Support unicast responses

[edaniels]

See https://rtcweb-wg.github.io/mdns-ice-candidates/mmusic/draft-ietf-mmusic-mdns-ice-candidates.html#implementation-guidance-1. It can help with security in addition to chattiness. I had implemented it in 17c664e but it was out of scope of the actual fix and I did not have a test to complement it. Also see https://datatracker.ietf.org/doc/html/draft-ietf-rtcweb-mdns-ice-candidates-04#section-3.2.1 point 2

Good reference at https://pkg.go.dev/golang.org/x/net/ipv4

[edaniels]

Did some initial investigation and I think this should slot behind IPv6 support which may end up exposing a new way to construct a server and possibly a client too with specific ipv4/6 sockets.

[edaniels]

For future Eric, here's some useful unicast and multicast reading/writing:

package main

import (
	"fmt"
	"net"
	"time"

	"golang.org/x/net/ipv4"
)

func main() {
	c, err := net.ListenPacket("udp4", "0.0.0.0:1024")
	if err != nil {
		fmt.Println("err0", err)
		return
	}
	defer c.Close()

	fmt.Println("Local Address", c.LocalAddr())

	en0, err := net.InterfaceByName("en0")
	if err != nil {
		// error handling
	}
	group := net.IPv4(224, 0, 0, 250)

	addrs, err := en0.Addrs()
	fmt.Println("en0 address", addrs, err)

	p := ipv4.NewPacketConn(c)
	if err := p.JoinGroup(en0, &net.UDPAddr{IP: group}); err != nil {
		// error handling
	}

	go func() {
		b := make([]byte, 1500)
		for {
			fmt.Println("Reading...")
			n, cm, src, err := p.ReadFrom(b)
			if err != nil {
				fmt.Println("err1", err)
				continue
			}
			fmt.Println("Read!", b[:n], n, cm, src)
		}
	}()

	// TODO(erd): helpful?
	if err := p.SetControlMessage(ipv4.FlagDst, true); err != nil {
		fmt.Println("p.SetControlMessage(ipv4.FlagDst, true)", err)
	}

	// unicast writer
	go func() {
		data := []byte{1, 2, 3}
		for {
			time.Sleep(time.Second)
			// p.SetTOS(0x0)
			// p.SetTTL(16)
			var cm ipv4.ControlMessage
			cm.Dst = addrs[0].(*net.IPNet).IP
			someAddr := &net.UDPAddr{
				IP:   cm.Dst,
				Port: 1024,
			}
			fmt.Println("writing to", someAddr)
			if n, err := p.WriteTo(data, &cm, someAddr); err != nil {
				// error handling
				fmt.Println("err2", err)
				continue
			} else {
				fmt.Println("wrote unicast1", n)
			}
		}
	}()

	c2, err := net.ListenPacket("udp4", "0.0.0.0:0")
	if err != nil {
		fmt.Println("err0", err)
		return
	}
	defer c2.Close()
	p2 := ipv4.NewPacketConn(c2)
	if err := p2.JoinGroup(en0, &net.UDPAddr{IP: group}); err != nil {
		// error handling
	}

	// unicast writer
	go func() {
		data := []byte{7, 8, 9}
		for {
			time.Sleep(time.Second)
			// p.SetTOS(0x0)
			// p.SetTTL(16)
			var cm ipv4.ControlMessage
			cm.Dst = addrs[0].(*net.IPNet).IP
			someAddr := &net.UDPAddr{
				IP:   cm.Dst,
				Port: 1024,
			}
			fmt.Println("writing to", someAddr)
			if n, err := p2.WriteTo(data, &cm, someAddr); err != nil {
				// error handling
				fmt.Println("err5", err)
				continue
			} else {
				fmt.Println("wrote unicast2", n)
			}
		}
	}()

	// multicast writer
	go func() {
		data := []byte{4, 5, 6}
		dst := &net.UDPAddr{IP: group, Port: 1024}
		for {
			time.Sleep(time.Second)
			for _, ifi := range []*net.Interface{en0} {
				if err := p.SetMulticastInterface(ifi); err != nil {
					fmt.Println("err3", err)
				}
				// p.SetMulticastTTL(2)
				fmt.Println("writing to", dst)
				if n, err := p.WriteTo(data, nil, dst); err != nil {
					fmt.Println("err4", err)
					continue
				} else {
					fmt.Println("wrote multicast", n)
				}
			}
		}
	}()

	select {}
}

[edaniels]

Working branch at https://github.com/edaniels/mdns/tree/unicast_support