Network Scanner
Network scanning involves using a port scanner to identify all hosts potentially connected to an organization's network, the network services operating on those hosts, such as the file transfer protocol (FTP) and hypertext transfer protocol (HTTP), and the specific application running the identified service, such as Internet Information Server (IIS) and Apache for the HTTP service. The result of the scan is a comprehensive list of all active hosts and services, printers, switches, and routers operating in the address space scanned by the port-scanning tool, i.e., any device that has a network address or is accessible to any other device.
Port scanners, such as nmap, first identify active hosts in the address range specified by the user using Transport Control Protocol/Internet Protocol (TCP/IP) Internet Control Message Protocol (ICMP) ECHO and ICMP ECHO_REPLY packets. Once active hosts have been identified, they are scanned for open TCP and User Datagram Protocol (UDP) ports that will then identify the network services operating on that host.

All basic scanners will identify active hosts and open ports, but some scanners provide additional information on the scanned hosts. The information gathered during this open port scan will often identify the target operating system. This process is called operating system fingerprinting. For example, if a host has TCP port 135 and 139 open, it is most likely a Windows NT or 2000 host.

While port scanners identify active hosts, services, applications and operating systems, they do NOT identify vulnerabilities (beyond some common Trojan ports). Vulnerabilities can only be identified by a human who interprets the mapping and scanning results. From these results, a qualified individual can ascertain what services are vulnerable and the presence of Trojans. Although the scanning process itself is highly automated, the interpretation of scanned data is not.

Purpose of network port scanning is to
  • Check for unauthorized hosts connected to the organization’s network
  • Identify vulnerable services
  • Identify deviations from the allowed services defined in the organization’s security policy
  • Prepare for penetration testing
  • Assist in the configuration of the intrusion detection system (IDS)
  • Collect forensics evidence.
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