Nmap detects ports by sending specially crafted packets to the target system and analyzing the responses. Here’s a basic overview of how it works:
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Port Scanning: Nmap sends packets to a range of ports on the target system. This can include TCP, UDP, or other types of packets depending on the scan type.
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Response Analysis: Nmap listens for responses from the target. The responses can indicate whether a port is open, closed, or filtered:
- Open: The target responds positively, indicating that a service is listening on that port.
- Closed: The target responds with a reset (RST) packet, indicating that no service is listening on that port.
- Filtered: No response is received, or an ICMP unreachable message is returned, suggesting that a firewall or filter is blocking access to the port.
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Service Detection: If a port is open, Nmap may also attempt to identify the service running on that port by sending additional probes and analyzing the responses.
By using various scanning techniques (like TCP Connect, SYN scan, etc.), Nmap can effectively determine the state of ports on a target system.
