Nmap determines the state of a port during a scan using various techniques, primarily based on the responses it receives from the target. Here are the main states and how Nmap identifies them:
Open:
- The port is accepting connections. Nmap sends a SYN packet (for TCP scans) and receives a SYN-ACK response, indicating the port is open.
Closed:
- The port is not accepting connections. Nmap sends a SYN packet and receives a RST (reset) response, indicating the port is closed.
Filtered:
- Nmap cannot determine if the port is open or closed because a firewall or filter is blocking the response. This can happen if no response is received or if an ICMP unreachable message is returned.
Open|Filtered:
- Nmap cannot determine if the port is open or filtered. This typically occurs when no response is received for a SYN packet.
Closed|Filtered:
- This state is less common but can occur in specific scenarios where Nmap cannot definitively classify the port.
Techniques Used:
- TCP Connect Scan: Establishes a full TCP connection to determine if a port is open.
- SYN Scan: Sends SYN packets and analyzes the responses without completing the TCP handshake.
- UDP Scan: Sends UDP packets and checks for responses or lack thereof to determine the state.
These methods allow Nmap to provide a comprehensive view of the network's port states. If you want to learn more about Nmap scanning techniques, consider exploring related resources or labs on LabEx!
