From f8c31c9f015770a3e88b9b82fcbf3fd40759f5c1 Mon Sep 17 00:00:00 2001
From: Vladimir Avtsenov <vladimir.lsk.cool@gmail.com>
Date: Sat, 24 Aug 2024 01:16:10 +0300
Subject: [PATCH] first dirty implementation of dns proxy

---
 go.mod  |   3 +
 main.go | 176 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 179 insertions(+)
 create mode 100644 go.mod
 create mode 100644 main.go

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..514b814
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module kvas2-go
+
+go 1.21
\ No newline at end of file
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..5381213
--- /dev/null
+++ b/main.go
@@ -0,0 +1,176 @@
+package main
+
+import (
+	"encoding/binary"
+	"encoding/hex"
+	"fmt"
+	"log"
+	"net"
+	"strings"
+	"time"
+)
+
+var (
+	ListenPort             = 7548
+	UsableDNSServerAddress = "127.0.0.1"
+	UsableDNSServerPort    = 53
+	DNSMaxPackageSize      = 4096
+)
+
+func parseName(response []byte, pos int) (string, int) {
+	var nameParts []string
+	var jumped bool
+	var outPos int
+	responseLen := len(response)
+
+	for {
+		length := int(response[pos])
+		pos++
+		if length == 0 {
+			break
+		}
+
+		if length&0xC0 == 0xC0 {
+			if !jumped {
+				outPos = pos + 1
+			}
+			pos = int(binary.BigEndian.Uint16(response[pos-1:pos+1]) & 0x3FFF)
+			jumped = true
+			continue
+		}
+
+		if pos+length > responseLen {
+			break
+		}
+
+		nameParts = append(nameParts, string(response[pos:pos+length]))
+		pos += length
+	}
+
+	if !jumped {
+		outPos = pos
+	}
+	return strings.Join(nameParts, "."), outPos
+}
+
+func sendToUpstream(upstreamAddr string, request []byte) ([]byte, error) {
+	conn, err := net.Dial("udp", upstreamAddr)
+	if err != nil {
+		return nil, fmt.Errorf("failed to dial upstream DNS: %w", err)
+	}
+	defer conn.Close()
+
+	_, err = conn.Write(request)
+	if err != nil {
+		return nil, fmt.Errorf("failed to send request to upstream DNS: %w", err)
+	}
+
+	err = conn.SetReadDeadline(time.Now().Add(5 * time.Second))
+	if err != nil {
+		return nil, fmt.Errorf("failed to set timeout: %w", err)
+	}
+
+	response := make([]byte, DNSMaxPackageSize)
+	n, err := conn.Read(response)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read response from upstream DNS: %w", err)
+	}
+
+	return response[:n], nil
+}
+
+func main() {
+	addr := fmt.Sprintf(":%d", ListenPort)
+	udpAddr, err := net.ResolveUDPAddr("udp", addr)
+	if err != nil {
+		log.Fatalf("Failed to resolve address: %v", err)
+	}
+
+	conn, err := net.ListenUDP("udp", udpAddr)
+	if err != nil {
+		log.Fatalf("Failed to listen on UDP: %v", err)
+	}
+	defer conn.Close()
+
+	fmt.Printf("DNS server is running on %s...\n", addr)
+
+	for {
+		buffer := make([]byte, DNSMaxPackageSize)
+		n, clientAddr, err := conn.ReadFromUDP(buffer)
+		if err != nil {
+			log.Printf("Failed to read from UDP: %v", err)
+			continue
+		}
+
+		go handleDNSRequest(conn, clientAddr, buffer[:n])
+	}
+}
+
+func process(response []byte) {
+	responseLen := len(response)
+	if responseLen <= 12 {
+		return
+	}
+
+	qCount := int(binary.LittleEndian.Uint16(response[5:7]))
+	aCount := int(binary.LittleEndian.Uint16(response[7:9]))
+
+	pos := 12
+
+	for i := 0; i < qCount; i++ {
+		var name string
+		name, pos = parseName(response, pos)
+		fmt.Printf("Requested name: %s\n", name)
+		pos += 4
+	}
+
+	for i := 0; i < aCount; i++ {
+		name, newPos := parseName(response, pos)
+		pos = newPos
+
+		if pos+10 > responseLen {
+			break
+		}
+
+		qtype := binary.BigEndian.Uint16(response[pos : pos+2])
+		pos += 2
+
+		qclass := binary.BigEndian.Uint16(response[pos : pos+2])
+		pos += 2
+
+		ttl := binary.BigEndian.Uint32(response[pos : pos+4])
+		pos += 4
+
+		rdlength := binary.BigEndian.Uint16(response[pos : pos+2])
+		pos += 2
+
+		if pos+int(rdlength) > responseLen {
+			break
+		}
+
+		if qtype == 1 && qclass == 1 && rdlength == 4 {
+			ip := net.IPv4(response[pos], response[pos+1], response[pos+2], response[pos+3])
+			fmt.Printf("Parsed A record: %s -> %s, TTL: %d\n", name, ip, ttl)
+		}
+
+		pos += int(rdlength)
+	}
+}
+
+func handleDNSRequest(conn *net.UDPConn, clientAddr *net.UDPAddr, buffer []byte) {
+	upstreamAddr := fmt.Sprintf("%s:%d", UsableDNSServerAddress, UsableDNSServerPort)
+
+	upstreamResponse, err := sendToUpstream(upstreamAddr, buffer)
+	if err != nil {
+		log.Printf("Failed to get response from upstream DNS: %v", err)
+		return
+	}
+	log.Printf("Response: %s", hex.EncodeToString(upstreamResponse))
+
+	process(upstreamResponse)
+
+	_, err = conn.WriteToUDP(upstreamResponse, clientAddr)
+	if err != nil {
+		log.Printf("Failed to send DNS response: %v", err)
+	}
+}