func fragment(eth layers.Ethernet, ip layers.IPv4, mtu int,
forward func([]byte)) 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 := (mtu - 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
}
forward(buf.Bytes())
}
return nil
}