CEH

Checking for Open Ports 111

TA B LE 5.1 TCP flags

Flag Use

SYN Initiates a connection between two hosts to facilitate communication.

ACK Acknowledges the receipt of a packet of information.

URG Indicates that the data contained in the packet is urgent and should be

processed immediately.

PSH Instructs the sending system to send all buffered data immediately.

FIN Tells the remote system that no more information will be sent. In essence

this gracefully closes a connection.

RST Resets a connection.

These flags will figure prominently in this section as well as on the CEH

exam in several areas such as sniffing and intrusion detection systems.

Study and memorize each of them.

FIGURE 5.1 The three-way handshake

Host A Host B

Send SYN seq=x In the Network

Receive SYN

Send SYN seq=y, ACK x+1

Receive ACK

Receive SYN + ACK

Send ACK y+1

Time