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package main
import (
"flag"
"fmt"
"log"
"net"
"os"
"os/signal"
"strings"
"sync"
"syscall"
"time"
"golang.org/x/net/dns/dnsmessage"
)
type DNSForwarder struct {
upstreamDNS string
listenAddresses []ListenAddress
nat64Prefix *net.IPNet
}
type ListenAddress struct {
IP string
Port int
}
func NewDNSForwarder(upstreamDNS string, listenAddresses []ListenAddress) *DNSForwarder {
_, nat64Network, _ := net.ParseCIDR("64:ff9b::/96")
return &DNSForwarder{
upstreamDNS: upstreamDNS,
listenAddresses: listenAddresses,
nat64Prefix: nat64Network,
}
}
func (df *DNSForwarder) createSocket(listenIP string, listenPort int) (*net.UDPConn, error) {
addr := &net.UDPAddr{
Port: listenPort,
}
if listenIP == "::" {
addr.IP = net.IPv6zero
} else {
addr.IP = net.ParseIP(listenIP)
}
conn, err := net.ListenUDP("udp", addr)
if err != nil {
return nil, err
}
log.Printf("DNS forwarder listening on %s:%d", listenIP, listenPort)
return conn, nil
}
func (df *DNSForwarder) handleSocket(conn *net.UDPConn) {
buffer := make([]byte, 4096)
for {
n, remoteAddr, err := conn.ReadFromUDP(buffer)
if err != nil {
log.Printf("Error reading from socket: %v", err)
continue
}
go func(data []byte, addr *net.UDPAddr) {
response := df.processQuery(data[:n])
if response != nil {
_, err := conn.WriteToUDP(response, addr)
if err != nil {
log.Printf("Error sending response: %v", err)
}
}
}(buffer[:n], remoteAddr)
}
}
func (df *DNSForwarder) processQuery(queryData []byte) []byte {
// Check minimum DNS message size
if len(queryData) < 12 {
log.Printf("Query too short to be valid DNS message")
return nil
}
var msg dnsmessage.Message
if err := msg.Unpack(queryData); err != nil {
log.Printf("Error unpacking DNS message: %v", err)
errorMsg := &dnsmessage.Message{
Header: dnsmessage.Header{
ID: msg.Header.ID,
Response: true,
RCode: dnsmessage.RCodeFormatError,
},
}
response, packErr := errorMsg.Pack()
if packErr != nil {
log.Printf("Error packing error response: %v", packErr)
return nil
}
return response
}
if len(msg.Questions) == 0 {
log.Printf("DNS query contains no questions")
return nil
}
upstreamConn, err := net.Dial("udp", df.upstreamDNS)
if err != nil {
log.Printf("Error connecting to upstream DNS: %v", err)
return nil
}
defer upstreamConn.Close()
// Set a reasonable timeout for upstream queries (5 seconds)
upstreamConn.SetDeadline(time.Now().Add(5 * time.Second))
_, err = upstreamConn.Write(queryData)
if err != nil {
log.Printf("Error sending query to upstream: %v", err)
return nil
}
responseBuffer := make([]byte, 4096)
n, err := upstreamConn.Read(responseBuffer)
if err != nil {
log.Printf("Error reading response from upstream: %v", err)
return nil
}
if n < 12 {
log.Printf("Response too short to be valid DNS message")
return nil
}
return df.filterDNS64Responses(responseBuffer[:n])
}
func (df *DNSForwarder) filterDNS64Responses(responseData []byte) []byte {
var msg dnsmessage.Message
if err := msg.Unpack(responseData); err != nil {
log.Printf("Error unpacking response: %v", err)
return responseData
}
var filteredAnswers []dnsmessage.Resource
for _, answer := range msg.Answers {
if answer.Header.Type == dnsmessage.TypeAAAA {
aaaa, ok := answer.Body.(*dnsmessage.AAAAResource)
if ok {
ip := net.IP(aaaa.AAAA[:])
if !df.nat64Prefix.Contains(ip) {
filteredAnswers = append(filteredAnswers, answer)
}
}
} else {
filteredAnswers = append(filteredAnswers, answer)
}
}
msg.Answers = filteredAnswers
response, err := msg.Pack()
if err != nil {
log.Printf("Error packing filtered response: %v", err)
return responseData
}
return response
}
func (df *DNSForwarder) Start() error {
var wg sync.WaitGroup
for _, addr := range df.listenAddresses {
conn, err := df.createSocket(addr.IP, addr.Port)
if err != nil {
return fmt.Errorf("failed to create socket for %s:%d: %v", addr.IP, addr.Port, err)
}
wg.Add(1)
go func() {
defer wg.Done()
df.handleSocket(conn)
}()
}
sigChan := make(chan os.Signal, 1)
signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
<-sigChan
log.Println("Shutting down...")
return nil
}
func parseListenAddress(addr string) ListenAddress {
parts := strings.Split(addr, ":")
if len(parts) < 2 {
return ListenAddress{IP: "::", Port: 53}
}
port := 53
fmt.Sscanf(parts[len(parts)-1], "%d", &port)
ip := strings.Join(parts[:len(parts)-1], ":")
if ip == "" {
ip = "::"
}
return ListenAddress{IP: ip, Port: port}
}
func main() {
upstreamDNS := flag.String("upstream", "[2606:4700:4700::1111]:53", "Upstream DNS server (format: ip:port or [ipv6]:port)")
listenAddrs := flag.String("listen", "[::]:53", "Comma-separated list of IP:PORT to listen on")
flag.Parse()
var addresses []ListenAddress
for _, addr := range strings.Split(*listenAddrs, ",") {
addresses = append(addresses, parseListenAddress(addr))
}
forwarder := NewDNSForwarder(*upstreamDNS, addresses)
if err := forwarder.Start(); err != nil {
log.Fatal(err)
}
}
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