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// Copyright 2018 The gVisor Authors.
// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package header
import (
const (
dstMAC = 0
srcMAC = 6
ethType = 12
// EthernetFields contains the fields of an ethernet frame header. It is used to
// describe the fields of a frame that needs to be encoded.
type EthernetFields struct {
// SrcAddr is the "MAC source" field of an ethernet frame header.
SrcAddr tcpip.LinkAddress
// DstAddr is the "MAC destination" field of an ethernet frame header.
DstAddr tcpip.LinkAddress
// Type is the "ethertype" field of an ethernet frame header.
Type tcpip.NetworkProtocolNumber
// Ethernet represents an ethernet frame header stored in a byte array.
type Ethernet []byte
const (
// EthernetMinimumSize is the minimum size of a valid ethernet frame.
EthernetMinimumSize = 14
// EthernetAddressSize is the size, in bytes, of an ethernet address.
EthernetAddressSize = 6
// unspecifiedEthernetAddress is the unspecified ethernet address
// (all bits set to 0).
unspecifiedEthernetAddress = tcpip.LinkAddress("\x00\x00\x00\x00\x00\x00")
// EthernetBroadcastAddress is an ethernet address that addresses every node
// on a local link.
EthernetBroadcastAddress = tcpip.LinkAddress("\xff\xff\xff\xff\xff\xff")
// unicastMulticastFlagMask is the mask of the least significant bit in
// the first octet (in network byte order) of an ethernet address that
// determines whether the ethernet address is a unicast or multicast. If
// the masked bit is a 1, then the address is a multicast, unicast
// otherwise.
// See the IEEE Std 802-2001 document for more details. Specifically,
// section 9.2.1 of
// "A 48-bit universal address consists of two parts. The first 24 bits
// correspond to the OUI as assigned by the IEEE, expect that the
// assignee may set the LSB of the first octet to 1 for group addresses
// or set it to 0 for individual addresses."
unicastMulticastFlagMask = 1
// unicastMulticastFlagByteIdx is the byte that holds the
// unicast/multicast flag. See unicastMulticastFlagMask.
unicastMulticastFlagByteIdx = 0
const (
// EthernetProtocolAll is a catch-all for all protocols carried inside
// an ethernet frame. It is mainly used to create packet sockets that
// capture all traffic.
EthernetProtocolAll tcpip.NetworkProtocolNumber = 0x0003
// EthernetProtocolPUP is the PARC Universial Packet protocol ethertype.
EthernetProtocolPUP tcpip.NetworkProtocolNumber = 0x0200
// Ethertypes holds the protocol numbers describing the payload of an ethernet
// frame. These types aren't necessarily supported by netstack, but can be used
// to catch all traffic of a type via packet endpoints.
var Ethertypes = []tcpip.NetworkProtocolNumber{
// SourceAddress returns the "MAC source" field of the ethernet frame header.
func (b Ethernet) SourceAddress() tcpip.LinkAddress {
return tcpip.LinkAddress(b[srcMAC:][:EthernetAddressSize])
// DestinationAddress returns the "MAC destination" field of the ethernet frame
// header.
func (b Ethernet) DestinationAddress() tcpip.LinkAddress {
return tcpip.LinkAddress(b[dstMAC:][:EthernetAddressSize])
// Type returns the "ethertype" field of the ethernet frame header.
func (b Ethernet) Type() tcpip.NetworkProtocolNumber {
return tcpip.NetworkProtocolNumber(binary.BigEndian.Uint16(b[ethType:]))
// Encode encodes all the fields of the ethernet frame header.
func (b Ethernet) Encode(e *EthernetFields) {
binary.BigEndian.PutUint16(b[ethType:], uint16(e.Type))
copy(b[srcMAC:][:EthernetAddressSize], e.SrcAddr)
copy(b[dstMAC:][:EthernetAddressSize], e.DstAddr)
// IsMulticastEthernetAddress returns true if the address is a multicast
// ethernet address.
func IsMulticastEthernetAddress(addr tcpip.LinkAddress) bool {
if len(addr) != EthernetAddressSize {
return false
return addr[unicastMulticastFlagByteIdx]&unicastMulticastFlagMask != 0
// IsValidUnicastEthernetAddress returns true if the address is a unicast
// ethernet address.
func IsValidUnicastEthernetAddress(addr tcpip.LinkAddress) bool {
if len(addr) != EthernetAddressSize {
return false
if addr == unspecifiedEthernetAddress {
return false
if addr[unicastMulticastFlagByteIdx]&unicastMulticastFlagMask != 0 {
return false
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[:])