Ping Sweep Techniques

The CEH scanning methodology starts with checking for systems that are live on the network, meaning that they respond to probes or connection requests. The simplest, although not necessarily the most accurate, way to determine whether systems are live is to perform a ping sweep of the IP address range. All systems that respond with a ping reply are considered live on the network. A ping sweep is also known as Internet Control Message Protocol (ICMP) scanning, as ICMP is the protocol used by the pingcommand.
ICMP scanning, or a ping sweep, is the process of sending an ICMP request or ping to all hosts on the network to determine which ones are up and responding to pings. ICMP began as a protocol used to send test and error messages between hosts on the Internet. It has evolved as a protocol utilized by every operating system, router, switch or Internet Protocol (IP)-based device. The ability to use the ICMP Echo request and Echo reply as a connectivity test between hosts is built into every IP-enabled device via the pingcommand. It is a quick and dirty test to see if two hosts have connectivity and is used extensively for troubleshooting.
A benefit of ICMP scanning is that it can be run in parallel, meaning all systems are scanned at the same time; thus it can run quickly on an entire network. Most hacking tools include a ping sweep option, which essentially means performing an ICMP request to every host on the network. Systems that respond with a ping response are alive and listening on the network. 
A timeout indicates that the remote system is not responding or turned off or that the ping was blocked. A reply indicates that the system is alive and responding to ICMP requests.

Detecting Ping Sweeps

Almost any IDS or intrusion prevention system (IPS) system will detect and alert the security administrator to a ping sweep occurring on the network. Most firewall and proxy servers block ping responses so a hacker can't accurately determine whether systems are available using a ping sweep alone. More intense port scanning must be used if systems don't respond to a ping sweep. Just because a ping sweep doesn't return any active hosts on the network doesn't mean they aren't available—you need to try an alternate method of identification. Remember, hacking takes time, patience, and persistence.

Scanning Ports and Identifying Services

Checking for open ports is the second step in the CEH scanning methodology. Port scanning is the method used to check for open ports. The process of port scanning involves probing each port on a host to determine which ports are open. Port scanning generally yields more valuable information than a ping sweep about the host and vulnerabilities on the system.
Service identification is the third step in the CEH scanning methodology; it's usually performed using the same tools as port scanning. By identifying open ports, a hacker can usually also identify the services associated with that port number.

Port-Scan Countermeasures

Countermeasures are processes or toolsets used by security administrators to detect and possibly thwart port scanning of hosts on their network. The following list of countermeasures should be implemented to prevent a hacker from acquiring information during a port scan:
  • Proper security architecture, such as implementation of IDS and firewalls, should be followed.
  • Ethical hackers use their toolset to test the scanning countermeasures that have been implemented. Once a firewall is in place, a port-scanning tool should be run against hosts on the network to determine whether the firewall correctly detects and stops the port-scanning activity.
  • The firewall should be able to detect the probes sent by port-scanning tools. The firewall should carry out stateful inspections, which means it examines the data of the packet and not just the TCP header to determine whether the traffic is allowed to pass through the firewall.
  • Network IDS should be used to identify the OS-detection method used by some common hackers tools.
  • Only needed ports should be kept open. The rest should be filtered or blocked.
  • The staff of the organization using the systems should be given appropriate training on security awareness. They should also know the various security policies they're required to follow.

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