github.com/google/netstack@v0.0.0-20191123085552-55fcc16cd0eb/tcpip/header/eth.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package header
    16  
    17  import (
    18  	"encoding/binary"
    19  
    20  	"github.com/google/netstack/tcpip"
    21  )
    22  
    23  const (
    24  	dstMAC  = 0
    25  	srcMAC  = 6
    26  	ethType = 12
    27  )
    28  
    29  // EthernetFields contains the fields of an ethernet frame header. It is used to
    30  // describe the fields of a frame that needs to be encoded.
    31  type EthernetFields struct {
    32  	// SrcAddr is the "MAC source" field of an ethernet frame header.
    33  	SrcAddr tcpip.LinkAddress
    34  
    35  	// DstAddr is the "MAC destination" field of an ethernet frame header.
    36  	DstAddr tcpip.LinkAddress
    37  
    38  	// Type is the "ethertype" field of an ethernet frame header.
    39  	Type tcpip.NetworkProtocolNumber
    40  }
    41  
    42  // Ethernet represents an ethernet frame header stored in a byte array.
    43  type Ethernet []byte
    44  
    45  const (
    46  	// EthernetMinimumSize is the minimum size of a valid ethernet frame.
    47  	EthernetMinimumSize = 14
    48  
    49  	// EthernetAddressSize is the size, in bytes, of an ethernet address.
    50  	EthernetAddressSize = 6
    51  
    52  	// unspecifiedEthernetAddress is the unspecified ethernet address
    53  	// (all bits set to 0).
    54  	unspecifiedEthernetAddress = tcpip.LinkAddress("\x00\x00\x00\x00\x00\x00")
    55  
    56  	// unicastMulticastFlagMask is the mask of the least significant bit in
    57  	// the first octet (in network byte order) of an ethernet address that
    58  	// determines whether the ethernet address is a unicast or multicast. If
    59  	// the masked bit is a 1, then the address is a multicast, unicast
    60  	// otherwise.
    61  	//
    62  	// See the IEEE Std 802-2001 document for more details. Specifically,
    63  	// section 9.2.1 of http://ieee802.org/secmail/pdfocSP2xXA6d.pdf:
    64  	// "A 48-bit universal address consists of two parts. The first 24 bits
    65  	// correspond to the OUI as assigned by the IEEE, expect that the
    66  	// assignee may set the LSB of the first octet to 1 for group addresses
    67  	// or set it to 0 for individual addresses."
    68  	unicastMulticastFlagMask = 1
    69  
    70  	// unicastMulticastFlagByteIdx is the byte that holds the
    71  	// unicast/multicast flag. See unicastMulticastFlagMask.
    72  	unicastMulticastFlagByteIdx = 0
    73  )
    74  
    75  const (
    76  	// EthernetProtocolAll is a catch-all for all protocols carried inside
    77  	// an ethernet frame. It is mainly used to create packet sockets that
    78  	// capture all traffic.
    79  	EthernetProtocolAll tcpip.NetworkProtocolNumber = 0x0003
    80  
    81  	// EthernetProtocolPUP is the PARC Universial Packet protocol ethertype.
    82  	EthernetProtocolPUP tcpip.NetworkProtocolNumber = 0x0200
    83  )
    84  
    85  // Ethertypes holds the protocol numbers describing the payload of an ethernet
    86  // frame. These types aren't necessarily supported by netstack, but can be used
    87  // to catch all traffic of a type via packet endpoints.
    88  var Ethertypes = []tcpip.NetworkProtocolNumber{
    89  	EthernetProtocolAll,
    90  	EthernetProtocolPUP,
    91  }
    92  
    93  // SourceAddress returns the "MAC source" field of the ethernet frame header.
    94  func (b Ethernet) SourceAddress() tcpip.LinkAddress {
    95  	return tcpip.LinkAddress(b[srcMAC:][:EthernetAddressSize])
    96  }
    97  
    98  // DestinationAddress returns the "MAC destination" field of the ethernet frame
    99  // header.
   100  func (b Ethernet) DestinationAddress() tcpip.LinkAddress {
   101  	return tcpip.LinkAddress(b[dstMAC:][:EthernetAddressSize])
   102  }
   103  
   104  // Type returns the "ethertype" field of the ethernet frame header.
   105  func (b Ethernet) Type() tcpip.NetworkProtocolNumber {
   106  	return tcpip.NetworkProtocolNumber(binary.BigEndian.Uint16(b[ethType:]))
   107  }
   108  
   109  // Encode encodes all the fields of the ethernet frame header.
   110  func (b Ethernet) Encode(e *EthernetFields) {
   111  	binary.BigEndian.PutUint16(b[ethType:], uint16(e.Type))
   112  	copy(b[srcMAC:][:EthernetAddressSize], e.SrcAddr)
   113  	copy(b[dstMAC:][:EthernetAddressSize], e.DstAddr)
   114  }
   115  
   116  // IsValidUnicastEthernetAddress returns true if addr is a valid unicast
   117  // ethernet address.
   118  func IsValidUnicastEthernetAddress(addr tcpip.LinkAddress) bool {
   119  	// Must be of the right length.
   120  	if len(addr) != EthernetAddressSize {
   121  		return false
   122  	}
   123  
   124  	// Must not be unspecified.
   125  	if addr == unspecifiedEthernetAddress {
   126  		return false
   127  	}
   128  
   129  	// Must not be a multicast.
   130  	if addr[unicastMulticastFlagByteIdx]&unicastMulticastFlagMask != 0 {
   131  		return false
   132  	}
   133  
   134  	// addr is a valid unicast ethernet address.
   135  	return true
   136  }