Fix UDP Socket Binding Issue on Linux for IPv4 and IPv6 (#582)

* Fix UDP Socket Binding Issue on Linux for IPv4 and IPv6

On Linux, when using UDP sockets, both IPv6 and IPv4 addresses are
bound to the same socket by default. This can cause issues when
sending and receiving packets across different address families.
Specifically, if we receive a packet from an IPv4 address,
attempting to send a response using an IPv6 address fails.

net/connUDP.go

@@ -516,6 +516,13 @@ func (c *UDPConn) writeTo(raddr *net.UDPAddr, cm *ControlMessage, buffer []byte)
 		// because the connection is already established.
 		return c.connection.Write(buffer)
 	}
+	// On Linux, UDP network binds both IPv6 and IPv4 addresses to the same socket.
+	// When receiving a packet from an IPv4 address, we cannot send a packet from an IPv6 address.
+	// Therefore, we wrap the connection using an IPv4 packet connection (packetConn).
+	if !IsIPv6(raddr.IP) && c.packetConn.IsIPv6() {
+		pc := packetConnIPv4{packetConn: ipv4.NewPacketConn(c.connection)}
+		return pc.WriteTo(buffer, cm, raddr)
+	}
 	return c.packetConn.WriteTo(buffer, cm, raddr)
 }
 

net/connUDP_internal_test.go

@@ -3,6 +3,7 @@ package net
 import (
 	"context"
 	"net"
+	"runtime"
 	"strconv"
 	"sync"
 	"testing"
@@ -19,53 +20,63 @@ const (
 )
 
 func TestUDPConnWriteWithContext(t *testing.T) {
-	peerAddr := "127.0.0.1:2154"
+	iface := getInterfaceIndex(t)
+	ifaceIpv4 := getIfaceAddr(t, iface, true)
+	peerAddr := ifaceIpv4.String() + ":2154"
 	b, err := net.ResolveUDPAddr(udpNetwork, peerAddr)
 	require.NoError(t, err)
 
 	ctxCanceled, ctxCancel := context.WithCancel(context.Background())
 	ctxCancel()
 
 	type args struct {
-		ctx     context.Context
-		udpAddr *net.UDPAddr
-		buffer  []byte
+		ctx           context.Context
+		listenNetwork string
+		udpAddr       *net.UDPAddr
+		buffer        []byte
 	}
 	tests := []struct {
 		name    string
 		args    args
 		wantErr bool
 	}{
 		{
-			name: "valid",
+			name: "valid - udp4 network",
 			args: args{
-				ctx:     context.Background(),
-				udpAddr: b,
-				buffer:  []byte("hello world"),
+				ctx:           context.Background(),
+				listenNetwork: udp4Network,
+				udpAddr:       b,
+				buffer:        []byte("hello world"),
 			},
 		},
 		{
 			name: "cancelled",
 			args: args{
-				ctx:    ctxCanceled,
-				buffer: []byte("hello world"),
+				ctx:           ctxCanceled,
+				listenNetwork: udp4Network,
+				buffer:        []byte("hello world"),
 			},
 			wantErr: true,
 		},
 	}
+	if runtime.GOOS == "linux" {
+		tests = append(tests, struct {
+			name    string
+			args    args
+			wantErr bool
+		}{
+			name: "valid - udp network",
+			args: args{
+				ctx:           context.Background(),
+				listenNetwork: udpNetwork,
+				udpAddr:       b,
+				buffer:        []byte("hello world"),
+			},
+		})
+	}
 
-	a, err := net.ResolveUDPAddr(udpNetwork, "127.0.0.1:")
-	require.NoError(t, err)
-	l1, err := net.ListenUDP(udpNetwork, a)
-	require.NoError(t, err)
-	c1 := NewUDPConn(udpNetwork, l1, WithErrors(func(err error) { t.Log(err) }))
-	defer func() {
-		errC := c1.Close()
-		require.NoError(t, errC)
-	}()
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-
 	l2, err := net.ListenUDP(udpNetwork, b)
 	require.NoError(t, err)
 	c2 := NewUDPConn(udpNetwork, l2, WithErrors(func(err error) { t.Log(err) }))
@@ -84,7 +95,19 @@ func TestUDPConnWriteWithContext(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			err = c1.WriteWithContext(tt.args.ctx, tt.args.udpAddr, tt.args.buffer)
+			a, err := net.ResolveUDPAddr(tt.args.listenNetwork, ":")
+			require.NoError(t, err)
+			l1, err := net.ListenUDP(tt.args.listenNetwork, a)
+			require.NoError(t, err)
+			c1 := NewUDPConn(tt.args.listenNetwork, l1, WithErrors(func(err error) { t.Log(err) }))
+			defer func() {
+				errC := c1.Close()
+				require.NoError(t, errC)
+			}()
+
+			err = c1.WriteWithOptions(tt.args.buffer, WithContext(ctx), WithRemoteAddr(tt.args.udpAddr), WithControlMessage(&ControlMessage{
+				IfIndex: iface.Index,
+			}))
 
 			c1.LocalAddr()
 			c1.RemoteAddr()
@@ -312,18 +335,21 @@ func isActiveMulticastInterface(iface net.Interface) bool {
 	return false
 }
 
-func TestUDPConnWriteToAddr(t *testing.T) {
+func getInterfaceIndex(t *testing.T) net.Interface {
 	ifaces, err := net.Interfaces()
 	require.NoError(t, err)
-	var iface net.Interface
 	for _, i := range ifaces {
 		t.Logf("interface name:%v, flags: %v", i.Name, i.Flags)
 		if isActiveMulticastInterface(i) {
-			iface = i
-			break
+			return i
 		}
 	}
-	require.NotEmpty(t, iface)
+	require.Fail(t, "No suitable interface found")
+	return net.Interface{}
+}
+
+func TestUDPConnWriteToAddr(t *testing.T) {
+	iface := getInterfaceIndex(t)
 	type args struct {
 		iface             *net.Interface
 		src               net.IP