Add multicast support

PiperOrigin-RevId: 212750821
Change-Id: I822fd63e48c684b45fd91f9ce057867b7eceb792

23 files changed, 602 insertions, 150 deletions

diff --git a/pkg/tcpip/checker/checker.go b/pkg/tcpip/checker/checker.go
index 518719de4..8e0e49efa 100644
--- a/pkg/tcpip/checker/checker.go
+++ b/pkg/tcpip/checker/checker.go
@@ -86,6 +86,22 @@ func DstAddr(addr tcpip.Address) NetworkChecker {
 	}
 }
 
+// TTL creates a checker that checks the TTL (ipv4) or HopLimit (ipv6).
+func TTL(ttl uint8) NetworkChecker {
+	return func(t *testing.T, h []header.Network) {
+		var v uint8
+		switch ip := h[0].(type) {
+		case header.IPv4:
+			v = ip.TTL()
+		case header.IPv6:
+			v = ip.HopLimit()
+		}
+		if v != ttl {
+			t.Fatalf("Bad TTL, got %v, want %v", v, ttl)
+		}
+	}
+}
+
 // PayloadLen creates a checker that checks the payload length.
 func PayloadLen(plen int) NetworkChecker {
 	return func(t *testing.T, h []header.Network) {
diff --git a/pkg/tcpip/header/ipv4.go b/pkg/tcpip/header/ipv4.go
index 950c54f74..29570cc34 100644
--- a/pkg/tcpip/header/ipv4.go
+++ b/pkg/tcpip/header/ipv4.go
@@ -93,6 +93,12 @@ const (
 
 	// IPv4Version is the version of the ipv4 protocol.
 	IPv4Version = 4
+
+	// IPv4Broadcast is the broadcast address of the IPv4 procotol.
+	IPv4Broadcast tcpip.Address = "\xff\xff\xff\xff"
+
+	// IPv4Any is the non-routable IPv4 "any" meta address.
+	IPv4Any tcpip.Address = "\x00\x00\x00\x00"
 )
 
 // Flags that may be set in an IPv4 packet.
@@ -259,3 +265,13 @@ func (b IPv4) IsValid(pktSize int) bool {
 
 	return true
 }
+
+// IsV4MulticastAddress determines if the provided address is an IPv4 multicast
+// address (range 224.0.0.0 to 239.255.255.255). The four most significant bits
+// will be 1110 = 0xe0.
+func IsV4MulticastAddress(addr tcpip.Address) bool {
+	if len(addr) != IPv4AddressSize {
+		return false
+	}
+	return (addr[0] & 0xf0) == 0xe0
+}
diff --git a/pkg/tcpip/header/ipv6.go b/pkg/tcpip/header/ipv6.go
index 62bc23c27..066e48a9a 100644
--- a/pkg/tcpip/header/ipv6.go
+++ b/pkg/tcpip/header/ipv6.go
@@ -193,3 +193,12 @@ func IsV4MappedAddress(addr tcpip.Address) bool {
 
 	return strings.HasPrefix(string(addr), "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff")
 }
+
+// IsV6MulticastAddress determines if the provided address is an IPv6
+// multicast address (anything starting with FF).
+func IsV6MulticastAddress(addr tcpip.Address) bool {
+	if len(addr) != IPv6AddressSize {
+		return false
+	}
+	return addr[0] == 0xff
+}
diff --git a/pkg/tcpip/network/arp/arp.go b/pkg/tcpip/network/arp/arp.go
index 6bf4be868..8f64e3f42 100644
--- a/pkg/tcpip/network/arp/arp.go
+++ b/pkg/tcpip/network/arp/arp.go
@@ -51,6 +51,11 @@ type endpoint struct {
 	linkAddrCache stack.LinkAddressCache
 }
 
+// DefaultTTL is unused for ARP. It implements stack.NetworkEndpoint.
+func (e *endpoint) DefaultTTL() uint8 {
+	return 0
+}
+
 func (e *endpoint) MTU() uint32 {
 	lmtu := e.linkEP.MTU()
 	return lmtu - uint32(e.MaxHeaderLength())
@@ -74,7 +79,7 @@ func (e *endpoint) MaxHeaderLength() uint16 {
 
 func (e *endpoint) Close() {}
 
-func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber) *tcpip.Error {
+func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
 	return tcpip.ErrNotSupported
 }
 
diff --git a/pkg/tcpip/network/ip_test.go b/pkg/tcpip/network/ip_test.go
index 1e92b7ae9..0cb53fb42 100644
--- a/pkg/tcpip/network/ip_test.go
+++ b/pkg/tcpip/network/ip_test.go
@@ -222,7 +222,7 @@ func TestIPv4Send(t *testing.T) {
 		t.Fatalf("could not find route: %v", err)
 	}
 	vv := buffer.NewVectorisedView(len(payload), []buffer.View{payload})
-	if err := ep.WritePacket(&r, &hdr, vv, 123); err != nil {
+	if err := ep.WritePacket(&r, &hdr, vv, 123, 123); err != nil {
 		t.Fatalf("WritePacket failed: %v", err)
 	}
 }
@@ -461,7 +461,7 @@ func TestIPv6Send(t *testing.T) {
 		t.Fatalf("could not find route: %v", err)
 	}
 	vv := buffer.NewVectorisedView(len(payload), []buffer.View{payload})
-	if err := ep.WritePacket(&r, &hdr, vv, 123); err != nil {
+	if err := ep.WritePacket(&r, &hdr, vv, 123, 123); err != nil {
 		t.Fatalf("WritePacket failed: %v", err)
 	}
 }
diff --git a/pkg/tcpip/network/ipv4/BUILD b/pkg/tcpip/network/ipv4/BUILD
index 19314e9bd..90d65d531 100644
--- a/pkg/tcpip/network/ipv4/BUILD
+++ b/pkg/tcpip/network/ipv4/BUILD
@@ -1,6 +1,6 @@
 package(licenses = ["notice"])  # Apache 2.0
 
-load("//tools/go_stateify:defs.bzl", "go_library")
+load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
 
 go_library(
     name = "ipv4",
@@ -21,3 +21,19 @@ go_library(
         "//pkg/tcpip/stack",
     ],
 )
+
+go_test(
+    name = "ipv4_test",
+    size = "small",
+    srcs = ["ipv4_test.go"],
+    deps = [
+        "//pkg/tcpip",
+        "//pkg/tcpip/header",
+        "//pkg/tcpip/link/channel",
+        "//pkg/tcpip/link/sniffer",
+        "//pkg/tcpip/network/ipv4",
+        "//pkg/tcpip/stack",
+        "//pkg/tcpip/transport/udp",
+        "//pkg/waiter",
+    ],
+)
diff --git a/pkg/tcpip/network/ipv4/icmp.go b/pkg/tcpip/network/ipv4/icmp.go
index de21e623e..74454f605 100644
--- a/pkg/tcpip/network/ipv4/icmp.go
+++ b/pkg/tcpip/network/ipv4/icmp.go
@@ -121,5 +121,5 @@ func sendPing4(r *stack.Route, code byte, data buffer.View) *tcpip.Error {
 	icmpv4.SetChecksum(^header.Checksum(icmpv4, header.Checksum(data, 0)))
 
 	vv := buffer.NewVectorisedView(len(data), []buffer.View{data})
-	return r.WritePacket(&hdr, vv, header.ICMPv4ProtocolNumber)
+	return r.WritePacket(&hdr, vv, header.ICMPv4ProtocolNumber, r.DefaultTTL())
 }
diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index 478957827..0a2378a6a 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -44,10 +44,6 @@ const (
 
 	// buckets is the number of identifier buckets.
 	buckets = 2048
-
-	// defaultIPv4TTL is the defautl TTL for IPv4 Packets egressed by
-	// Netstack.
-	defaultIPv4TTL = 255
 )
 
 type address [header.IPv4AddressSize]byte
@@ -78,6 +74,11 @@ func newEndpoint(nicid tcpip.NICID, addr tcpip.Address, dispatcher stack.Transpo
 	return e
 }
 
+// DefaultTTL is the default time-to-live value for this endpoint.
+func (e *endpoint) DefaultTTL() uint8 {
+	return 255
+}
+
 // MTU implements stack.NetworkEndpoint.MTU. It returns the link-layer MTU minus
 // the network layer max header length.
 func (e *endpoint) MTU() uint32 {
@@ -106,7 +107,7 @@ func (e *endpoint) MaxHeaderLength() uint16 {
 }
 
 // WritePacket writes a packet to the given destination address and protocol.
-func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber) *tcpip.Error {
+func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
 	ip := header.IPv4(hdr.Prepend(header.IPv4MinimumSize))
 	length := uint16(hdr.UsedLength() + payload.Size())
 	id := uint32(0)
@@ -119,7 +120,7 @@ func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload
 		IHL:         header.IPv4MinimumSize,
 		TotalLength: length,
 		ID:          uint16(id),
-		TTL:         defaultIPv4TTL,
+		TTL:         ttl,
 		Protocol:    uint8(protocol),
 		SrcAddr:     tcpip.Address(e.address[:]),
 		DstAddr:     r.RemoteAddress,
diff --git a/pkg/tcpip/network/ipv4/ipv4_test.go b/pkg/tcpip/network/ipv4/ipv4_test.go
new file mode 100644
index 000000000..2b7067a50
--- /dev/null
+++ b/pkg/tcpip/network/ipv4/ipv4_test.go
@@ -0,0 +1,92 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ipv4_test
+
+import (
+	"testing"
+
+	"gvisor.googlesource.com/gvisor/pkg/tcpip"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/header"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/link/channel"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/link/sniffer"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/network/ipv4"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/transport/udp"
+	"gvisor.googlesource.com/gvisor/pkg/waiter"
+)
+
+func TestExcludeBroadcast(t *testing.T) {
+	s := stack.New([]string{ipv4.ProtocolName}, []string{udp.ProtocolName}, stack.Options{})
+
+	const defaultMTU = 65536
+	id, _ := channel.New(256, defaultMTU, "")
+	if testing.Verbose() {
+		id = sniffer.New(id)
+	}
+	if err := s.CreateNIC(1, id); err != nil {
+		t.Fatalf("CreateNIC failed: %v", err)
+	}
+
+	if err := s.AddAddress(1, ipv4.ProtocolNumber, header.IPv4Broadcast); err != nil {
+		t.Fatalf("AddAddress failed: %v", err)
+	}
+	if err := s.AddAddress(1, ipv4.ProtocolNumber, header.IPv4Any); err != nil {
+		t.Fatalf("AddAddress failed: %v", err)
+	}
+
+	s.SetRouteTable([]tcpip.Route{{
+		Destination: "\x00\x00\x00\x00",
+		Mask:        "\x00\x00\x00\x00",
+		Gateway:     "",
+		NIC:         1,
+	}})
+
+	randomAddr := tcpip.FullAddress{NIC: 1, Addr: "\x0a\x00\x00\x01", Port: 53}
+
+	var wq waiter.Queue
+	t.Run("WithoutPrimaryAddress", func(t *testing.T) {
+		ep, err := s.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer ep.Close()
+
+		// Cannot connect using a broadcast address as the source.
+		if err := ep.Connect(randomAddr); err != tcpip.ErrNoRoute {
+			t.Errorf("got ep.Connect(...) = %v, want = %v", err, tcpip.ErrNoRoute)
+		}
+
+		// However, we can bind to a broadcast address to listen.
+		if err := ep.Bind(tcpip.FullAddress{Addr: header.IPv4Broadcast, Port: 53, NIC: 1}, nil); err != nil {
+			t.Errorf("Bind failed: %v", err)
+		}
+	})
+
+	t.Run("WithPrimaryAddress", func(t *testing.T) {
+		ep, err := s.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer ep.Close()
+
+		// Add a valid primary endpoint address, now we can connect.
+		if err := s.AddAddress(1, ipv4.ProtocolNumber, "\x0a\x00\x00\x02"); err != nil {
+			t.Fatalf("AddAddress failed: %v", err)
+		}
+		if err := ep.Connect(randomAddr); err != nil {
+			t.Errorf("Connect failed: %v", err)
+		}
+	})
+}
diff --git a/pkg/tcpip/network/ipv6/icmp.go b/pkg/tcpip/network/ipv6/icmp.go
index b18b78830..2158ba8f7 100644
--- a/pkg/tcpip/network/ipv6/icmp.go
+++ b/pkg/tcpip/network/ipv6/icmp.go
@@ -62,7 +62,6 @@ func (e *endpoint) handleControl(typ stack.ControlType, extra uint32, vv *buffer
 	e.dispatcher.DeliverTransportControlPacket(e.id.LocalAddress, h.DestinationAddress(), ProtocolNumber, p, typ, extra, vv)
 }
 
-// TODO: take buffer.VectorisedView by value.
 func (e *endpoint) handleICMP(r *stack.Route, vv *buffer.VectorisedView) {
 	v := vv.First()
 	if len(v) < header.ICMPv6MinimumSize {
@@ -107,7 +106,7 @@ func (e *endpoint) handleICMP(r *stack.Route, vv *buffer.VectorisedView) {
 		pkt[icmpV6LengthOffset] = 1
 		copy(pkt[icmpV6LengthOffset+1:], r.LocalLinkAddress[:])
 		pkt.SetChecksum(icmpChecksum(pkt, r.LocalAddress, r.RemoteAddress, buffer.VectorisedView{}))
-		r.WritePacket(&hdr, buffer.VectorisedView{}, header.ICMPv6ProtocolNumber)
+		r.WritePacket(&hdr, buffer.VectorisedView{}, header.ICMPv6ProtocolNumber, r.DefaultTTL())
 
 		e.linkAddrCache.AddLinkAddress(e.nicid, r.RemoteAddress, r.RemoteLinkAddress)
 
@@ -131,7 +130,7 @@ func (e *endpoint) handleICMP(r *stack.Route, vv *buffer.VectorisedView) {
 		copy(pkt, h)
 		pkt.SetType(header.ICMPv6EchoReply)
 		pkt.SetChecksum(icmpChecksum(pkt, r.LocalAddress, r.RemoteAddress, *vv))
-		r.WritePacket(&hdr, *vv, header.ICMPv6ProtocolNumber)
+		r.WritePacket(&hdr, *vv, header.ICMPv6ProtocolNumber, r.DefaultTTL())
 
 	case header.ICMPv6EchoReply:
 		if len(v) < header.ICMPv6EchoMinimumSize {
diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index 19dc1b49e..eb89168c3 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -43,17 +43,19 @@ const (
 	defaultIPv6HopLimit = 255
 )
 
-type address [header.IPv6AddressSize]byte
-
 type endpoint struct {
 	nicid         tcpip.NICID
 	id            stack.NetworkEndpointID
-	address       address
 	linkEP        stack.LinkEndpoint
 	linkAddrCache stack.LinkAddressCache
 	dispatcher    stack.TransportDispatcher
 }
 
+// DefaultTTL is the default hop limit for this endpoint.
+func (e *endpoint) DefaultTTL() uint8 {
+	return 255
+}
+
 // MTU implements stack.NetworkEndpoint.MTU. It returns the link-layer MTU minus
 // the network layer max header length.
 func (e *endpoint) MTU() uint32 {
@@ -82,14 +84,14 @@ func (e *endpoint) MaxHeaderLength() uint16 {
 }
 
 // WritePacket writes a packet to the given destination address and protocol.
-func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber) *tcpip.Error {
+func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
 	length := uint16(hdr.UsedLength() + payload.Size())
 	ip := header.IPv6(hdr.Prepend(header.IPv6MinimumSize))
 	ip.Encode(&header.IPv6Fields{
 		PayloadLength: length,
 		NextHeader:    uint8(protocol),
-		HopLimit:      defaultIPv6HopLimit,
-		SrcAddr:       tcpip.Address(e.address[:]),
+		HopLimit:      ttl,
+		SrcAddr:       e.id.LocalAddress,
 		DstAddr:       r.RemoteAddress,
 	})
 	r.Stats().IP.PacketsSent.Increment()
@@ -149,15 +151,13 @@ func (*protocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
 
 // NewEndpoint creates a new ipv6 endpoint.
 func (p *protocol) NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
-	e := &endpoint{
+	return &endpoint{
 		nicid:         nicid,
+		id:            stack.NetworkEndpointID{LocalAddress: addr},
 		linkEP:        linkEP,
 		linkAddrCache: linkAddrCache,
 		dispatcher:    dispatcher,
-	}
-	copy(e.address[:], addr)
-	e.id = stack.NetworkEndpointID{LocalAddress: tcpip.Address(e.address[:])}
-	return e, nil
+	}, nil
 }
 
 // SetOption implements NetworkProtocol.SetOption.
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 845b40f11..61afa673e 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -43,6 +43,25 @@ type NIC struct {
 	subnets     []tcpip.Subnet
 }
 
+// PrimaryEndpointBehavior is an enumeration of an endpoint's primacy behavior.
+type PrimaryEndpointBehavior int
+
+const (
+	// CanBePrimaryEndpoint indicates the endpoint can be used as a primary
+	// endpoint for new connections with no local address. This is the
+	// default when calling NIC.AddAddress.
+	CanBePrimaryEndpoint PrimaryEndpointBehavior = iota
+
+	// FirstPrimaryEndpoint indicates the endpoint should be the first
+	// primary endpoint considered. If there are multiple endpoints with
+	// this behavior, the most recently-added one will be first.
+	FirstPrimaryEndpoint
+
+	// NeverPrimaryEndpoint indicates the endpoint should never be a
+	// primary endpoint.
+	NeverPrimaryEndpoint
+)
+
 func newNIC(stack *Stack, id tcpip.NICID, name string, ep LinkEndpoint) *NIC {
 	return &NIC{
 		stack:     stack,
@@ -141,6 +160,11 @@ func (n *NIC) primaryEndpoint(protocol tcpip.NetworkProtocolNumber) *referencedN
 
 	for e := list.Front(); e != nil; e = e.Next() {
 		r := e.(*referencedNetworkEndpoint)
+		// TODO: allow broadcast address when SO_BROADCAST is set.
+		switch r.ep.ID().LocalAddress {
+		case header.IPv4Broadcast, header.IPv4Any:
+			continue
+		}
 		if r.tryIncRef() {
 			return r
 		}
@@ -150,7 +174,7 @@ func (n *NIC) primaryEndpoint(protocol tcpip.NetworkProtocolNumber) *referencedN
 }
 
 // findEndpoint finds the endpoint, if any, with the given address.
-func (n *NIC) findEndpoint(protocol tcpip.NetworkProtocolNumber, address tcpip.Address) *referencedNetworkEndpoint {
+func (n *NIC) findEndpoint(protocol tcpip.NetworkProtocolNumber, address tcpip.Address, peb PrimaryEndpointBehavior) *referencedNetworkEndpoint {
 	id := NetworkEndpointID{address}
 
 	n.mu.RLock()
@@ -171,7 +195,7 @@ func (n *NIC) findEndpoint(protocol tcpip.NetworkProtocolNumber, address tcpip.A
 	n.mu.Lock()
 	ref = n.endpoints[id]
 	if ref == nil || !ref.tryIncRef() {
-		ref, _ = n.addAddressLocked(protocol, address, true)
+		ref, _ = n.addAddressLocked(protocol, address, peb, true)
 		if ref != nil {
 			ref.holdsInsertRef = false
 		}
@@ -180,7 +204,7 @@ func (n *NIC) findEndpoint(protocol tcpip.NetworkProtocolNumber, address tcpip.A
 	return ref
 }
 
-func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, replace bool) (*referencedNetworkEndpoint, *tcpip.Error) {
+func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior, replace bool) (*referencedNetworkEndpoint, *tcpip.Error) {
 	netProto, ok := n.stack.networkProtocols[protocol]
 	if !ok {
 		return nil, tcpip.ErrUnknownProtocol
@@ -224,7 +248,12 @@ func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.
 		n.primary[protocol] = l
 	}
 
-	l.PushBack(ref)
+	switch peb {
+	case CanBePrimaryEndpoint:
+		l.PushBack(ref)
+	case FirstPrimaryEndpoint:
+		l.PushFront(ref)
+	}
 
 	return ref, nil
 }
@@ -232,9 +261,15 @@ func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.
 // AddAddress adds a new address to n, so that it starts accepting packets
 // targeted at the given address (and network protocol).
 func (n *NIC) AddAddress(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) *tcpip.Error {
+	return n.AddAddressWithOptions(protocol, addr, CanBePrimaryEndpoint)
+}
+
+// AddAddressWithOptions is the same as AddAddress, but allows you to specify
+// whether the new endpoint can be primary or not.
+func (n *NIC) AddAddressWithOptions(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior) *tcpip.Error {
 	// Add the endpoint.
 	n.mu.Lock()
-	_, err := n.addAddressLocked(protocol, addr, false)
+	_, err := n.addAddressLocked(protocol, addr, peb, false)
 	n.mu.Unlock()
 
 	return err
@@ -319,7 +354,11 @@ func (n *NIC) removeEndpointLocked(r *referencedNetworkEndpoint) {
 	}
 
 	delete(n.endpoints, id)
-	n.primary[r.protocol].Remove(r)
+	wasInList := r.Next() != nil || r.Prev() != nil || r == n.primary[r.protocol].Front()
+	if wasInList {
+		n.primary[r.protocol].Remove(r)
+	}
+
 	r.ep.Close()
 }
 
@@ -398,7 +437,7 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remoteLinkAddr tcpip.Lin
 			ref, ok = n.endpoints[id]
 			if !ok || !ref.tryIncRef() {
 				var err *tcpip.Error
-				ref, err = n.addAddressLocked(protocol, dst, true)
+				ref, err = n.addAddressLocked(protocol, dst, CanBePrimaryEndpoint, true)
 				if err == nil {
 					ref.holdsInsertRef = false
 				}
diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index b9e2cc045..7afde3598 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -124,6 +124,10 @@ type TransportDispatcher interface {
 // NetworkEndpoint is the interface that needs to be implemented by endpoints
 // of network layer protocols (e.g., ipv4, ipv6).
 type NetworkEndpoint interface {
+	// DefaultTTL is the default time-to-live value (or hop limit, in ipv6)
+	// for this endpoint.
+	DefaultTTL() uint8
+
 	// MTU is the maximum transmission unit for this endpoint. This is
 	// generally calculated as the MTU of the underlying data link endpoint
 	// minus the network endpoint max header length.
@@ -141,7 +145,7 @@ type NetworkEndpoint interface {
 
 	// WritePacket writes a packet to the given destination address and
 	// protocol.
-	WritePacket(r *Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber) *tcpip.Error
+	WritePacket(r *Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error
 
 	// ID returns the network protocol endpoint ID.
 	ID() *NetworkEndpointID
diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 533a0b560..9dfb95b4a 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -129,14 +129,19 @@ func (r *Route) IsResolutionRequired() bool {
 }
 
 // WritePacket writes the packet through the given route.
-func (r *Route) WritePacket(hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber) *tcpip.Error {
-	err := r.ref.ep.WritePacket(r, hdr, payload, protocol)
+func (r *Route) WritePacket(hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
+	err := r.ref.ep.WritePacket(r, hdr, payload, protocol, ttl)
 	if err == tcpip.ErrNoRoute {
 		r.Stats().IP.OutgoingPacketErrors.Increment()
 	}
 	return err
 }
 
+// DefaultTTL returns the default TTL of the underlying network endpoint.
+func (r *Route) DefaultTTL() uint8 {
+	return r.ref.ep.DefaultTTL()
+}
+
 // MTU returns the MTU of the underlying network endpoint.
 func (r *Route) MTU() uint32 {
 	return r.ref.ep.MTU()
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 675ccc6fa..789c819dd 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -607,6 +607,12 @@ type NICStateFlags struct {
 
 // AddAddress adds a new network-layer address to the specified NIC.
 func (s *Stack) AddAddress(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) *tcpip.Error {
+	return s.AddAddressWithOptions(id, protocol, addr, CanBePrimaryEndpoint)
+}
+
+// AddAddressWithOptions is the same as AddAddress, but allows you to specify
+// whether the new endpoint can be primary or not.
+func (s *Stack) AddAddressWithOptions(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior) *tcpip.Error {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
@@ -615,7 +621,7 @@ func (s *Stack) AddAddress(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber,
 		return tcpip.ErrUnknownNICID
 	}
 
-	return nic.AddAddress(protocol, addr)
+	return nic.AddAddressWithOptions(protocol, addr, peb)
 }
 
 // AddSubnet adds a subnet range to the specified NIC.
@@ -703,7 +709,7 @@ func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address, n
 
 		var ref *referencedNetworkEndpoint
 		if len(localAddr) != 0 {
-			ref = nic.findEndpoint(netProto, localAddr)
+			ref = nic.findEndpoint(netProto, localAddr, CanBePrimaryEndpoint)
 		} else {
 			ref = nic.primaryEndpoint(netProto)
 		}
@@ -746,7 +752,7 @@ func (s *Stack) CheckLocalAddress(nicid tcpip.NICID, protocol tcpip.NetworkProto
 			return 0
 		}
 
-		ref := nic.findEndpoint(protocol, addr)
+		ref := nic.findEndpoint(protocol, addr, CanBePrimaryEndpoint)
 		if ref == nil {
 			return 0
 		}
@@ -758,7 +764,7 @@ func (s *Stack) CheckLocalAddress(nicid tcpip.NICID, protocol tcpip.NetworkProto
 
 	// Go through all the NICs.
 	for _, nic := range s.nics {
-		ref := nic.findEndpoint(protocol, addr)
+		ref := nic.findEndpoint(protocol, addr, CanBePrimaryEndpoint)
 		if ref != nil {
 			ref.decRef()
 			return nic.id
@@ -926,3 +932,14 @@ func (s *Stack) RemoveTCPProbe() {
 	s.tcpProbeFunc = nil
 	s.mu.Unlock()
 }
+
+// JoinGroup joins the given multicast group on the given NIC.
+func (s *Stack) JoinGroup(protocol tcpip.NetworkProtocolNumber, nicID tcpip.NICID, multicastAddr tcpip.Address) *tcpip.Error {
+	// TODO: notify network of subscription via igmp protocol.
+	return s.AddAddressWithOptions(nicID, protocol, multicastAddr, NeverPrimaryEndpoint)
+}
+
+// LeaveGroup leaves the given multicast group on the given NIC.
+func (s *Stack) LeaveGroup(protocol tcpip.NetworkProtocolNumber, nicID tcpip.NICID, multicastAddr tcpip.Address) *tcpip.Error {
+	return s.RemoveAddress(nicID, multicastAddr)
+}
diff --git a/pkg/tcpip/stack/stack_test.go b/pkg/tcpip/stack/stack_test.go
index 02f02dfa2..816707d27 100644
--- a/pkg/tcpip/stack/stack_test.go
+++ b/pkg/tcpip/stack/stack_test.go
@@ -65,6 +65,10 @@ func (f *fakeNetworkEndpoint) NICID() tcpip.NICID {
 	return f.nicid
 }
 
+func (*fakeNetworkEndpoint) DefaultTTL() uint8 {
+	return 123
+}
+
 func (f *fakeNetworkEndpoint) ID() *stack.NetworkEndpointID {
 	return &f.id
 }
@@ -105,7 +109,7 @@ func (f *fakeNetworkEndpoint) Capabilities() stack.LinkEndpointCapabilities {
 	return f.linkEP.Capabilities()
 }
 
-func (f *fakeNetworkEndpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber) *tcpip.Error {
+func (f *fakeNetworkEndpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, _ uint8) *tcpip.Error {
 	// Increment the sent packet count in the protocol descriptor.
 	f.proto.sendPacketCount[int(r.RemoteAddress[0])%len(f.proto.sendPacketCount)]++
 
@@ -269,7 +273,7 @@ func sendTo(t *testing.T, s *stack.Stack, addr tcpip.Address) {
 	defer r.Release()
 
 	hdr := buffer.NewPrependable(int(r.MaxHeaderLength()))
-	if err := r.WritePacket(&hdr, buffer.VectorisedView{}, fakeTransNumber); err != nil {
+	if err := r.WritePacket(&hdr, buffer.VectorisedView{}, fakeTransNumber, 123); err != nil {
 		t.Errorf("WritePacket failed: %v", err)
 		return
 	}
diff --git a/pkg/tcpip/stack/transport_test.go b/pkg/tcpip/stack/transport_test.go
index 5ab485c98..e4607192f 100644
--- a/pkg/tcpip/stack/transport_test.go
+++ b/pkg/tcpip/stack/transport_test.go
@@ -71,7 +71,7 @@ func (f *fakeTransportEndpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions)
 		return 0, err
 	}
 	vv := buffer.NewVectorisedView(len(v), []buffer.View{v})
-	if err := f.route.WritePacket(&hdr, vv, fakeTransNumber); err != nil {
+	if err := f.route.WritePacket(&hdr, vv, fakeTransNumber, 123); err != nil {
 		return 0, err
 	}
 
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index 4065fc30f..51360b11f 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -453,6 +453,28 @@ type KeepaliveIntervalOption time.Duration
 // closed.
 type KeepaliveCountOption int
 
+// MulticastTTLOption is used by SetSockOpt/GetSockOpt to control the default
+// TTL value for multicast messages. The default is 1.
+type MulticastTTLOption uint8
+
+// MembershipOption is used by SetSockOpt/GetSockOpt as an argument to
+// AddMembershipOption and RemoveMembershipOption.
+type MembershipOption struct {
+	NIC           NICID
+	InterfaceAddr Address
+	MulticastAddr Address
+}
+
+// AddMembershipOption is used by SetSockOpt/GetSockOpt to join a multicast
+// group identified by the given multicast address, on the interface matching
+// the given interface address.
+type AddMembershipOption MembershipOption
+
+// RemoveMembershipOption is used by SetSockOpt/GetSockOpt to leave a multicast
+// group identified by the given multicast address, on the interface matching
+// the given interface address.
+type RemoveMembershipOption MembershipOption
+
 // Route is a row in the routing table. It specifies through which NIC (and
 // gateway) sets of packets should be routed. A row is considered viable if the
 // masked target address matches the destination adddress in the row.
diff --git a/pkg/tcpip/transport/ping/endpoint.go b/pkg/tcpip/transport/ping/endpoint.go
index fc98c41eb..7aaf2d9c6 100644
--- a/pkg/tcpip/transport/ping/endpoint.go
+++ b/pkg/tcpip/transport/ping/endpoint.go
@@ -385,7 +385,7 @@ func sendPing4(r *stack.Route, ident uint16, data buffer.View) *tcpip.Error {
 	icmpv4.SetChecksum(^header.Checksum(icmpv4, header.Checksum(data, 0)))
 
 	vv := buffer.NewVectorisedView(len(data), []buffer.View{data})
-	return r.WritePacket(&hdr, vv, header.ICMPv4ProtocolNumber)
+	return r.WritePacket(&hdr, vv, header.ICMPv4ProtocolNumber, r.DefaultTTL())
 }
 
 func sendPing6(r *stack.Route, ident uint16, data buffer.View) *tcpip.Error {
@@ -408,8 +408,9 @@ func sendPing6(r *stack.Route, ident uint16, data buffer.View) *tcpip.Error {
 
 	icmpv6.SetChecksum(0)
 	icmpv6.SetChecksum(^header.Checksum(icmpv6, header.Checksum(data, 0)))
+
 	vv := buffer.NewVectorisedView(len(data), []buffer.View{data})
-	return r.WritePacket(&hdr, vv, header.ICMPv6ProtocolNumber)
+	return r.WritePacket(&hdr, vv, header.ICMPv6ProtocolNumber, r.DefaultTTL())
 }
 
 func (e *endpoint) checkV4Mapped(addr *tcpip.FullAddress, allowMismatch bool) (tcpip.NetworkProtocolNumber, *tcpip.Error) {
diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index de5f963cf..ce87d5818 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -563,14 +563,14 @@ func sendSynTCP(r *stack.Route, id stack.TransportEndpointID, flags byte, seq, a
 	}
 
 	options := makeSynOptions(opts)
-	err := sendTCPWithOptions(r, id, buffer.VectorisedView{}, flags, seq, ack, rcvWnd, options)
+	err := sendTCP(r, id, buffer.VectorisedView{}, r.DefaultTTL(), flags, seq, ack, rcvWnd, options)
 	putOptions(options)
 	return err
 }
 
-// sendTCPWithOptions sends a TCP segment with the provided options via the
-// provided network endpoint and under the provided identity.
-func sendTCPWithOptions(r *stack.Route, id stack.TransportEndpointID, data buffer.VectorisedView, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size, opts []byte) *tcpip.Error {
+// sendTCP sends a TCP segment with the provided options via the provided
+// network endpoint and under the provided identity.
+func sendTCP(r *stack.Route, id stack.TransportEndpointID, data buffer.VectorisedView, ttl uint8, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size, opts []byte) *tcpip.Error {
 	optLen := len(opts)
 	// Allocate a buffer for the TCP header.
 	hdr := buffer.NewPrependable(header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen)
@@ -608,48 +608,7 @@ func sendTCPWithOptions(r *stack.Route, id stack.TransportEndpointID, data buffe
 		r.Stats().TCP.ResetsSent.Increment()
 	}
 
-	return r.WritePacket(&hdr, data, ProtocolNumber)
-}
-
-// sendTCP sends a TCP segment via the provided network endpoint and under the
-// provided identity.
-func sendTCP(r *stack.Route, id stack.TransportEndpointID, payload buffer.VectorisedView, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size) *tcpip.Error {
-	// Allocate a buffer for the TCP header.
-	hdr := buffer.NewPrependable(header.TCPMinimumSize + int(r.MaxHeaderLength()))
-
-	if rcvWnd > 0xffff {
-		rcvWnd = 0xffff
-	}
-
-	// Initialize the header.
-	tcp := header.TCP(hdr.Prepend(header.TCPMinimumSize))
-	tcp.Encode(&header.TCPFields{
-		SrcPort:    id.LocalPort,
-		DstPort:    id.RemotePort,
-		SeqNum:     uint32(seq),
-		AckNum:     uint32(ack),
-		DataOffset: header.TCPMinimumSize,
-		Flags:      flags,
-		WindowSize: uint16(rcvWnd),
-	})
-
-	// Only calculate the checksum if offloading isn't supported.
-	if r.Capabilities()&stack.CapabilityChecksumOffload == 0 {
-		length := uint16(hdr.UsedLength() + payload.Size())
-		xsum := r.PseudoHeaderChecksum(ProtocolNumber)
-		for _, v := range payload.Views() {
-			xsum = header.Checksum(v, xsum)
-		}
-
-		tcp.SetChecksum(^tcp.CalculateChecksum(xsum, length))
-	}
-
-	r.Stats().TCP.SegmentsSent.Increment()
-	if (flags & flagRst) != 0 {
-		r.Stats().TCP.ResetsSent.Increment()
-	}
-
-	return r.WritePacket(&hdr, payload, ProtocolNumber)
+	return r.WritePacket(&hdr, data, ProtocolNumber, ttl)
 }
 
 // makeOptions makes an options slice.
@@ -698,12 +657,7 @@ func (e *endpoint) sendRaw(data buffer.VectorisedView, flags byte, seq, ack seqn
 		sackBlocks = e.sack.Blocks[:e.sack.NumBlocks]
 	}
 	options := e.makeOptions(sackBlocks)
-	if len(options) > 0 {
-		err := sendTCPWithOptions(&e.route, e.id, data, flags, seq, ack, rcvWnd, options)
-		putOptions(options)
-		return err
-	}
-	err := sendTCP(&e.route, e.id, data, flags, seq, ack, rcvWnd)
+	err := sendTCP(&e.route, e.id, data, e.route.DefaultTTL(), flags, seq, ack, rcvWnd, options)
 	putOptions(options)
 	return err
 }
diff --git a/pkg/tcpip/transport/tcp/protocol.go b/pkg/tcpip/transport/tcp/protocol.go
index 006b2f074..fe21f2c78 100644
--- a/pkg/tcpip/transport/tcp/protocol.go
+++ b/pkg/tcpip/transport/tcp/protocol.go
@@ -147,7 +147,7 @@ func replyWithReset(s *segment) {
 
 	ack := s.sequenceNumber.Add(s.logicalLen())
 
-	sendTCP(&s.route, s.id, buffer.VectorisedView{}, flagRst|flagAck, seq, ack, 0)
+	sendTCP(&s.route, s.id, buffer.VectorisedView{}, s.route.DefaultTTL(), flagRst|flagAck, seq, ack, 0, nil)
 }
 
 // SetOption implements TransportProtocol.SetOption.
diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index d091a6196..5de518a55 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -70,19 +70,24 @@ type endpoint struct {
 	rcvTimestamp  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
+	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
 
 	// shutdownFlags represent the current shutdown state of the endpoint.
 	shutdownFlags tcpip.ShutdownFlags
 
+	// multicastMemberships that need to be remvoed when the endpoint is
+	// closed. Protected by the mu mutex.
+	multicastMemberships []multicastMembership
+
 	// effectiveNetProtos contains the network protocols actually in use. In
 	// most cases it will only contain "netProto", but in cases like IPv6
 	// endpoints with v6only set to false, this could include multiple
@@ -92,11 +97,29 @@ type endpoint struct {
 	effectiveNetProtos []tcpip.NetworkProtocolNumber
 }
 
+type multicastMembership struct {
+	nicID         tcpip.NICID
+	multicastAddr tcpip.Address
+}
+
 func newEndpoint(stack *stack.Stack, netProto tcpip.NetworkProtocolNumber, waiterQueue *waiter.Queue) *endpoint {
 	return &endpoint{
-		stack:         stack,
-		netProto:      netProto,
-		waiterQueue:   waiterQueue,
+		stack:       stack,
+		netProto:    netProto,
+		waiterQueue: waiterQueue,
+		// RFC 1075 section 5.4 recommends a TTL of 1 for membership
+		// requests.
+		//
+		// RFC 5135 4.2.1 appears to assume that IGMP messages have a
+		// TTL of 1.
+		//
+		// RFC 5135 Appendix A defines TTL=1: A multicast source that
+		// wants its traffic to not traverse a router (e.g., leave a
+		// home network) may find it useful to send traffic with IP
+		// TTL=1.
+		//
+		// Linux defaults to TTL=1.
+		multicastTTL:  1,
 		rcvBufSizeMax: 32 * 1024,
 		sndBufSize:    32 * 1024,
 	}
@@ -135,6 +158,11 @@ func (e *endpoint) Close() {
 		e.stack.ReleasePort(e.effectiveNetProtos, ProtocolNumber, e.id.LocalAddress, e.id.LocalPort)
 	}
 
+	for _, mem := range e.multicastMemberships {
+		e.stack.LeaveGroup(e.netProto, mem.nicID, mem.multicastAddr)
+	}
+	e.multicastMemberships = nil
+
 	// Close the receive list and drain it.
 	e.rcvMu.Lock()
 	e.rcvClosed = true
@@ -329,8 +357,13 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 		return 0, err
 	}
 
+	ttl := route.DefaultTTL()
+	if header.IsV4MulticastAddress(route.RemoteAddress) || header.IsV6MulticastAddress(route.RemoteAddress) {
+		ttl = e.multicastTTL
+	}
+
 	vv := buffer.NewVectorisedView(len(v), []buffer.View{v})
-	if err := sendUDP(route, vv, e.id.LocalPort, dstPort); err != nil {
+	if err := sendUDP(route, vv, e.id.LocalPort, dstPort, ttl); err != nil {
 		return 0, err
 	}
 	return uintptr(len(v)), nil
@@ -365,6 +398,56 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		e.rcvMu.Lock()
 		e.rcvTimestamp = v != 0
 		e.rcvMu.Unlock()
+
+	case tcpip.MulticastTTLOption:
+		e.mu.Lock()
+		defer e.mu.Unlock()
+		e.multicastTTL = uint8(v)
+
+	case tcpip.AddMembershipOption:
+		nicID := v.NIC
+		if v.InterfaceAddr != header.IPv4Any {
+			nicID = e.stack.CheckLocalAddress(nicID, e.netProto, v.InterfaceAddr)
+		}
+		if nicID == 0 {
+			return tcpip.ErrNoRoute
+		}
+
+		// TODO: check that v.MulticastAddr is a multicast address.
+		if err := e.stack.JoinGroup(e.netProto, nicID, v.MulticastAddr); err != nil {
+			return err
+		}
+
+		e.mu.Lock()
+		defer e.mu.Unlock()
+
+		e.multicastMemberships = append(e.multicastMemberships, multicastMembership{nicID, v.MulticastAddr})
+
+	case tcpip.RemoveMembershipOption:
+		nicID := v.NIC
+		if v.InterfaceAddr != header.IPv4Any {
+			nicID = e.stack.CheckLocalAddress(nicID, e.netProto, v.InterfaceAddr)
+		}
+		if nicID == 0 {
+			return tcpip.ErrNoRoute
+		}
+
+		// TODO: check that v.MulticastAddr is a multicast address.
+		if err := e.stack.LeaveGroup(e.netProto, nicID, v.MulticastAddr); err != nil {
+			return err
+		}
+
+		e.mu.Lock()
+		defer e.mu.Unlock()
+		for i, mem := range e.multicastMemberships {
+			if mem.nicID == nicID && mem.multicastAddr == v.MulticastAddr {
+				// Only remove the first match, so that each added membership above is
+				// paired with exactly 1 removal.
+				e.multicastMemberships[i] = e.multicastMemberships[len(e.multicastMemberships)-1]
+				e.multicastMemberships = e.multicastMemberships[:len(e.multicastMemberships)-1]
+				break
+			}
+		}
 	}
 	return nil
 }
@@ -421,6 +504,12 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 			*o = 1
 		}
 		e.rcvMu.Unlock()
+
+	case *tcpip.MulticastTTLOption:
+		e.mu.Lock()
+		*o = tcpip.MulticastTTLOption(e.multicastTTL)
+		e.mu.Unlock()
+		return nil
 	}
 
 	return tcpip.ErrUnknownProtocolOption
@@ -428,7 +517,7 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 
 // sendUDP sends a UDP segment via the provided network endpoint and under the
 // provided identity.
-func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort uint16) *tcpip.Error {
+func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort uint16, ttl uint8) *tcpip.Error {
 	// Allocate a buffer for the UDP header.
 	hdr := buffer.NewPrependable(header.UDPMinimumSize + int(r.MaxHeaderLength()))
 
@@ -454,7 +543,7 @@ func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort u
 	// Track count of packets sent.
 	r.Stats().UDP.PacketsSent.Increment()
 
-	return r.WritePacket(&hdr, data, ProtocolNumber)
+	return r.WritePacket(&hdr, data, ProtocolNumber, ttl)
 }
 
 func (e *endpoint) checkV4Mapped(addr *tcpip.FullAddress, allowMismatch bool) (tcpip.NetworkProtocolNumber, *tcpip.Error) {
@@ -581,7 +670,9 @@ func (e *endpoint) Shutdown(flags tcpip.ShutdownFlags) *tcpip.Error {
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
-	if e.state != stateConnected {
+	// A socket in the bound state can still receive multicast messages,
+	// so we need to notify waiters on shutdown.
+	if e.state != stateBound && e.state != stateConnected {
 		return tcpip.ErrNotConnected
 	}
 
diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index 4700193c2..6d7a737bd 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -34,16 +34,20 @@ import (
 )
 
 const (
-	stackV6Addr          = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01"
-	testV6Addr           = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02"
-	stackV4MappedAddr    = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff" + stackAddr
-	testV4MappedAddr     = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff" + testAddr
-	V4MappedWildcardAddr = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00"
-
-	stackAddr = "\x0a\x00\x00\x01"
-	stackPort = 1234
-	testAddr  = "\x0a\x00\x00\x02"
-	testPort  = 4096
+	stackV6Addr           = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01"
+	testV6Addr            = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02"
+	stackV4MappedAddr     = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff" + stackAddr
+	testV4MappedAddr      = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff" + testAddr
+	multicastV4MappedAddr = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff" + multicastAddr
+	V4MappedWildcardAddr  = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00"
+
+	stackAddr       = "\x0a\x00\x00\x01"
+	stackPort       = 1234
+	testAddr        = "\x0a\x00\x00\x02"
+	testPort        = 4096
+	multicastAddr   = "\xe8\x2b\xd3\xea"
+	multicastV6Addr = "\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
+	multicastPort   = 1234
 
 	// defaultMTU is the MTU, in bytes, used throughout the tests, except
 	// where another value is explicitly used. It is chosen to match the MTU
@@ -128,37 +132,35 @@ func (c *testContext) createV6Endpoint(v6only bool) {
 	}
 }
 
-func (c *testContext) getV6Packet() []byte {
+func (c *testContext) getPacket(protocolNumber tcpip.NetworkProtocolNumber, multicast bool) []byte {
 	select {
 	case p := <-c.linkEP.C:
-		if p.Proto != ipv6.ProtocolNumber {
-			c.t.Fatalf("Bad network protocol: got %v, wanted %v", p.Proto, ipv6.ProtocolNumber)
+		if p.Proto != protocolNumber {
+			c.t.Fatalf("Bad network protocol: got %v, wanted %v", p.Proto, protocolNumber)
 		}
 		b := make([]byte, len(p.Header)+len(p.Payload))
 		copy(b, p.Header)
 		copy(b[len(p.Header):], p.Payload)
 
-		checker.IPv6(c.t, b, checker.SrcAddr(stackV6Addr), checker.DstAddr(testV6Addr))
-		return b
-
-	case <-time.After(2 * time.Second):
-		c.t.Fatalf("Packet wasn't written out")
-	}
-
-	return nil
-}
-
-func (c *testContext) getPacket() []byte {
-	select {
-	case p := <-c.linkEP.C:
-		if p.Proto != ipv4.ProtocolNumber {
-			c.t.Fatalf("Bad network protocol: got %v, wanted %v", p.Proto, ipv4.ProtocolNumber)
+		var checkerFn func(*testing.T, []byte, ...checker.NetworkChecker)
+		var srcAddr, dstAddr tcpip.Address
+		switch protocolNumber {
+		case ipv4.ProtocolNumber:
+			checkerFn = checker.IPv4
+			srcAddr, dstAddr = stackAddr, testAddr
+			if multicast {
+				dstAddr = multicastAddr
+			}
+		case ipv6.ProtocolNumber:
+			checkerFn = checker.IPv6
+			srcAddr, dstAddr = stackV6Addr, testV6Addr
+			if multicast {
+				dstAddr = multicastV6Addr
+			}
+		default:
+			c.t.Fatalf("unknown protocol %d", protocolNumber)
 		}
-		b := make([]byte, len(p.Header)+len(p.Payload))
-		copy(b, p.Header)
-		copy(b[len(p.Header):], p.Payload)
-
-		checker.IPv4(c.t, b, checker.SrcAddr(stackAddr), checker.DstAddr(testAddr))
+		checkerFn(c.t, b, checker.SrcAddr(srcAddr), checker.DstAddr(dstAddr))
 		return b
 
 	case <-time.After(2 * time.Second):
@@ -495,7 +497,7 @@ func testV4Write(c *testContext) uint16 {
 	}
 
 	// Check that we received the packet.
-	b := c.getPacket()
+	b := c.getPacket(ipv4.ProtocolNumber, false)
 	udp := header.UDP(header.IPv4(b).Payload())
 	checker.IPv4(c.t, b,
 		checker.UDP(
@@ -525,7 +527,7 @@ func testV6Write(c *testContext) uint16 {
 	}
 
 	// Check that we received the packet.
-	b := c.getV6Packet()
+	b := c.getPacket(ipv6.ProtocolNumber, false)
 	udp := header.UDP(header.IPv6(b).Payload())
 	checker.IPv6(c.t, b,
 		checker.UDP(
@@ -682,7 +684,7 @@ func TestV6WriteOnConnected(t *testing.T) {
 	}
 
 	// Check that we received the packet.
-	b := c.getV6Packet()
+	b := c.getPacket(ipv6.ProtocolNumber, false)
 	udp := header.UDP(header.IPv6(b).Payload())
 	checker.IPv6(c.t, b,
 		checker.UDP(
@@ -718,7 +720,7 @@ func TestV4WriteOnConnected(t *testing.T) {
 	}
 
 	// Check that we received the packet.
-	b := c.getPacket()
+	b := c.getPacket(ipv4.ProtocolNumber, false)
 	udp := header.UDP(header.IPv4(b).Payload())
 	checker.IPv4(c.t, b,
 		checker.UDP(
@@ -769,3 +771,162 @@ func TestWriteIncrementsPacketsSent(t *testing.T) {
 		c.t.Fatalf("Write did not increment PacketsSent: got %v, want %v", got, want)
 	}
 }
+
+func TestTTL(t *testing.T) {
+	payload := tcpip.SlicePayload(buffer.View(newPayload()))
+
+	for _, name := range []string{"v4", "v6", "dual"} {
+		t.Run(name, func(t *testing.T) {
+			var networkProtocolNumber tcpip.NetworkProtocolNumber
+			switch name {
+			case "v4":
+				networkProtocolNumber = ipv4.ProtocolNumber
+			case "v6", "dual":
+				networkProtocolNumber = ipv6.ProtocolNumber
+			default:
+				t.Fatal("unknown test variant")
+			}
+
+			var variants []string
+			switch name {
+			case "v4":
+				variants = []string{"v4"}
+			case "v6":
+				variants = []string{"v6"}
+			case "dual":
+				variants = []string{"v6", "mapped"}
+			}
+
+			for _, variant := range variants {
+				t.Run(variant, func(t *testing.T) {
+					for _, typ := range []string{"unicast", "multicast"} {
+						t.Run(typ, func(t *testing.T) {
+							var addr tcpip.Address
+							var port uint16
+							switch typ {
+							case "unicast":
+								port = testPort
+								switch variant {
+								case "v4":
+									addr = testAddr
+								case "mapped":
+									addr = testV4MappedAddr
+								case "v6":
+									addr = testV6Addr
+								default:
+									t.Fatal("unknown test variant")
+								}
+							case "multicast":
+								port = multicastPort
+								switch variant {
+								case "v4":
+									addr = multicastAddr
+								case "mapped":
+									addr = multicastV4MappedAddr
+								case "v6":
+									addr = multicastV6Addr
+								default:
+									t.Fatal("unknown test variant")
+								}
+							default:
+								t.Fatal("unknown test variant")
+							}
+
+							c := newDualTestContext(t, defaultMTU)
+							defer c.cleanup()
+
+							var err *tcpip.Error
+							c.ep, err = c.s.NewEndpoint(udp.ProtocolNumber, networkProtocolNumber, &c.wq)
+							if err != nil {
+								c.t.Fatalf("NewEndpoint failed: %v", err)
+							}
+
+							switch name {
+							case "v4":
+							case "v6":
+								if err := c.ep.SetSockOpt(tcpip.V6OnlyOption(1)); err != nil {
+									c.t.Fatalf("SetSockOpt failed: %v", err)
+								}
+							case "dual":
+								if err := c.ep.SetSockOpt(tcpip.V6OnlyOption(0)); err != nil {
+									c.t.Fatalf("SetSockOpt failed: %v", err)
+								}
+							default:
+								t.Fatal("unknown test variant")
+							}
+
+							const multicastTTL = 42
+							if err := c.ep.SetSockOpt(tcpip.MulticastTTLOption(multicastTTL)); err != nil {
+								c.t.Fatalf("SetSockOpt failed: %v", err)
+							}
+
+							n, err := c.ep.Write(payload, tcpip.WriteOptions{To: &tcpip.FullAddress{Addr: addr, Port: port}})
+							if err != nil {
+								c.t.Fatalf("Write failed: %v", err)
+							}
+							if n != uintptr(len(payload)) {
+								c.t.Fatalf("got c.ep.Write(...) = %d, want = %d", n, len(payload))
+							}
+
+							checkerFn := checker.IPv4
+							switch variant {
+							case "v4", "mapped":
+							case "v6":
+								checkerFn = checker.IPv6
+							default:
+								t.Fatal("unknown test variant")
+							}
+							var wantTTL uint8
+							var multicast bool
+							switch typ {
+							case "unicast":
+								multicast = false
+								switch variant {
+								case "v4", "mapped":
+									ep, err := ipv4.NewProtocol().NewEndpoint(0, "", nil, nil, nil)
+									if err != nil {
+										t.Fatal(err)
+									}
+									wantTTL = ep.DefaultTTL()
+									ep.Close()
+								case "v6":
+									ep, err := ipv6.NewProtocol().NewEndpoint(0, "", nil, nil, nil)
+									if err != nil {
+										t.Fatal(err)
+									}
+									wantTTL = ep.DefaultTTL()
+									ep.Close()
+								default:
+									t.Fatal("unknown test variant")
+								}
+							case "multicast":
+								wantTTL = multicastTTL
+								multicast = true
+							default:
+								t.Fatal("unknown test variant")
+							}
+
+							var networkProtocolNumber tcpip.NetworkProtocolNumber
+							switch variant {
+							case "v4", "mapped":
+								networkProtocolNumber = ipv4.ProtocolNumber
+							case "v6":
+								networkProtocolNumber = ipv6.ProtocolNumber
+							default:
+								t.Fatal("unknown test variant")
+							}
+
+							b := c.getPacket(networkProtocolNumber, multicast)
+							checkerFn(c.t, b,
+								checker.TTL(wantTTL),
+								checker.UDP(
+									checker.DstPort(port),
+								),
+							)
+						})
+					}
+				})
+			}
+		})
+	}
+}