A TCP scan, also known as a TCP port scan, is a technique used in network reconnaissance to identify open ports on a target system. It involves sending TCP (Transmission Control Protocol) packets to various port numbers on a target host and analyzing the responses to determine which ports are open, closed, or filtered.
Types of TCP Scans:
- TCP Connect Scan (also known as a full-connect scan): In this type of scan, the scanner attempts to establish a full TCP connection with each port on the target. If a connection is successfully established, it indicates that the port is open and accepting connections.
- TCP SYN Scan (also known as half-open scan or stealth scan): This scan sends SYN (synchronize) packets to the target ports. If the port is open, it responds with a SYN-ACK packet. If the port is closed, it responds with a RST (reset) packet. This scan technique is stealthier than the TCP Connect Scan because it does not complete the TCP handshake.
- TCP FIN Scan: This scan sends FIN (finish) packets to the target ports. If the port is closed, it should respond with a RST packet. However, some systems may not respond to a FIN packet at all, making it difficult to determine if the port is open or closed accurately.
- TCP XMAS Scan: Similar to the FIN scan, this scan sends packets with the FIN, URG (urgent), and PSH (push) flags set. It aims to elicit responses from closed ports, which may vary depending on the target system’s behavior.
- TCP NULL Scan: This scan sends packets with no TCP flags set. Like the FIN scan, it aims to elicit responses from closed ports. If the port is open, it should not respond at all. However, some systems may respond differently, making interpretation challenging.
Purposes of TCP Scans:
- Network reconnaissance: TCP scans are commonly used by network administrators and security professionals to identify potential vulnerabilities in their systems and network infrastructure.
- Security assessments: Penetration testers and ethical hackers use TCP scans to assess the security posture of target systems and identify entry points for further exploitation.
- Intrusion detection: TCP scans can also be used by intrusion detection systems (IDS) and intrusion prevention systems (IPS) to detect and block suspicious or unauthorized network activity.
Legal and Ethical Considerations:
While TCP scanning itself is not inherently malicious, using it without proper authorization can violate laws and regulations related to unauthorized access to computer systems and networks. It’s essential to obtain permission from the system owner or administrator before conducting any type of network scanning.
In summary, TCP scanning is a valuable tool for network reconnaissance and security assessment, but it should be used responsibly and within the bounds of applicable laws and ethical guidelines.
