Hello, World!

main.go

@@ -0,0 +1,182 @@
+package main
+
+import (
+	"encoding/base64"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"os"
+	"strings"
+)
+
+const (
+	TYPE_A    = 1
+	TYPE_AAAA = 28
+	CLASS_IN  = 1
+)
+
+const TTL uint32 = 60
+
+var records = map[string]net.IP{}
+
+func main() {
+	data, err := os.ReadFile("records.ini")
+	if err != nil {
+		panic("failed to open records.ini")
+	}
+	lines := strings.Split(strings.TrimSpace(string(data)), "\n")
+	for _, line := range lines {
+		parts := strings.Split(strings.TrimSpace(line), "=")
+		ip := net.ParseIP(parts[1])
+		if ip == nil {
+			panic("failed to parse IP")
+		}
+		records[strings.ToLower(parts[0])] = ip
+	}
+
+	go func() {
+		fmt.Println("Listening on :8053...")
+		http.HandleFunc("/dns-query", handleHTTP)
+		if err := http.ListenAndServe(":8053", nil); err != nil {
+			panic(err)
+		}
+	}()
+
+	addr, err := net.ResolveUDPAddr("udp", "0.0.0.0:53")
+	if err != nil {
+		panic(err)
+	}
+	listener, err := net.ListenUDP("udp", addr)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println("Listening on :53...")
+	for {
+		req := make([]byte, 4096)
+		n, addr, err := listener.ReadFromUDP(req)
+		if err != nil {
+			fmt.Println("ERROR:", err)
+			continue
+		}
+
+		go handleUDP(listener, req[:n], addr)
+	}
+}
+
+func handleHTTP(w http.ResponseWriter, r *http.Request) {
+	var req []byte
+	var err error
+
+	switch r.Method {
+	case http.MethodGet:
+		dnsParam := r.URL.Query().Get("dns")
+		if dnsParam == "" {
+			http.Error(w, "missing dns parameter", http.StatusBadRequest)
+			return
+		}
+		req, err = base64.RawURLEncoding.DecodeString(dnsParam)
+		if err != nil {
+			http.Error(w, "invalid base64 encoding", http.StatusBadRequest)
+			return
+		}
+	case http.MethodPost:
+		req, err = io.ReadAll(io.LimitReader(r.Body, 65536))
+		if err != nil {
+			http.Error(w, "failed to read request body", http.StatusInternalServerError)
+			return
+		}
+		defer r.Body.Close()
+	default:
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	ip, _, _ := net.SplitHostPort(r.RemoteAddr)
+	resp := handleReq(ip, req)
+	w.Header().Set("Content-Type", "application/dns-message")
+	w.WriteHeader(http.StatusOK)
+	w.Write(resp)
+}
+
+func handleUDP(listener *net.UDPConn, req []byte, addr *net.UDPAddr) {
+	resp := handleReq(addr.IP.String(), req)
+	if _, err := listener.WriteToUDP(resp, addr); err != nil {
+		fmt.Println(err)
+	}
+}
+
+func handleReq(ip string, req []byte) []byte {
+	defer func() {
+		if r := recover(); r != nil {
+			fmt.Printf("Recovered from panic: CLIENT=%s: %v\n", ip, r)
+		}
+	}()
+
+	// we assume that QDCOUNT=1 but i think that's reasonable
+
+	// https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.2
+	// TODO: we should probably handle compression but from what i know clients don't generally use it
+	labels := []string{}
+	offset := 12
+	for {
+		length := int(req[offset])
+		offset++
+		if length == 0 || length > 63 {
+			break
+		}
+		labels = append(labels, string(req[offset:offset+length]))
+		offset += length
+	}
+	rawName := req[12:offset]
+	name := strings.ToLower(strings.Join(labels, "."))
+
+	// https://datatracker.ietf.org/doc/html/rfc1035#section-3.2.2
+	questionType := binary.BigEndian.Uint16(req[offset:])
+	offset += 2
+	// https://datatracker.ietf.org/doc/html/rfc1035#section-3.2.4
+	questionClass := binary.BigEndian.Uint16(req[offset:])
+	offset += 2
+
+	fmt.Printf("CLIENT=%s QNAME=%s TYPE=%d CLASS=%d\n", ip, name, questionType, questionClass)
+
+	resp := make([]byte, offset)
+	copy(resp, req[:offset])
+
+	resp[2] |= 0x80 // QR=1 (response)
+	resp[3] &= 0xf0 // clear RCODE
+	resp[2] |= 0x04 // authoritative answer
+	resp[3] &= 0x7f // recursion not available
+
+	binary.BigEndian.PutUint16(resp[6:], 0)  // ANCOUNT = 0
+	binary.BigEndian.PutUint16(resp[8:], 0)  // NSCOUNT = 0
+	binary.BigEndian.PutUint16(resp[10:], 0) // ARCOUNT = 0
+
+	data := []byte{}
+	if record, ok := records[name]; questionClass == CLASS_IN && ok {
+		ip4 := record.To4()
+		if questionType == TYPE_A && ip4 != nil {
+			data = ip4
+		} else if questionType == TYPE_AAAA && ip4 == nil {
+			data = record
+		}
+	} else {
+		resp[3] |= 0x03 // RCODE=NXDOMAIN
+	}
+
+	if len(data) > 0 {
+		// https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.3
+		resp = append(resp, rawName...)
+		resp = binary.BigEndian.AppendUint16(resp, questionType)
+		resp = binary.BigEndian.AppendUint16(resp, questionClass)
+		resp = binary.BigEndian.AppendUint32(resp, TTL)
+		resp = binary.BigEndian.AppendUint16(resp, uint16(len(data)))
+		resp = append(resp, data...)
+
+		binary.BigEndian.PutUint16(resp[6:], uint16(1)) // ANCOUNT=1
+	}
+
+	return resp
+}

records.ini

@@ -0,0 +1,2 @@
+four.su=123.234.123.234
+six.local=fd00:1234::1