Implement IP_MULTICAST_IF.

This allows setting a default send interface for IPv4 multicast. IPv6 support
will come later.

PiperOrigin-RevId: 234251379
Change-Id: I65922341cd8b8880f690fae3eeb7ddfa47c8c173

3 files changed, 105 insertions, 28 deletions

diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 854ebe1bb..252c79317 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -565,6 +565,17 @@ func (s *Stack) EnableNIC(id tcpip.NICID) *tcpip.Error {
 	return nil
 }
 
+// CheckNIC checks if a NIC is usable.
+func (s *Stack) CheckNIC(id tcpip.NICID) bool {
+	s.mu.RLock()
+	nic, ok := s.nics[id]
+	s.mu.RUnlock()
+	if ok {
+		return nic.linkEP.IsAttached()
+	}
+	return false
+}
+
 // NICSubnets returns a map of NICIDs to their associated subnets.
 func (s *Stack) NICSubnets() map[tcpip.NICID][]tcpip.Subnet {
 	s.mu.RLock()
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index 3cd431d4c..a6e47397a 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -473,6 +473,13 @@ type KeepaliveCountOption int
 // TTL value for multicast messages. The default is 1.
 type MulticastTTLOption uint8
 
+// MulticastInterfaceOption is used by SetSockOpt/GetSockOpt to specify a
+// default interface for multicast.
+type MulticastInterfaceOption struct {
+	NIC           NICID
+	InterfaceAddr Address
+}
+
 // MembershipOption is used by SetSockOpt/GetSockOpt as an argument to
 // AddMembershipOption and RemoveMembershipOption.
 type MembershipOption struct {
diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index fa8f02e46..9c3881d63 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -69,17 +69,19 @@ type endpoint struct {
 	rcvClosed     bool
 
 	// The following fields are protected by the mu mutex.
-	mu           sync.RWMutex `state:"nosave"`
-	sndBufSize   int
-	id           stack.TransportEndpointID
-	state        endpointState
-	bindNICID    tcpip.NICID
-	regNICID     tcpip.NICID
-	route        stack.Route `state:"manual"`
-	dstPort      uint16
-	v6only       bool
-	multicastTTL uint8
-	reusePort    bool
+	mu             sync.RWMutex `state:"nosave"`
+	sndBufSize     int
+	id             stack.TransportEndpointID
+	state          endpointState
+	bindNICID      tcpip.NICID
+	regNICID       tcpip.NICID
+	route          stack.Route `state:"manual"`
+	dstPort        uint16
+	v6only         bool
+	multicastTTL   uint8
+	multicastAddr  tcpip.Address
+	multicastNICID tcpip.NICID
+	reusePort      bool
 
 	// shutdownFlags represent the current shutdown state of the endpoint.
 	shutdownFlags tcpip.ShutdownFlags
@@ -251,6 +253,33 @@ func (e *endpoint) prepareForWrite(to *tcpip.FullAddress) (retry bool, err *tcpi
 	return true, nil
 }
 
+// connectRoute establishes a route to the specified interface or the
+// configured multicast interface if no interface is specified and the
+// specified address is a multicast address.
+func (e *endpoint) connectRoute(nicid tcpip.NICID, addr tcpip.FullAddress) (stack.Route, tcpip.NICID, tcpip.NetworkProtocolNumber, *tcpip.Error) {
+	netProto, err := e.checkV4Mapped(&addr, false)
+	if err != nil {
+		return stack.Route{}, 0, 0, err
+	}
+
+	localAddr := e.id.LocalAddress
+	if header.IsV4MulticastAddress(addr.Addr) || header.IsV6MulticastAddress(addr.Addr) {
+		if nicid == 0 {
+			nicid = e.multicastNICID
+		}
+		if localAddr == "" {
+			localAddr = e.multicastAddr
+		}
+	}
+
+	// Find a route to the desired destination.
+	r, err := e.stack.FindRoute(nicid, localAddr, addr.Addr, netProto)
+	if err != nil {
+		return stack.Route{}, 0, 0, err
+	}
+	return r, nicid, netProto, nil
+}
+
 // Write writes data to the endpoint's peer. This method does not block
 // if the data cannot be written.
 func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, <-chan struct{}, *tcpip.Error) {
@@ -318,15 +347,7 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, <-c
 			nicid = e.bindNICID
 		}
 
-		toCopy := *to
-		to = &toCopy
-		netProto, err := e.checkV4Mapped(to, false)
-		if err != nil {
-			return 0, nil, err
-		}
-
-		// Find the enpoint.
-		r, err := e.stack.FindRoute(nicid, e.id.LocalAddress, to.Addr, netProto)
+		r, _, _, err := e.connectRoute(nicid, *to)
 		if err != nil {
 			return 0, nil, err
 		}
@@ -394,6 +415,42 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		e.multicastTTL = uint8(v)
 		e.mu.Unlock()
 
+	case tcpip.MulticastInterfaceOption:
+		e.mu.Lock()
+		defer e.mu.Unlock()
+
+		fa := tcpip.FullAddress{Addr: v.InterfaceAddr}
+		netProto, err := e.checkV4Mapped(&fa, false)
+		if err != nil {
+			return err
+		}
+		nic := v.NIC
+		addr := fa.Addr
+
+		if nic == 0 && addr == "" {
+			e.multicastAddr = ""
+			e.multicastNICID = 0
+			break
+		}
+
+		if nic != 0 {
+			if !e.stack.CheckNIC(nic) {
+				return tcpip.ErrBadLocalAddress
+			}
+		} else {
+			nic = e.stack.CheckLocalAddress(0, netProto, addr)
+			if nic == 0 {
+				return tcpip.ErrBadLocalAddress
+			}
+		}
+
+		if e.bindNICID != 0 && e.bindNICID != nic {
+			return tcpip.ErrInvalidEndpointState
+		}
+
+		e.multicastNICID = nic
+		e.multicastAddr = addr
+
 	case tcpip.AddMembershipOption:
 		nicID := v.NIC
 		if v.InterfaceAddr != header.IPv4Any {
@@ -445,7 +502,6 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		e.mu.Lock()
 		e.reusePort = v != 0
 		e.mu.Unlock()
-		return nil
 	}
 	return nil
 }
@@ -501,6 +557,15 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 		e.mu.Unlock()
 		return nil
 
+	case *tcpip.MulticastInterfaceOption:
+		e.mu.Lock()
+		*o = tcpip.MulticastInterfaceOption{
+			e.multicastNICID,
+			e.multicastAddr,
+		}
+		e.mu.Unlock()
+		return nil
+
 	case *tcpip.ReusePortOption:
 		e.mu.RLock()
 		v := e.reusePort
@@ -610,13 +675,7 @@ func (e *endpoint) Connect(addr tcpip.FullAddress) *tcpip.Error {
 		return tcpip.ErrInvalidEndpointState
 	}
 
-	netProto, err := e.checkV4Mapped(&addr, false)
-	if err != nil {
-		return err
-	}
-
-	// Find a route to the desired destination.
-	r, err := e.stack.FindRoute(nicid, e.id.LocalAddress, addr.Addr, netProto)
+	r, nicid, netProto, err := e.connectRoute(nicid, addr)
 	if err != nil {
 		return err
 	}