Add starting TCP support

protocols/tcp.go

@@ -0,0 +1,78 @@
+package protocols
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+)
+
+type TCPPacket struct {
+	ipPacket IPPacket
+	header   tcpHeader
+}
+
+type tcpHeader struct {
+	sourcePort      uint16
+	destinationPort uint16
+	sequenceNumber  uint32
+	ackNumber       uint32
+	rawOffset       uint8
+	reserved        uint8
+	flags           uint16
+	windowSize      uint16
+	checksum        uint16
+	urgentPointer   uint16
+	options         []byte
+}
+
+var errTCPHeaderTooShort = errors.New("TCP header must be at least 20 bytes")
+var errTCPHeaderLenMismatch = errors.New("TCP header length less than raw Offset")
+
+func TCPPacketFromIPPacket(ip IPPacket) (*TCPPacket, error) {
+	tcpHeader, err := tcpHeaderFromBytes(ip.Payload())
+
+	return &TCPPacket{
+		ipPacket: ip,
+		header:   *tcpHeader,
+	}, err
+}
+
+func tcpHeaderFromBytes(raw []byte) (*tcpHeader, error) {
+	if len(raw) < 20 {
+		return nil, errTCPHeaderTooShort
+	}
+	offset := (raw[12] >> 4)
+	hLen := int(offset * 4)
+	if len(raw) < hLen {
+		return nil, errTCPHeaderLenMismatch
+	}
+	return &tcpHeader{
+		sourcePort:      binary.BigEndian.Uint16(raw[0:2]),
+		destinationPort: binary.BigEndian.Uint16(raw[2:4]),
+		sequenceNumber:  binary.BigEndian.Uint32(raw[4:8]),
+		ackNumber:       binary.BigEndian.Uint32(raw[8:12]),
+		rawOffset:       offset,
+		reserved:        (raw[12] & 0x0E) >> 1,
+		flags:           binary.BigEndian.Uint16(raw[12:14]) & 0x1FF,
+		windowSize:      binary.BigEndian.Uint16(raw[14:16]),
+		checksum:        binary.BigEndian.Uint16(raw[16:18]),
+		urgentPointer:   binary.BigEndian.Uint16(raw[18:20]),
+		options:         raw[20:hLen],
+	}, nil
+}
+
+func (p TCPPacket) Info() string {
+	return fmt.Sprintf(`
+TCP packet
+
+Source Port: %d
+Destination Port: %d
+Checksum: %d
+
+===============================
+%s
+===============================
+`,
+		p.header.sourcePort, p.header.sourcePort, p.header.checksum, p.ipPacket.Info(),
+	)
+}

sockets/raw_socket.go

@@ -120,7 +120,13 @@ func (rs *RawSocket) ReadPackets() {
 					continue
 				}
 				log.Println(udpPacket.Info())
-			case "TCP":
+			case "tcp":
+				tcpPacket, err := protocols.TCPPacketFromIPPacket(packet)
+				if err != nil {
+					log.Println("Error reading TCP packet:", err)
+					continue
+				}
+				log.Println(tcpPacket.Info())
 			}
 		}
 	}