From 50349f400f17c5ed2fe875a46d97995f5f623320 Mon Sep 17 00:00:00 2001
From: Nicolai Ommer <nicolai.ommer@gmail.com>
Date: Sun, 24 Mar 2024 13:48:32 +0100
Subject: [PATCH] Use golang.org/x/net/ipv4 to receive multicast messages in
 ssl-multicast-sources

With the previous approach, message from different interfaces were received by all interfaces. With the higher level net/ipv4, suggested by the GO documentation, this can be done in a more sane way.
---
 cmd/ssl-multicast-sources/main.go | 70 ++++++++++++++++++-------------
 go.mod                            |  5 +++
 go.sum                            |  4 ++
 3 files changed, 51 insertions(+), 28 deletions(-)

diff --git a/cmd/ssl-multicast-sources/main.go b/cmd/ssl-multicast-sources/main.go
index 1eed308..eb9caa8 100644
--- a/cmd/ssl-multicast-sources/main.go
+++ b/cmd/ssl-multicast-sources/main.go
@@ -2,10 +2,12 @@ package main
 
 import (
 	"flag"
+	"golang.org/x/net/ipv4"
 	"log"
 	"net"
 	"os"
 	"os/signal"
+	"strings"
 	"sync"
 	"syscall"
 )
@@ -13,9 +15,8 @@ import (
 const maxDatagramSize = 8192
 
 type Source struct {
-	nif  string
-	ip   string
-	port int
+	cm  string
+	src string
 }
 
 var detectedRemotes map[string][]Source
@@ -30,11 +31,8 @@ func main() {
 		mcAddresses = []string{"224.5.23.1:10003", "224.5.23.2:10006", "224.5.23.2:10010", "224.5.23.2:10012"}
 	}
 
-	ifiList := interfaces()
 	for _, address := range mcAddresses {
-		for _, ifi := range ifiList {
-			go receiveOnInterface(address, ifi)
-		}
+		go receiveOnInterfaceIpv4(address)
 	}
 
 	signals := make(chan os.Signal, 1)
@@ -54,38 +52,54 @@ func interfaces() (interfaces []net.Interface) {
 	return
 }
 
-func receiveOnInterface(address string, ifi net.Interface) {
-	addr, err := net.ResolveUDPAddr("udp", address)
+func receiveOnInterfaceIpv4(address string) {
+	group := net.ParseIP(strings.Split(address, ":")[0])
+	c, err := net.ListenPacket("udp4", address)
 	if err != nil {
-		log.Printf("Could resolve multicast address %v: %v", address, err)
+		log.Printf("Could not start listening on %v: %v", address, err)
 		return
 	}
+	defer func(c net.PacketConn) {
+		if err := c.Close(); err != nil {
+			log.Println("Failed to close connection:", err)
+		}
+	}(c)
 
-	conn, err := net.ListenMulticastUDP("udp", &ifi, addr)
-	if err != nil {
-		log.Printf("Could not listen at %v: %v", address, err)
-		return
-	}
+	log.Printf("Listening on %s", address)
+
+	p := ipv4.NewPacketConn(c)
 
-	if err := conn.SetReadBuffer(maxDatagramSize); err != nil {
-		log.Println("Could not set read buffer: ", err)
+	if err := p.SetControlMessage(ipv4.FlagDst, true); err != nil {
+		log.Println("Failed to set control message flag 'FlagDst':", err)
 	}
 
-	log.Printf("Listening on %s (%s)", address, ifi.Name)
+	ifiList := interfaces()
+	for _, ifi := range ifiList {
+		if err := p.JoinGroup(&ifi, &net.UDPAddr{IP: group}); err != nil {
+			log.Printf("Failed to join multicast group %v: %v", group, err)
+		} else {
+			log.Printf("Joined multicast group %v on interface %+v", group, ifi)
+		}
+	}
 
 	data := make([]byte, maxDatagramSize)
 	for {
-		_, remoteAddr, err := conn.ReadFromUDP(data)
+		_, cm, src, err := p.ReadFrom(data)
 		if err != nil {
-			log.Println("ReadFromUDP failed:", err)
-			return
+			log.Printf("Could not read from %v: %v", address, err)
+			continue
+		}
+		if cm.Dst.IsMulticast() {
+			if cm.Dst.Equal(group) {
+				addRemote(address, Source{
+					cm:  cm.String(),
+					src: src.String(),
+				})
+			} else {
+				// unknown group, discard
+				continue
+			}
 		}
-
-		addRemote(address, Source{
-			nif:  ifi.Name,
-			ip:   remoteAddr.IP.String(),
-			port: remoteAddr.Port,
-		})
 	}
 }
 
@@ -104,5 +118,5 @@ func addRemote(address string, source Source) {
 	}
 
 	detectedRemotes[address] = append(detectedRemotes[address], source)
-	log.Printf("New source on %v: %+v\n", address, source)
+	log.Printf("New source on %v: %+v", address, source)
 }
diff --git a/go.mod b/go.mod
index 5c5c26d..81c0cc8 100644
--- a/go.mod
+++ b/go.mod
@@ -6,3 +6,8 @@ require (
 	github.com/pkg/errors v0.9.1
 	google.golang.org/protobuf v1.33.0
 )
+
+require (
+	golang.org/x/net v0.22.0 // indirect
+	golang.org/x/sys v0.18.0 // indirect
+)
diff --git a/go.sum b/go.sum
index 7f7e0ac..5ec8775 100644
--- a/go.sum
+++ b/go.sum
@@ -3,6 +3,10 @@ github.com/google/go-cmp v0.5.5 h1:Khx7svrCpmxxtHBq5j2mp/xVjsi8hQMfNLvJFAlrGgU=
 github.com/google/go-cmp v0.5.5/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
+golang.org/x/net v0.22.0 h1:9sGLhx7iRIHEiX0oAJ3MRZMUCElJgy7Br1nO+AMN3Tc=
+golang.org/x/net v0.22.0/go.mod h1:JKghWKKOSdJwpW2GEx0Ja7fmaKnMsbu+MWVZTokSYmg=
+golang.org/x/sys v0.18.0 h1:DBdB3niSjOA/O0blCZBqDefyWNYveAYMNF1Wum0DYQ4=
+golang.org/x/sys v0.18.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 google.golang.org/protobuf v1.26.0-rc.1/go.mod h1:jlhhOSvTdKEhbULTjvd4ARK9grFBp09yW+WbY/TyQbw=