feat: add VRRP unicast peer support for macvlan and restricted network environments

- Add unicast_src_ip and unicast_peers config fields
- NewUnicastSocket binds to source IP for proper packet reception
- SendTo() sends VRRP advertisements directly to peer IPs
- Receive loop filters packets from non-peer sources in unicast mode
- Backward compatible: multicast mode used when unicast_peers is empty
- Add unicast config example (etc/config.unicast.yaml)
- Update README with unicast documentation and examples

internal/vrrp/socket.go

@@ -47,29 +47,11 @@ func NewSocket(ifaceName string) (*Socket, error) {
 		return nil, fmt.Errorf("no IPv4 address found on interface %s", ifaceName)
 	}
 
-	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, VRRPProtocolNumber)
+	rawConn, packetConn, err := createRawConn()
 	if err != nil {
 		return nil, fmt.Errorf("failed to create raw socket: %w", err)
 	}
 
-	if err := syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1); err != nil {
-		syscall.Close(fd)
-		return nil, fmt.Errorf("failed to set SO_REUSEADDR: %w", err)
-	}
-
-	file := os.NewFile(uintptr(fd), "vrrp-socket")
-	packetConn, err := net.FilePacketConn(file)
-	file.Close()
-	if err != nil {
-		return nil, fmt.Errorf("failed to create packet connection: %w", err)
-	}
-
-	rawConn, err := ipv4.NewRawConn(packetConn)
-	if err != nil {
-		packetConn.Close()
-		return nil, fmt.Errorf("failed to create raw connection: %w", err)
-	}
-
 	groupIP := net.ParseIP(VRRPMulticastAddr).To4()
 	if groupIP == nil {
 		rawConn.Close()
@@ -95,6 +77,71 @@ func NewSocket(ifaceName string) (*Socket, error) {
 	}, nil
 }
 
+func NewUnicastSocket(ifaceName string, srcIP net.IP) (*Socket, error) {
+	iface, err := net.InterfaceByName(ifaceName)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get interface %s: %w", ifaceName, err)
+	}
+
+	rawConn, packetConn, err := createRawConn()
+	if err != nil {
+		return nil, err
+	}
+
+	// Bind to the specific interface and source IP so we receive unicast VRRP packets
+	bindAddr := &syscall.SockaddrInet4{}
+	copy(bindAddr.Addr[:], srcIP.To4())
+	fd, _ := rawConn.SyscallConn()
+	var bindErr error
+	fd.Control(func(fd uintptr) {
+		bindErr = syscall.Bind(int(fd), bindAddr)
+	})
+	if bindErr != nil {
+		rawConn.Close()
+		return nil, fmt.Errorf("failed to bind to %s: %w", srcIP, bindErr)
+	}
+
+	if err := rawConn.SetControlMessage(ipv4.FlagTTL|ipv4.FlagSrc|ipv4.FlagDst|ipv4.FlagInterface, true); err != nil {
+		rawConn.Close()
+		return nil, fmt.Errorf("failed to set control message: %w", err)
+	}
+
+	return &Socket{
+		conn:       rawConn,
+		packetConn: packetConn,
+		iface:      iface,
+		localIP:    srcIP,
+		groupIP:    nil,
+	}, nil
+}
+
+func createRawConn() (*ipv4.RawConn, net.PacketConn, error) {
+	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, VRRPProtocolNumber)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to create raw socket: %w", err)
+	}
+
+	if err := syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1); err != nil {
+		syscall.Close(fd)
+		return nil, nil, fmt.Errorf("failed to set SO_REUSEADDR: %w", err)
+	}
+
+	file := os.NewFile(uintptr(fd), "vrrp-socket")
+	packetConn, err := net.FilePacketConn(file)
+	file.Close()
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to create packet connection: %w", err)
+	}
+
+	rawConn, err := ipv4.NewRawConn(packetConn)
+	if err != nil {
+		packetConn.Close()
+		return nil, nil, fmt.Errorf("failed to create raw connection: %w", err)
+	}
+
+	return rawConn, packetConn, nil
+}
+
 func (s *Socket) Send(pkt *VRRPPacket) error {
 	data, err := pkt.Marshal()
 	if err != nil {
@@ -119,6 +166,30 @@ func (s *Socket) Send(pkt *VRRPPacket) error {
 	return nil
 }
 
+func (s *Socket) SendTo(pkt *VRRPPacket, dst net.IP) error {
+	data, err := pkt.Marshal()
+	if err != nil {
+		return fmt.Errorf("failed to marshal packet: %w", err)
+	}
+
+	header := &ipv4.Header{
+		Version:  ipv4.Version,
+		Len:      ipv4.HeaderLen,
+		TOS:      0xC0,
+		TotalLen: ipv4.HeaderLen + len(data),
+		TTL:      255,
+		Protocol: VRRPProtocolNumber,
+		Dst:      dst,
+		Src:      s.localIP,
+	}
+
+	if err := s.conn.WriteTo(header, data, nil); err != nil {
+		return fmt.Errorf("failed to send packet to %s: %w", dst, err)
+	}
+
+	return nil
+}
+
 func (s *Socket) Receive() (*VRRPPacket, net.IP, error) {
 	buf := make([]byte, 1500)
 
@@ -140,8 +211,10 @@ func (s *Socket) SetReadDeadline(t time.Time) error {
 }
 
 func (s *Socket) Close() error {
-	if err := s.conn.LeaveGroup(s.iface, &net.IPAddr{IP: s.groupIP}); err != nil {
-		return err
+	if s.groupIP != nil {
+		if err := s.conn.LeaveGroup(s.iface, &net.IPAddr{IP: s.groupIP}); err != nil {
+			return err
+		}
 	}
 	return s.conn.Close()
-}
+}