Golang IPv4.Length方法代码示例

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, &eth, &ip,
			&segPayload)
		if err != nil {
			return err
		}

		forward(buf.Bytes())
	}
	return nil
}

/* Adds the SIFF header to a packet, or modifies it in the case that it already
exists. Pass in the NFPacket, the flags (bitwise OR them if you need both), and
the capabilities and capability updates arrays. If only IsSiff is set, just fill
the last 4 bytes with dummy data, it'll be ignored. If you want to update specific
fields, then use the [update function name here] function */
func setSiffFields(packet *netfilter.NFPacket, flags uint8, capabilities []byte, updoots []byte) {
	var ipLayer *layers.IPv4
	var option [1]layers.IPv4Option
	option[0].OptionType = 86
	option[0].OptionLength = 8

	/* Get the IPv4 layer, and if it doesn't exist, keep doing shit
	   I can't be arsed for proper response outside the bounds of this project */
	if layer := packet.Packet.Layer(layers.LayerTypeIPv4); layer != nil {
		ipLayer = layer.(*layers.IPv4)
	} else {
		// maybe do something?
	}

	/* Modify the ip layer information */
	var IHLchange uint16 = uint16(ipLayer.IHL)

	// compute new IHL and length
	if (flags & CapabilityUpdate) == CapabilityUpdate {
		ipLayer.IHL = 8
		option[0].OptionLength = 12
	} else if (flags&IsSiff) == IsSiff || (flags&Exp) == Exp {
		ipLayer.IHL = 7
	} else {
		ipLayer.IHL = 5
	}

	IHLchange = uint16(ipLayer.IHL) - IHLchange
	if IHLchange != 0 {
		ipLayer.Length += IHLchange * 4
	}

	if (flags & Evil) == Evil {
		// set the evil flag. If we do this, we don't need to do anything else,
		// since evil packets are legacy, and don't have other flags
		ipLayer.Flags |= layers.IPv4EvilBit
	} else {
		// set the flags option
		option[0].OptionData = []byte{0, 0}
		if (flags & Exp) == Exp {
			option[0].OptionData[0] = byte(Exp)
		}
		if (flags & CapabilityUpdate) == CapabilityUpdate {
			option[0].OptionData[0] |= byte(IsSiff | CapabilityUpdate)
		} else if (flags & IsSiff) == IsSiff {
			option[0].OptionData[0] |= byte(IsSiff)
		}

		// handle the options
		if flags != 0 {
			for _, b := range capabilities {
				option[0].OptionData = append(option[0].OptionData, b)
			}
		}

		if (flags & CapabilityUpdate) == CapabilityUpdate {
			for _, b := range updoots {
				option[0].OptionData = append(option[0].OptionData, b)
			}
		}
		// add options
		if flags != 0 {
			ipLayer.Options = append([]layers.IPv4Option{option[0]}, ipLayer.Options...)
		}
	}

	// we're done
}