Use link-local source address for link-local multicast

Tests:
- header_test.TestIsV6LinkLocalMulticastAddress
- header_test.TestScopeForIPv6Address
- stack_test.TestIPv6SourceAddressSelectionScopeAndSameAddress
PiperOrigin-RevId: 297215576

2 files changed, 114 insertions, 6 deletions

diff --git a/pkg/tcpip/header/ipv6.go b/pkg/tcpip/header/ipv6.go
index 70e6ce095..76e88e9b3 100644
--- a/pkg/tcpip/header/ipv6.go
+++ b/pkg/tcpip/header/ipv6.go
@@ -115,6 +115,19 @@ const (
 	// for the secret key used to generate an opaque interface identifier as
 	// outlined by RFC 7217.
 	OpaqueIIDSecretKeyMinBytes = 16
+
+	// ipv6MulticastAddressScopeByteIdx is the byte where the scope (scop) field
+	// is located within a multicast IPv6 address, as per RFC 4291 section 2.7.
+	ipv6MulticastAddressScopeByteIdx = 1
+
+	// ipv6MulticastAddressScopeMask is the mask for the scope (scop) field,
+	// within the byte holding the field, as per RFC 4291 section 2.7.
+	ipv6MulticastAddressScopeMask = 0xF
+
+	// ipv6LinkLocalMulticastScope is the value of the scope (scop) field within
+	// a multicast IPv6 address that indicates the address has link-local scope,
+	// as per RFC 4291 section 2.7.
+	ipv6LinkLocalMulticastScope = 2
 )
 
 // IPv6EmptySubnet is the empty IPv6 subnet. It may also be known as the
@@ -340,6 +353,12 @@ func IsV6LinkLocalAddress(addr tcpip.Address) bool {
 	return addr[0] == 0xfe && (addr[1]&0xc0) == 0x80
 }
 
+// IsV6LinkLocalMulticastAddress determines if the provided address is an IPv6
+// link-local multicast address.
+func IsV6LinkLocalMulticastAddress(addr tcpip.Address) bool {
+	return IsV6MulticastAddress(addr) && addr[ipv6MulticastAddressScopeByteIdx]&ipv6MulticastAddressScopeMask == ipv6LinkLocalMulticastScope
+}
+
 // IsV6UniqueLocalAddress determines if the provided address is an IPv6
 // unique-local address (within the prefix FC00::/7).
 func IsV6UniqueLocalAddress(addr tcpip.Address) bool {
@@ -411,6 +430,9 @@ func ScopeForIPv6Address(addr tcpip.Address) (IPv6AddressScope, *tcpip.Error) {
 	}
 
 	switch {
+	case IsV6LinkLocalMulticastAddress(addr):
+		return LinkLocalScope, nil
+
 	case IsV6LinkLocalAddress(addr):
 		return LinkLocalScope, nil
 
diff --git a/pkg/tcpip/header/ipv6_test.go b/pkg/tcpip/header/ipv6_test.go
index c3ad503aa..426a873b1 100644
--- a/pkg/tcpip/header/ipv6_test.go
+++ b/pkg/tcpip/header/ipv6_test.go
@@ -27,11 +27,12 @@ import (
 )
 
 const (
-	linkAddr         = tcpip.LinkAddress("\x02\x02\x03\x04\x05\x06")
-	linkLocalAddr    = tcpip.Address("\xfe\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
-	uniqueLocalAddr1 = tcpip.Address("\xfc\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
-	uniqueLocalAddr2 = tcpip.Address("\xfd\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02")
-	globalAddr       = tcpip.Address("\xa0\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+	linkAddr               = tcpip.LinkAddress("\x02\x02\x03\x04\x05\x06")
+	linkLocalAddr          = tcpip.Address("\xfe\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+	linkLocalMulticastAddr = tcpip.Address("\xff\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+	uniqueLocalAddr1       = tcpip.Address("\xfc\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+	uniqueLocalAddr2       = tcpip.Address("\xfd\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02")
+	globalAddr             = tcpip.Address("\xa0\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
 )
 
 func TestEthernetAdddressToModifiedEUI64(t *testing.T) {
@@ -256,6 +257,85 @@ func TestIsV6UniqueLocalAddress(t *testing.T) {
 	}
 }
 
+func TestIsV6LinkLocalMulticastAddress(t *testing.T) {
+	tests := []struct {
+		name     string
+		addr     tcpip.Address
+		expected bool
+	}{
+		{
+			name:     "Valid Link Local Multicast",
+			addr:     linkLocalMulticastAddr,
+			expected: true,
+		},
+		{
+			name:     "Valid Link Local Multicast with flags",
+			addr:     "\xff\xf2\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01",
+			expected: true,
+		},
+		{
+			name:     "Link Local Unicast",
+			addr:     linkLocalAddr,
+			expected: false,
+		},
+		{
+			name:     "IPv4 Multicast",
+			addr:     "\xe0\x00\x00\x01",
+			expected: false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := header.IsV6LinkLocalMulticastAddress(test.addr); got != test.expected {
+				t.Errorf("got header.IsV6LinkLocalMulticastAddress(%s) = %t, want = %t", test.addr, got, test.expected)
+			}
+		})
+	}
+}
+
+func TestIsV6LinkLocalAddress(t *testing.T) {
+	tests := []struct {
+		name     string
+		addr     tcpip.Address
+		expected bool
+	}{
+		{
+			name:     "Valid Link Local Unicast",
+			addr:     linkLocalAddr,
+			expected: true,
+		},
+		{
+			name:     "Link Local Multicast",
+			addr:     linkLocalMulticastAddr,
+			expected: false,
+		},
+		{
+			name:     "Unique Local",
+			addr:     uniqueLocalAddr1,
+			expected: false,
+		},
+		{
+			name:     "Global",
+			addr:     globalAddr,
+			expected: false,
+		},
+		{
+			name:     "IPv4 Link Local",
+			addr:     "\xa9\xfe\x00\x01",
+			expected: false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := header.IsV6LinkLocalAddress(test.addr); got != test.expected {
+				t.Errorf("got header.IsV6LinkLocalAddress(%s) = %t, want = %t", test.addr, got, test.expected)
+			}
+		})
+	}
+}
+
 func TestScopeForIPv6Address(t *testing.T) {
 	tests := []struct {
 		name  string
@@ -270,12 +350,18 @@ func TestScopeForIPv6Address(t *testing.T) {
 			err:   nil,
 		},
 		{
-			name:  "Link Local",
+			name:  "Link Local Unicast",
 			addr:  linkLocalAddr,
 			scope: header.LinkLocalScope,
 			err:   nil,
 		},
 		{
+			name:  "Link Local Multicast",
+			addr:  linkLocalMulticastAddr,
+			scope: header.LinkLocalScope,
+			err:   nil,
+		},
+		{
 			name:  "Global",
 			addr:  globalAddr,
 			scope: header.GlobalScope,