Add system stack

internal/clashtcpip/tcp.go

@@ -0,0 +1,90 @@
+package clashtcpip
+
+import (
+	"encoding/binary"
+	"net"
+)
+
+const (
+	TCPFin uint16 = 1 << 0
+	TCPSyn uint16 = 1 << 1
+	TCPRst uint16 = 1 << 2
+	TCPPuh uint16 = 1 << 3
+	TCPAck uint16 = 1 << 4
+	TCPUrg uint16 = 1 << 5
+	TCPEce uint16 = 1 << 6
+	TCPEwr uint16 = 1 << 7
+	TCPNs  uint16 = 1 << 8
+)
+
+const TCPHeaderSize = 20
+
+type TCPPacket []byte
+
+func (p TCPPacket) SourcePort() uint16 {
+	return binary.BigEndian.Uint16(p)
+}
+
+func (p TCPPacket) SetSourcePort(port uint16) {
+	binary.BigEndian.PutUint16(p, port)
+}
+
+func (p TCPPacket) DestinationPort() uint16 {
+	return binary.BigEndian.Uint16(p[2:])
+}
+
+func (p TCPPacket) SetDestinationPort(port uint16) {
+	binary.BigEndian.PutUint16(p[2:], port)
+}
+
+func (p TCPPacket) Flags() uint16 {
+	return uint16(p[13] | (p[12] & 0x1))
+}
+
+func (p TCPPacket) Checksum() uint16 {
+	return binary.BigEndian.Uint16(p[16:])
+}
+
+func (p TCPPacket) SetChecksum(sum [2]byte) {
+	p[16] = sum[0]
+	p[17] = sum[1]
+}
+
+func (p TCPPacket) ResetChecksum(psum uint32) {
+	p.SetChecksum(zeroChecksum)
+	p.SetChecksum(Checksum(psum, p))
+}
+
+func (p TCPPacket) Valid() bool {
+	return len(p) >= TCPHeaderSize
+}
+
+func (p TCPPacket) Verify(sourceAddress net.IP, targetAddress net.IP) error {
+	var checksum [2]byte
+	checksum[0] = p[16]
+	checksum[1] = p[17]
+
+	// reset checksum
+	p[16] = 0
+	p[17] = 0
+
+	// restore checksum
+	defer func() {
+		p[16] = checksum[0]
+		p[17] = checksum[1]
+	}()
+
+	// check checksum
+	s := uint32(0)
+	s += Sum(sourceAddress)
+	s += Sum(targetAddress)
+	s += uint32(TCP)
+	s += uint32(len(p))
+
+	check := Checksum(s, p)
+	if checksum[0] != check[0] || checksum[1] != check[1] {
+		return ErrInvalidChecksum
+	}
+
+	return nil
+}