Quick Reference on Port Scanning
Intro This article is about basic types of port scanning. Port States (taking from Nmap man page) open An application is actively accepting TCP connections, UDP datagrams or SCTP associations on this port. closed A closed port is accessible (it receives and responds to Nmap probe packets), but there is no application listening on it. They can be helpful in showing that a host is up on an IP address (host discovery, or ping scanning), and as part of OS detection. Because closed ports are reachable, it may be worth scanning later in case some open up. Administrators may want to consider blocking such ports with a firewall. Then they would appear in the filtered state, discussed next. filtered Nmap cannot determine whether the port is open because packet filtering prevents its probes from reaching the port. The filtering could be from a dedicated firewall device, router rules, or host-based firewall software. Sometimes they respond with ICMP error messages such as type 3 c...