add ethernet/trailer.go

pkg/util/ethernet/trailer.go

@@ -0,0 +1,86 @@
+package ethernet
+
+import (
+	"encoding/binary"
+	"fmt"
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"net"
+)
+
+// EthernetBroadcast is the broadcast MAC address used by Ethernet.
+// var EthernetBroadcast = net.HardwareAddr{0xff, 0xff, 0xff, 0xff, 0xff, 0xff}
+
+// EthernetWithTrailer is the layer for Ethernet frame headers.
+type EthernetWithTrailer struct {
+	layers.BaseLayer
+	SrcMAC, DstMAC net.HardwareAddr
+	EthernetType   layers.EthernetType
+	// Length is only set if a length field exists within this header.  Ethernet
+	// headers follow two different standards, one that uses an EthernetType, the
+	// other which defines a length the follows with a LLC header (802.3).  If the
+	// former is the case, we set EthernetType and Length stays 0.  In the latter
+	// case, we set Length and EthernetType = EthernetTypeLLC.
+	Length  uint16
+	Trailer []byte
+}
+
+// LayerType returns LayerTypeEthernet
+func (e *EthernetWithTrailer) LayerType() gopacket.LayerType { return layers.LayerTypeEthernet }
+
+// SerializeTo writes the serialized form of this layer into the
+// SerializationBuffer, implementing gopacket.SerializableLayer.
+// See the docs for gopacket.SerializableLayer for more info.
+func (e *EthernetWithTrailer) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
+	if len(e.DstMAC) != 6 {
+		return fmt.Errorf("invalid dst MAC: %v", e.DstMAC)
+	}
+	if len(e.SrcMAC) != 6 {
+		return fmt.Errorf("invalid src MAC: %v", e.SrcMAC)
+	}
+	payload := b.Bytes()
+	bytes, err := b.PrependBytes(14)
+	if err != nil {
+		return err
+	}
+	copy(bytes, e.DstMAC)
+	copy(bytes[6:], e.SrcMAC)
+	if e.Length != 0 || e.EthernetType == layers.EthernetTypeLLC {
+		if opts.FixLengths {
+			e.Length = uint16(len(payload))
+		}
+		if e.EthernetType != layers.EthernetTypeLLC {
+			return fmt.Errorf("ethernet type %v not compatible with length value %v", e.EthernetType, e.Length)
+		} else if e.Length > 0x0600 {
+			return fmt.Errorf("invalid ethernet length %v", e.Length)
+		}
+		binary.BigEndian.PutUint16(bytes[12:], e.Length)
+	} else {
+		binary.BigEndian.PutUint16(bytes[12:], uint16(e.EthernetType))
+	}
+	length := len(b.Bytes())
+	if length < 60 {
+		// Pad out to 60 bytes.
+		padding, err := b.AppendBytes(60 - length)
+		if err != nil {
+			return err
+		}
+		copy(padding, lotsOfZeros[:])
+	}
+
+	//todo: find a way to put the trailer here
+	trailer, err := b.AppendBytes(len(e.Trailer))
+	if err != nil {
+		return err
+	}
+	copy(trailer, e.Trailer)
+	// todo: some of this gets gobbled up as framecheck sequence, putting a 4 byte 0 in the trailer to avoid that
+	checksum, err := b.AppendBytes(4)
+	if err != nil {
+		return err
+	}
+	copy(checksum, lotsOfZeros[:])
+	return nil
+}
+
+var lotsOfZeros [1024]byte