There are various port scanners which uses simple method of scanning. These applications uses the normal connect method to scan open ports and it takes long time. This scanner is faster than normal scanner as it uses faster 'TCP Half Open' Scanning or 'TCP SYN' scanning technique. This method is less detectable than the simple port scanner.

When any two hosts wants to communicate together connection must be established between them. In case of TCP connection, three way handshake takes place between source and destination hosts. Below is the exact sequence of packets exchanged between the hosts.

Once the connection is established, both machines can transmit data packet until one of them ends the connection by sending FIN packet. Some of the simple port scanners use this technique. It can be implemented by creating socket and calling 'Connect method' on each port. This is simple to implement but very slow method and more over it can be easily detected. Half scanning is more fast and efficient than full scanning technique. Half open connection is explained below.  

Since the host A does not send any additional ACK packet , it is called half open connection. Now the host can easily find out if the target port is open or closed. It it receives TCP packet with SYN+ACK flag set, then it means that target port is open. If it receives RST+ACK packet, it implies that target port is closed.

In this method full handshake does not take place , hence it is quite faster than full scanning method. Since the implementation has to be done at the protocol level , knowledge of TCP/IP protocol suite is essential.

Core part of the implementation is sending the TCP packet and ARP packet. This involves building the raw packet by filling all headers. For this we must know MAC address of the source and destination machine. MAC address also called Ethernet address, is the address associated with Ethernet adapter.

This is done by sending ARP packet. ARP packet is used to determine the host's MAC address when its IP address is known. First ARP request packet is sent by specifying the source MAC address, source IP address and destination IP Address. The ARP reply packet contains the destination MAC address. This method also prevents the target host from sending ARP packet to source host when the source host sends the first SYN packet during scanning process. From the ARP request packet that we have sent, target host will come to know about the MAC address of the source host.

Scanning process involves building TCP packet. For this one has to prepare the Ethernet Header, IP header and TCP header. Header file 'packet.h' contains the format details for each of these headers. You can refer RFC for details regarding these formats.

Each time during scanning TCP SYN packet is sent with different port numbers. Then the corresponding reply packet is checked for the flag RST+ACK or SYN+ACK. Based upon this flag target port status is determined.