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// Copyright 2012 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.
package layers
import (
"encoding/binary"
"github.com/google/gopacket"
)
// GRE is a Generic Routing Encapsulation header.
type GRE struct {
BaseLayer
ChecksumPresent, RoutingPresent, KeyPresent, SeqPresent, StrictSourceRoute bool
RecursionControl, Flags, Version uint8
Protocol EthernetType
Checksum, Offset uint16
Key, Seq uint32
*GRERouting
}
// GRERouting is GRE routing information, present if the RoutingPresent flag is
// set.
type GRERouting struct {
AddressFamily uint16
SREOffset, SRELength uint8
RoutingInformation []byte
Next *GRERouting
}
// LayerType returns gopacket.LayerTypeGRE.
func (g *GRE) LayerType() gopacket.LayerType { return LayerTypeGRE }
// DecodeFromBytes decodes the given bytes into this layer.
func (g *GRE) DecodeFromBytes(data []byte, df gopacket.DecodeFeedback) error {
g.ChecksumPresent = data[0]&0x80 != 0
g.RoutingPresent = data[0]&0x40 != 0
g.KeyPresent = data[0]&0x20 != 0
g.SeqPresent = data[0]&0x10 != 0
g.StrictSourceRoute = data[0]&0x08 != 0
g.RecursionControl = data[0] & 0x7
g.Flags = data[1] >> 3
g.Version = data[1] & 0x7
g.Protocol = EthernetType(binary.BigEndian.Uint16(data[2:4]))
offset := 4
if g.ChecksumPresent || g.RoutingPresent {
g.Checksum = binary.BigEndian.Uint16(data[offset : offset+2])
g.Offset = binary.BigEndian.Uint16(data[offset+2 : offset+4])
offset += 4
}
if g.KeyPresent {
g.Key = binary.BigEndian.Uint32(data[offset : offset+4])
offset += 4
}
if g.SeqPresent {
g.Seq = binary.BigEndian.Uint32(data[offset : offset+4])
offset += 4
}
if g.RoutingPresent {
tail := &g.GRERouting
for {
sre := &GRERouting{
AddressFamily: binary.BigEndian.Uint16(data[offset : offset+2]),
SREOffset: data[offset+2],
SRELength: data[offset+3],
}
sre.RoutingInformation = data[offset+4 : offset+4+int(sre.SRELength)]
offset += 4 + int(sre.SRELength)
if sre.AddressFamily == 0 && sre.SRELength == 0 {
break
}
(*tail) = sre
tail = &sre.Next
}
}
g.BaseLayer = BaseLayer{data[:offset], data[offset:]}
return nil
}
// SerializeTo writes the serialized form of this layer into the SerializationBuffer,
// implementing gopacket.SerializableLayer. See the docs for gopacket.SerializableLayer for more info.
func (g *GRE) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
size := 4
if g.ChecksumPresent || g.RoutingPresent {
size += 4
}
if g.KeyPresent {
size += 4
}
if g.SeqPresent {
size += 4
}
if g.RoutingPresent {
r := g.GRERouting
for r != nil {
size += 4 + int(r.SRELength)
r = r.Next
}
size += 4
}
buf, err := b.PrependBytes(size)
if err != nil {
return err
}
// Reset any potentially dirty memory in the first 2 bytes, as these use OR to set flags.
buf[0] = 0
buf[1] = 0
if g.ChecksumPresent {
buf[0] |= 0x80
}
if g.RoutingPresent {
buf[0] |= 0x40
}
if g.KeyPresent {
buf[0] |= 0x20
}
if g.SeqPresent {
buf[0] |= 0x10
}
if g.StrictSourceRoute {
buf[0] |= 0x08
}
buf[0] |= g.RecursionControl
buf[1] |= g.Flags << 3
buf[1] |= g.Version
binary.BigEndian.PutUint16(buf[2:4], uint16(g.Protocol))
offset := 4
if g.ChecksumPresent || g.RoutingPresent {
// Don't write the checksum value yet, as we may need to compute it,
// which requires the entire header be complete.
// Instead we zeroize the memory in case it is dirty.
buf[offset] = 0
buf[offset+1] = 0
binary.BigEndian.PutUint16(buf[offset+2:offset+4], g.Offset)
offset += 4
}
if g.KeyPresent {
binary.BigEndian.PutUint32(buf[offset:offset+4], g.Key)
offset += 4
}
if g.SeqPresent {
binary.BigEndian.PutUint32(buf[offset:offset+4], g.Seq)
offset += 4
}
if g.RoutingPresent {
sre := g.GRERouting
for sre != nil {
binary.BigEndian.PutUint16(buf[offset:offset+2], sre.AddressFamily)
buf[offset+2] = sre.SREOffset
buf[offset+3] = sre.SRELength
copy(buf[offset+4:offset+4+int(sre.SRELength)], sre.RoutingInformation)
offset += 4 + int(sre.SRELength)
sre = sre.Next
}
// Terminate routing field with a "NULL" SRE.
binary.BigEndian.PutUint32(buf[offset:offset+4], 0)
}
if g.ChecksumPresent {
if opts.ComputeChecksums {
g.Checksum = tcpipChecksum(b.Bytes(), 0)
}
binary.BigEndian.PutUint16(buf[4:6], g.Checksum)
}
return nil
}
// CanDecode returns the set of layer types that this DecodingLayer can decode.
func (g *GRE) CanDecode() gopacket.LayerClass {
return LayerTypeGRE
}
// NextLayerType returns the layer type contained by this DecodingLayer.
func (g *GRE) NextLayerType() gopacket.LayerType {
return g.Protocol.LayerType()
}
func decodeGRE(data []byte, p gopacket.PacketBuilder) error {
g := &GRE{}
return decodingLayerDecoder(g, data, p)
}