How do you know that a port being scanned is open

Question

Scanners send specific packets to determine port status. What responses indicate that a port is open, and how are they interpreted?

Answer 1

Port scanners determine the status of a port by analyzing the target system's responses to specific probe packets. The nature of these responses indicates whether a port is open, closed, or filtered. The interpretation varies between the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).​

TCP Port Scanning:

One of the most common techniques for TCP is the SYN scan, often referred to as a "half-open" scan. The process involves:​

1. Sending a SYN Packet: The scanner transmits a TCP packet with the SYN flag set to the target port, initiating a connection request.​

2. Analyzing Responses:

   • SYN-ACK Response: If the target port is open, it replies with a SYN-ACK packet, indicating readiness to establish a connection. The scanner typically responds by sending a RST (reset) packet to terminate the handshake before completion.

   • RST Response: If the port is closed, the target responds with a RST packet, indicating no service is listening on that port.

   • No Response or ICMP Error: If no response is received, or an ICMP unreachable error is returned, it suggests the port is filtered by a firewall or other network device, preventing the probe from reaching the port.

UDP Port Scanning:

UDP scanning is more challenging due to the connectionless nature of the protocol. The typical steps include:​

1. Sending a UDP Packet: The scanner sends a UDP packet to the target port.​

2. Analyzing Responses:

   • ICMP Port Unreachable Message: If the port is closed, the target often responds with an ICMP message indicating the port is unreachable.

   • No Response: An open port may not respond at all, making it difficult to distinguish between open and filtered ports. Some services might reply with a UDP packet, confirming the port is open. ​

   • Rate Limiting: Many systems implement rate limiting for ICMP responses, which can slow down UDP scanning and lead to inaccurate results. ​

Considerations:

• Accuracy: The reliability of scan results can be affected by network configurations, firewalls, and intrusion detection systems that may block or manipulate responses.​

• Legal Implications: Conducting port scans without permission can be considered intrusive and may violate legal statutes or organizational policies. Always ensure you have authorization before performing such scans.​

Understanding these response patterns enables network administrators and security professionals to assess the security posture of systems and identify potential vulnerabilities effectively.