Extract multicast IP to Ethernet address mapping

Test: header.TestEthernetAddressFromMulticastIPAddress
PiperOrigin-RevId: 292604649

2 files changed, 75 insertions, 0 deletions

diff --git a/pkg/tcpip/header/eth.go b/pkg/tcpip/header/eth.go
index f5d2c127f..b1e92d2d7 100644
--- a/pkg/tcpip/header/eth.go
+++ b/pkg/tcpip/header/eth.go
@@ -134,3 +134,44 @@ func IsValidUnicastEthernetAddress(addr tcpip.LinkAddress) bool {
 	// addr is a valid unicast ethernet address.
 	return true
 }
+
+// EthernetAddressFromMulticastIPv4Address returns a multicast Ethernet address
+// for a multicast IPv4 address.
+//
+// addr MUST be a multicast IPv4 address.
+func EthernetAddressFromMulticastIPv4Address(addr tcpip.Address) tcpip.LinkAddress {
+	var linkAddrBytes [EthernetAddressSize]byte
+	// RFC 1112 Host Extensions for IP Multicasting
+	//
+	// 6.4. Extensions to an Ethernet Local Network Module:
+	//
+	// An IP host group address is mapped to an Ethernet multicast
+	// address by placing the low-order 23-bits of the IP address
+	// into the low-order 23 bits of the Ethernet multicast address
+	// 01-00-5E-00-00-00 (hex).
+	linkAddrBytes[0] = 0x1
+	linkAddrBytes[2] = 0x5e
+	linkAddrBytes[3] = addr[1] & 0x7F
+	copy(linkAddrBytes[4:], addr[IPv4AddressSize-2:])
+	return tcpip.LinkAddress(linkAddrBytes[:])
+}
+
+// EthernetAddressFromMulticastIPv6Address returns a multicast Ethernet address
+// for a multicast IPv6 address.
+//
+// addr MUST be a multicast IPv6 address.
+func EthernetAddressFromMulticastIPv6Address(addr tcpip.Address) tcpip.LinkAddress {
+	// RFC 2464 Transmission of IPv6 Packets over Ethernet Networks
+	//
+	// 7. Address Mapping -- Multicast
+	//
+	// An IPv6 packet with a multicast destination address DST,
+	// consisting of the sixteen octets DST[1] through DST[16], is
+	// transmitted to the Ethernet multicast address whose first
+	// two octets are the value 3333 hexadecimal and whose last
+	// four octets are the last four octets of DST.
+	linkAddrBytes := []byte(addr[IPv6AddressSize-EthernetAddressSize:])
+	linkAddrBytes[0] = 0x33
+	linkAddrBytes[1] = 0x33
+	return tcpip.LinkAddress(linkAddrBytes[:])
+}
diff --git a/pkg/tcpip/header/eth_test.go b/pkg/tcpip/header/eth_test.go
index 6634c90f5..7a0014ad9 100644
--- a/pkg/tcpip/header/eth_test.go
+++ b/pkg/tcpip/header/eth_test.go
@@ -66,3 +66,37 @@ func TestIsValidUnicastEthernetAddress(t *testing.T) {
 		})
 	}
 }
+
+func TestEthernetAddressFromMulticastIPv4Address(t *testing.T) {
+	tests := []struct {
+		name             string
+		addr             tcpip.Address
+		expectedLinkAddr tcpip.LinkAddress
+	}{
+		{
+			name:             "IPv4 Multicast without 24th bit set",
+			addr:             "\xe0\x7e\xdc\xba",
+			expectedLinkAddr: "\x01\x00\x5e\x7e\xdc\xba",
+		},
+		{
+			name:             "IPv4 Multicast with 24th bit set",
+			addr:             "\xe0\xfe\xdc\xba",
+			expectedLinkAddr: "\x01\x00\x5e\x7e\xdc\xba",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := EthernetAddressFromMulticastIPv4Address(test.addr); got != test.expectedLinkAddr {
+				t.Fatalf("got EthernetAddressFromMulticastIPv4Address(%s) = %s, want = %s", got, test.expectedLinkAddr)
+			}
+		})
+	}
+}
+
+func TestEthernetAddressFromMulticastIPv6Address(t *testing.T) {
+	addr := tcpip.Address("\xff\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x1a")
+	if got, want := EthernetAddressFromMulticastIPv6Address(addr), tcpip.LinkAddress("\x33\x33\x0d\x0e\x0f\x1a"); got != want {
+		t.Fatalf("got EthernetAddressFromMulticastIPv6Address(%s) = %s, want = %s", addr, got, want)
+	}
+}