feat: add fallback logic

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"encoding/binary"
+	"errors"
 	"flag"
 	"io"
 	"math"
@@ -28,10 +29,14 @@ var (
 				_ = c.Close()
 			}, nil,
 		})
+	fallback *net.UDPAddr
+	forcefb  bool
 )
 
 func main() {
-	iphost := flag.String("l", "127.0.0.1:5345", "listen DNS UDP port")
+	iphost := flag.String("l", "0.0.0.0:53", "listen DNS UDP port")
+	fbsrv := flag.String("fb", "127.0.0.1:5345", "fallback to DNS UDP port")
+	flag.BoolVar(&forcefb, "ffb", false, "force using fallback")
 	frag := flag.Uint("frag", 3, "TLS first fragemt size")
 	flag.Parse()
 
@@ -39,6 +44,15 @@ func main() {
 		terasu.DefaultFirstFragmentLen = uint8(*frag)
 	}
 
+	if *fbsrv != "" {
+		addrport, err := netip.ParseAddrPort(*fbsrv)
+		if err != nil {
+			logrus.Fatal("ParseAddrPort err:", err)
+		}
+		fallback = net.UDPAddrFromAddrPort(addrport)
+		logrus.Infoln("Set fallback server to", fallback)
+	}
+
 	addrport, err := netip.ParseAddrPort(*iphost)
 	if err != nil {
 		logrus.Fatal("ParseAddrPort err:", err)
@@ -74,12 +88,21 @@ RECONN:
 }
 
 func response(cnt uint8, conn *net.UDPConn, addr *net.UDPAddr, payload pbuf.Bytes) {
+	var (
+		err     error
+		tlsconn net.Conn
+		loopcnt = 0
+	)
+
+	if forcefb {
+		goto FALLBACK
+	}
+
 	defer releasefree(cnt)
 	logrus.Debugln(addr, "Run on lock", cnt)
 
-	loopcnt := 0
 REDAIL:
-	tlsconn, err := dialtls(cnt)
+	tlsconn, err = dialtls(cnt)
 	if err != nil {
 		logrus.Warnln(addr, "Dial DNS server err:", err)
 		return
@@ -118,15 +141,60 @@ REDAIL:
 			logrus.Debugln("Write response to", addr)
 		})
 	})
-	if err != nil {
+	if err == nil {
-		logrus.Warnln(addr, "Proxy to DNS server err:", err)
-		tlsconnCache.Delete(cnt)
-		loopcnt++
-		if loopcnt < 3 {
-			goto REDAIL
-		}
 		return
 	}
+	logrus.Warnln(addr, "Proxy to DNS server err:", err)
+	tlsconnCache.Delete(cnt)
+	loopcnt++
+	if loopcnt < 2 {
+		goto REDAIL
+	}
+FALLBACK:
+	fbconn, err := net.DialUDP("udp", nil, fallback) // dummy fill
+	if err != nil {
+		logrus.Warnln(addr, "Fallback DialUDP err:", err)
+		return
+	}
+	logrus.Warnln(addr, "Fallback from", fbconn.LocalAddr(), "to", fbconn.RemoteAddr())
+	defer fbconn.Close()
+	payload.V(func(b []byte) {
+		_, err = fbconn.Write(b)
+		if err != nil {
+			logrus.Warnln(addr, "Write to fallback err:", err)
+			return
+		}
+	})
+	if err != nil {
+		return
+	}
+	_ = fbconn.SetReadDeadline(time.Now().Add(time.Second * 4))
+	pbuf.NewBytes(4096).V(func(b []byte) {
+		var (
+			n       int
+			srvaddr *net.UDPAddr
+		)
+		for {
+			n, srvaddr, err = fbconn.ReadFromUDP(b)
+			if err != nil {
+				logrus.Warnln(addr, "Read from fallback err:", err)
+				return
+			}
+			if !srvaddr.IP.Equal(fallback.IP) || srvaddr.Port != fallback.Port {
+				logrus.Warnln(addr, "Read expect fallback addr but got", srvaddr)
+				err = errors.New("wrong endpoint")
+				continue
+			}
+			break
+		}
+		if err == nil {
+			_, err = conn.WriteToUDP(b[:n], addr)
+			if err != nil {
+				return
+			}
+			logrus.Debugln("Write fallback response to", addr)
+		}
+	})
 }
 
 // lockfree is spin update