Relay cmd to tunnel dns-sd over TCP

New cmdline tool I made (based on cmd/debug) to relay dns-sd
to docker containers on darwin to help deal with some limitations
of docker desktop (docker/for-mac#68).

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

cmd/relay/main.go

@@ -0,0 +1,258 @@
+// Relay packets to/from a tunneled interface
+// Intended for use with Docker Deskstop on MacOSX which doesn't properly handle this
+package main
+
+import (
+	"context"
+	"encoding/binary"
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"os"
+	"os/signal"
+	"strconv"
+	"time"
+
+	"github.com/brutella/dnssd"
+)
+
+const (
+	SERVER_TYPE = "tcp"
+	MAXDGRAMSZ  = 8192
+	TIMEFORMAT  = "15:04:05.000"
+)
+
+var (
+	mode       = flag.String("mode", "server", "mode (client or server")
+	debug      = flag.Bool("d", false, "enable debug prints")
+	dump       = flag.Bool("dump", false, "dump all traffic")
+	serverIp   = flag.String("serverIp", "127.0.0.1", "IP to listen on for new clients")
+	relayHost  = flag.String("host", "host.docker.internal", "host the client mode connects to")
+	relayPort  = flag.Int("port", 53535, "port connected to for relay traffic")
+	relayFPort = flag.Int("from", 35353, "port to send relayed messages from")
+	dnssdAddr  = flag.String("dnssdAddr", "224.0.0.251", "multicast address to use for dns-sd")
+	dnssdPort  = flag.Int("dnssdPort", 5353, "port to use for mDNS dns-sd")
+	v          = func(string, ...interface{}) {}
+)
+
+func flags() {
+	flag.Parse()
+	if *debug {
+		v = log.Printf
+	}
+}
+
+// provide a reliable data-gram service over TCP
+type TcpDGC struct {
+	connection net.Conn
+}
+
+func (c *TcpDGC) readN(msg []byte, total uint32) error {
+	totalread := uint32(0)
+
+	for totalread < total {
+		n, err := c.connection.Read(msg[totalread:total])
+		if err != nil {
+			v("readN: short read error")
+			return err
+		}
+		if n < 0 {
+			v("n returned from read is negative")
+			return fmt.Errorf("negative return in readN")
+		}
+		totalread += uint32(n)
+	}
+	return nil
+}
+
+func (c *TcpDGC) Send(msg []byte) (int, error) {
+	hdr := make([]byte, 4)
+	binary.LittleEndian.PutUint32(hdr, uint32(len(msg)))
+	actualBuf := append(hdr[:], msg[:]...)
+	v("Sending msg size %x", uint32(len(actualBuf)))
+	n, err := c.connection.Write(actualBuf)
+
+	return (n - 4), err
+}
+
+func (c *TcpDGC) Recv(msg []byte) (uint32, error) {
+	hdr := make([]byte, 4)
+	err := c.readN(hdr, 4)
+	if err != nil {
+		return 0, err
+	}
+
+	buflen := binary.LittleEndian.Uint32(hdr)
+	if buflen > uint32(len(msg)) {
+		return 0, fmt.Errorf("buffer not big enough to store datagram size %x", buflen)
+	}
+	v("receiving message size %x", buflen)
+	err = c.readN(msg, buflen)
+	if err != nil {
+		return 0, err
+	}
+
+	return buflen, err
+}
+
+func relay(connection net.Conn, tunChan chan *dnssd.Request, ctlChan chan int) {
+	c := &TcpDGC{connection}
+
+	for {
+		select {
+		case <-ctlChan:
+			v("Received request to stop in relay")
+			os.Exit(1)
+		case r := <-tunChan:
+			if r.From().Port == *relayFPort {
+				// Drop multicasts from me
+				continue
+			}
+			m := r.Raw()
+			rm, err := m.Pack()
+			if err != nil {
+				log.Printf("repacking DNS msg: %s\n", err.Error())
+				return
+			}
+
+			n, err := c.Send(rm)
+			if err != nil {
+				log.Printf("Forwarding repacked DNS msg: %s\n", err.Error())
+				return
+			}
+			if n < len(rm) {
+				log.Printf("Forwarding repacked DNS msg returned partial send of %d/%d bytes\n", n, len(rm))
+				return
+			}
+		}
+	}
+}
+
+func relayToPeer(connection net.Conn, stop chan os.Signal) {
+	tunChan := make(chan *dnssd.Request, 64)
+	ctlChan := make(chan int, 1)
+
+	v("Relaying…\n")
+
+	go relay(connection, tunChan, ctlChan)
+
+	fn := func(req *dnssd.Request) {
+		tunChan <- req
+		if *dump {
+			log.Printf("-------------------------------------------\n")
+			log.Printf("%s\n%v\n", time.Now().Format(TIMEFORMAT), req)
+		}
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	if rsp, err := dnssd.NewResponder(); err != nil {
+		log.Println(err)
+	} else {
+		rsp.Debug(ctx, fn)
+
+		<-stop
+
+		v("Interrupt detected")
+		cancel()
+		ctlChan <- 1
+		os.Exit(1)
+	}
+}
+
+func server(stop chan os.Signal) {
+	v("Server Running...")
+
+	server, err := net.Listen(SERVER_TYPE, *serverIp+":"+strconv.Itoa(*relayPort))
+	if err != nil {
+		log.Println("error listening:", err.Error())
+		os.Exit(1)
+	}
+	defer server.Close()
+	v("Listening on " + *serverIp + ":" + strconv.Itoa(*relayPort))
+	v("Waiting for client...")
+	go func() {
+		<-stop
+		server.Close()
+		os.Exit(1)
+	}()
+	for {
+		connection, err := server.Accept()
+		if err != nil {
+			log.Printf("error accepting: %s\n", err.Error())
+			os.Exit(1)
+		}
+		log.Printf("new client connected %s\n", connection.RemoteAddr().String())
+		go relayToPeer(connection, stop)
+		go relayFromPeer(connection)
+	}
+}
+
+func relayFromPeer(connection net.Conn) {
+	maddr, err := net.ResolveUDPAddr("udp", *dnssdAddr+":"+strconv.Itoa(*dnssdPort))
+	if err != nil {
+		log.Fatal(err)
+	}
+	myaddr, err := net.ResolveUDPAddr("udp", ":"+strconv.Itoa(*relayFPort))
+	if err != nil {
+		log.Fatal(err)
+	}
+	mc, err := net.DialUDP("udp", myaddr, maddr)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer mc.Close()
+
+	c := &TcpDGC{connection}
+	for {
+		buffer := make([]byte, MAXDGRAMSZ)
+
+		mLen, err := c.Recv(buffer)
+		if err != nil {
+			log.Printf("error reading from %s: %s\n", connection.RemoteAddr().String(), err.Error())
+			return
+		}
+
+		n, err := mc.Write(buffer[:mLen])
+		if err != nil {
+			log.Printf("error forwarding message as %s: %s\n", mc.LocalAddr().String(), err.Error())
+			return
+		}
+		if uint32(n) < mLen {
+			log.Printf("short write as %s: %d\n", mc.LocalAddr().String(), n)
+		}
+	}
+}
+
+func client(stop chan os.Signal) {
+	connection, err := net.Dial(SERVER_TYPE, *relayHost+":"+strconv.Itoa(*relayPort))
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer connection.Close()
+	go func() {
+		<-stop
+		connection.Close()
+		os.Exit(1)
+	}()
+
+	go relayToPeer(connection, stop)
+	relayFromPeer(connection)
+}
+
+func main() {
+	flags()
+	stop := make(chan os.Signal, 1)
+	signal.Notify(stop, os.Interrupt)
+
+	if *mode == "server" {
+		v("starting server")
+		server(stop)
+	}
+	if *mode == "client" {
+		v("starting client")
+		client(stop)
+	}
+}