Add support for multicast (#243)

core/nic.go

@@ -2,8 +2,11 @@ package core
 
 import (
 	"fmt"
+	"net"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/network/ipv4"
+	"gvisor.dev/gvisor/pkg/tcpip/network/ipv6"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 
 	"github.com/xjasonlyu/tun2socks/v2/core/option"
@@ -56,3 +59,61 @@ func withSpoofing(nicID tcpip.NICID, v bool) option.Option {
 		return nil
 	}
 }
+
+// withMulticastGroups adds a NIC to the given multicast groups.
+func withMulticastGroups(nicID tcpip.NICID, multicastGroups []net.IP) option.Option {
+	return func(s *stack.Stack) error {
+		if multicastGroups == nil {
+			return nil
+		}
+		// The default NIC of tun2 is working on Spoofing mode. When the UDP Endpoint
+		// tries to use a non-local address to connect, the network stack will
+		// generate a temporary addressState to build the route, which can be primary
+		// but is ephemeral. Nevertheless, when the UDP Endpoint tries to use a
+		// multicast address to connect, the network stack will select an available
+		// primary addressState to build the route. However, when tun2socks is in the
+		// just-initialized or idle state, there will be no available primary addressState,
+		// and the connect operation will fail. Therefore, we need to add permanent addresses,
+		// e.g. 10.0.0.1/8 and fd00:1/8, to the default NIC, which are only used to build
+		// routes for multicast response and do not affect other connections.
+		//
+		// In fact, for multicast, the sender normally does not expect a response.
+		// So, the ep.net.Connect is unnecessary. If we implement a custom UDP Forwarder
+		// and ForwarderRequest in the future, we can remove these code.
+		s.AddProtocolAddress(
+			nicID,
+			tcpip.ProtocolAddress{
+				Protocol: ipv4.ProtocolNumber,
+				AddressWithPrefix: tcpip.AddressWithPrefix{
+					Address:   "\x0A\x00\x00\x01",
+					PrefixLen: 8,
+				},
+			},
+			stack.AddressProperties{PEB: stack.CanBePrimaryEndpoint},
+		)
+		s.AddProtocolAddress(
+			nicID,
+			tcpip.ProtocolAddress{
+				Protocol: ipv6.ProtocolNumber,
+				AddressWithPrefix: tcpip.AddressWithPrefix{
+					Address:   "\xfd\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01",
+					PrefixLen: 8,
+				},
+			},
+			stack.AddressProperties{PEB: stack.CanBePrimaryEndpoint},
+		)
+		for _, multicastGroup := range multicastGroups {
+			if tcpIpAddr := multicastGroup.To4(); tcpIpAddr != nil {
+				if err := s.JoinGroup(ipv4.ProtocolNumber, nicID, tcpip.Address(tcpIpAddr)); err != nil {
+					return fmt.Errorf("join multicast groups: %s", err)
+				}
+			} else {
+				tcpIpAddr := multicastGroup.To16()
+				if err := s.JoinGroup(ipv6.ProtocolNumber, nicID, tcpip.Address(tcpIpAddr)); err != nil {
+					return fmt.Errorf("join multicast groups: %s", err)
+				}
+			}
+		}
+		return nil
+	}
+}

core/stack.go

@@ -1,6 +1,8 @@
 package core
 
 import (
+	"net"
+
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/network/ipv4"
 	"gvisor.dev/gvisor/pkg/tcpip/network/ipv6"
@@ -23,6 +25,10 @@ type Config struct {
 	// stack to set transport handlers.
 	TransportHandler adapter.TransportHandler
 
+	// MulticastGroups is used by internal stack to add
+	// nic to given groups
+	MulticastGroups []net.IP
+
 	// Options are supplement options to apply settings
 	// for the internal stack.
 	Options []option.Option
@@ -88,6 +94,9 @@ func CreateStack(cfg *Config) (*stack.Stack, error) {
 		// Add default route table for IPv4 and IPv6. This will handle
 		// all incoming ICMP packets.
 		withRouteTable(nicID),
+
+		// Adds default NIC to the given multicast groups.
+		withMulticastGroups(nicID, cfg.MulticastGroups),
 	)
 
 	for _, opt := range opts {

engine/engine.go

@@ -214,9 +214,15 @@ func netstack(k *Key) (err error) {
 		opts = append(opts, option.WithTCPReceiveBufferSize(int(size)))
 	}
 
+	var multicastGroups []net.IP = nil
+	if multicastGroups, err = parseMulticastGroups(k.MulticastGroups); err != nil {
+		return err
+	}
+
 	if _defaultStack, err = core.CreateStack(&core.Config{
 		LinkEndpoint:     _defaultDevice,
 		TransportHandler: &mirror.Tunnel{},
+		MulticastGroups:  multicastGroups,
 		Options:          opts,
 	}); err != nil {
 		return

engine/key.go

@@ -13,6 +13,7 @@ type Key struct {
 	TCPModerateReceiveBuffer bool          `yaml:"tcp-moderate-receive-buffer"`
 	TCPSendBufferSize        string        `yaml:"tcp-send-buffer-size"`
 	TCPReceiveBufferSize     string        `yaml:"tcp-receive-buffer-size"`
+	MulticastGroups          string        `yaml:"multicast-groups"`
 	TUNPreUp                 string        `yaml:"tun-pre-up"`
 	TUNPostUp                string        `yaml:"tun-post-up"`
 	UDPTimeout               time.Duration `yaml:"udp-timeout"`

engine/parse.go

@@ -150,3 +150,22 @@ func parseShadowsocks(u *url.URL) (address, method, password, obfsMode, obfsHost
 
 	return
 }
+
+func parseMulticastGroups(multicastGroupsString string) ([]net.IP, error) {
+	if multicastGroupsString == "" {
+		return nil, nil
+	}
+	ipStrings := strings.Split(multicastGroupsString, ",")
+	multicastGroups := []net.IP{}
+	for _, ipString := range ipStrings {
+		ip := net.ParseIP(ipString)
+		if ip == nil {
+			return nil, fmt.Errorf("unsupported ip format: %s", ipString)
+		}
+		if !ip.IsMulticast() {
+			return nil, fmt.Errorf("invalid multicast address: %s", ipString)
+		}
+		multicastGroups = append(multicastGroups, ip)
+	}
+	return multicastGroups, nil
+}

main.go

@@ -36,6 +36,7 @@ func init() {
 	flag.StringVar(&key.TCPSendBufferSize, "tcp-sndbuf", "", "Set TCP send buffer size for netstack")
 	flag.StringVar(&key.TCPReceiveBufferSize, "tcp-rcvbuf", "", "Set TCP receive buffer size for netstack")
 	flag.BoolVar(&key.TCPModerateReceiveBuffer, "tcp-auto-tuning", false, "Enable TCP receive buffer auto-tuning")
+	flag.StringVar(&key.MulticastGroups, "multicast-groups", "", "Join these multicast groups ip1,ip2,...")
 	flag.StringVar(&key.TUNPreUp, "tun-pre-up", "", "Execute a command before TUN device setup")
 	flag.StringVar(&key.TUNPostUp, "tun-post-up", "", "Execute a command after TUN device setup")
 	flag.BoolVar(&versionFlag, "version", false, "Show version and then quit")