func fragment(eth layers.Ethernet, ip layers.IPv4, pmtu int, frame *ForwardedFrame, forward func(*ForwardedFrame)) error {
// We are not doing any sort of NAT, so we don't need to worry
// about checksums of IP payload (eg UDP checksum).
headerSize := int(ip.IHL) * 4
// &^ is bit clear (AND NOT). So here we're clearing the lowest 3
// bits.
maxSegmentSize := (pmtu - headerSize) &^ 7
opts := gopacket.SerializeOptions{
FixLengths: false,
ComputeChecksums: true}
payloadSize := int(ip.Length) - headerSize
payload := ip.BaseLayer.Payload[:payloadSize]
offsetBase := int(ip.FragOffset) << 3
origFlags := ip.Flags
ip.Flags = ip.Flags | layers.IPv4MoreFragments
ip.Length = uint16(headerSize + maxSegmentSize)
if eth.EthernetType == layers.EthernetTypeLLC {
// using LLC, so must set eth length correctly. eth length
// is just the length of the payload
eth.Length = ip.Length
} else {
eth.Length = 0
}
for offset := 0; offset < payloadSize; offset += maxSegmentSize {
var segmentPayload []byte
if len(payload) <= maxSegmentSize {
// last one
segmentPayload = payload
ip.Length = uint16(len(payload) + headerSize)
ip.Flags = origFlags
if eth.EthernetType == layers.EthernetTypeLLC {
eth.Length = ip.Length
} else {
eth.Length = 0
}
} else {
segmentPayload = payload[:maxSegmentSize]
payload = payload[maxSegmentSize:]
}
ip.FragOffset = uint16((offset + offsetBase) >> 3)
buf := gopacket.NewSerializeBuffer()
segPayload := gopacket.Payload(segmentPayload)
err := gopacket.SerializeLayers(buf, opts, ð, &ip, &segPayload)
if err != nil {
return err
}
// make copies of the frame we received
segFrame := *frame
segFrame.frame = buf.Bytes()
forward(&segFrame)
}
return nil
}
/*Shim inserts the given router into the shim layer route record of the given
IPv4 packet, creating a new route record if it's not already present.*/
func Shim(ipLayer *layers.IPv4, r Router) {
ipPayload := bytes.NewBuffer(ipLayer.LayerPayload())
var modifiedIPPayload bytes.Buffer
var rr RouteRecord
if Shimmed(ipLayer) {
rr.ReadFrom(ipPayload)
ipLayer.Length -= uint16(rr.Len())
} else {
rr.Protocol = uint8(ipLayer.Protocol)
}
/*Add the specified router to the route record and put the record at the
beginning of the payload.*/
rr.AddRouter(r)
rr.WriteTo(&modifiedIPPayload)
ipPayload.WriteTo(&modifiedIPPayload)
ipLayer.Length += uint16(rr.Len())
ipLayer.Protocol = layers.IPProtocol(IPProtocolAITFRouteRecord)
ipLayer.Checksum = 0
ipLayer.Payload = modifiedIPPayload.Bytes()
}
/*Unshim removes the shim layer from an IPv4 packet, if it's present.*/
func Unshim(ipLayer *layers.IPv4) *RouteRecord {
if Shimmed(ipLayer) {
/*Remove the route record from the payload*/
ipPayload := bytes.NewBuffer(ipLayer.LayerPayload())
var rr RouteRecord
rr.ReadFrom(ipPayload)
ipLayer.Length -= uint16(rr.Len())
ipLayer.Protocol = layers.IPProtocol(rr.Protocol)
ipLayer.Checksum = 0
ipLayer.Payload = ipPayload.Bytes()
return &rr
}
return nil
}