Passive reconnaissance is an attempt to gain information about targeted computers and networks without actively engaging with the systems. In active reconnaissance, in contrast, the attacker engages with the target system, typically conducting a port scan to determine find any open ports.
The term reconnaissance comes from its military use to describe an information-gathering mission. Both types of reconnaissance are sometimes referred to as passive attacks because the purpose is simply to obtain information, rather than to actively exploit the target. However, reconnaissance is often a preliminary step towards an active attempt to exploit the target system.
Methods of passive reconnaissance include:
- War driving to detect vulnerable wireless networks.
- Looking for information stored on discarded computers and other devices.
- Masquerading as an authorized network user.
Both active and passive reconnaissance are also used for ethical hacking, in which white hat hackers use attack methods to determine system vulnerabilities so that problems can be taken care of before the system falls prey to a real attack.
The simplest way to protect yourself from port scan attacks or reconnaissance attacks is to use a good firewall and intrusion prevention system (IPS). The firewall controls which ports are exposed and to whom they are visible, while the IPS will detect port scans in progress and shut them down before they are able to gain a full map of your network.
Continue reading about active reconnaissance:
> Reconnaissance attacks are covered in this introduction to network security.
>eTorials also covers reconnaissance attacks.