Add starting UDP support

protocols/udp.go

@@ -0,0 +1,96 @@
+package protocols
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+)
+
+type UDPPacket struct {
+	ipPacket IPPacket
+	header   udpHeader
+}
+
+type udpHeader struct {
+	sourcePort      uint16
+	destinationPort uint16
+	length          uint16
+	checksum        uint16
+}
+
+var errInvalidUDPHeader = errors.New("UDP header must be 8 bytes")
+
+func UDPPacketFromIPPacket(ip IPPacket) (*UDPPacket, error) {
+	udpHeader, err := udpHeaderFromBytes(ip.Payload())
+
+	return &UDPPacket{
+		ipPacket: ip,
+		header:   *udpHeader,
+	}, err
+}
+
+func udpv4PacketFromBytes(raw []byte) (*UDPPacket, error) {
+	ipPacket, err := ipv4PacketFromBytes(raw)
+	if err != nil {
+		return nil, err
+	}
+
+	// 14 bytes eth frame + (IPv4 IHL * 4)
+	offset := 14 + (ipPacket.header.ihl * 4)
+	h, err := udpHeaderFromBytes(raw[offset:])
+	if err != nil {
+		return nil, err
+	}
+
+	return &UDPPacket{
+		ipPacket: ipPacket,
+		header:   *h,
+	}, nil
+}
+
+func udpv6PacketFromBytes(raw []byte) (*UDPPacket, error) {
+	ipPacket, err := ipv6PacketFromBytes(raw)
+	if err != nil {
+		return nil, err
+	}
+	// 14 bytes eth frame + 40 IPv6 header
+	h, err := udpHeaderFromBytes(raw[54:])
+	if err != nil {
+		return nil, err
+	}
+
+	return &UDPPacket{
+		ipPacket: ipPacket,
+		header:   *h,
+	}, nil
+}
+
+func (p UDPPacket) Info() string {
+	return fmt.Sprintf(`
+UDP packet
+
+Source Port: %d
+Destination Port: %d
+Length: %d
+Checksum: %d
+
+===============================
+%s
+===============================
+`,
+		p.header.sourcePort, p.header.sourcePort, p.header.length, p.header.checksum, p.ipPacket.Info(),
+	)
+}
+
+func udpHeaderFromBytes(raw []byte) (*udpHeader, error) {
+	if len(raw) < 8 {
+		return nil, errInvalidUDPHeader
+	}
+
+	return &udpHeader{
+		sourcePort:      binary.BigEndian.Uint16(raw[0:2]),
+		destinationPort: binary.BigEndian.Uint16(raw[2:4]),
+		length:          binary.BigEndian.Uint16(raw[4:6]),
+		checksum:        binary.BigEndian.Uint16(raw[6:8]),
+	}, nil
+}