(buf)
    return buf
}

type DNSServer struct {
    addr string
}

Next, we implement the processing method of DNS query request , we will use the Miekg/dns library to send requests to another DNS server:

func (s *DNSServer) handleDNSQuery(w dns.ResponseWriter, r *dns.Msg) {
    msg := dns.Msg{}
    msg.SetReply(r)
    
    client := DNSClient{Conn: nil}
    defer client.Close()
    
    for _, a := range msg.Answer {
        if a.Header().Class == dns.ClassINET {
            switch a.Header().Rrtype {
            case dns.TypeA:
                q := dns.Question{Name: a.Header().Name, Qtype: dns.TypeA, Qclass: dns.ClassINET}
                client.Exchange(&q) // DNS 转发
            }
        }
    }
    
    w.WriteMsg(&msg)
}

Finally, we adjust the main function to listen for DNS requests and forward them to other DNS servers:

func main() {
    server := DNSServer{addr: "127.0.0.1:53"}
    serverHandler := dns.NewServeMux()
    serverHandler.HandleFunc(".", server.handleDNSQuery)
    
    go func() {
        if err := dns.ListenAndServe(server.addr, "udp", serverHandler); err != nil {
            panic(err)
        }
    }()
    
    time.Sleep(time.Second * 1000)
}

Now, we have successfully implemented a simple DNS forwarding service. When a DNS query cannot be resolved by the local DNS server, it queries other DNS servers until an answer is found. During actual operation, we need to consider issues such as caching and the maximum number of queries to ensure the stability and performance of the service.

Summary

In this article, we discussed the mechanism of DNS forwarding and the process of implementing DNS forwarding service using Go. We use the Miekg/dns library to handle the parsing and forwarding of DNS requests and explain the details required for implementation.

For developers who need to implement DNS servers or DNS forwarding services, the Go language provides a reliable and efficient solution. A closer look at the implementation in this article will help you understand the details of the DNS protocol and its implementation.

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