Banner Grabbing and OS Fingerprinting Techniques

Banner grabbing and operating system identification—which can also be defined as fingerprinting the TCP/IP stack—is the fourth step in the CEH scanning methodology. The process of fingerprinting allows the hacker to identify particularly vulnerable or high-value targets on the network. Hackers are looking for the easiest way to gain access to a system or network. Banner grabbing is the process of opening a connection and reading the banner or response sent by the application. Many email, FTP, and web servers will respond to a telnet connection with the name and version of the software. This aids a hacker in fingerprinting the OS and application software. For example, a Microsoft Exchange email server would only be installed on a Windows OS.

Active stack fingerprinting is the most common form of fingerprinting. It involves sending data to a system to see how the system responds. It's based on the fact that various operating system vendors implement the TCP stack differently, and responses will differ based on the operating system. The responses are then compared to a database to determine the operating system. Active stack fingerprinting is detectable because it repeatedly attempts to connect with the same target system.

Passive stack fingerprinting is stealthier and involves examining traffic on the network to determine the operating system. It uses sniffing techniques instead of scanning techniques. Passive stack fingerprinting usually goes undetected by an IDS or other security system but is less accurate than active fingerprinting.

---

Drawing Network Diagrams of Vulnerable Hosts

Although it isn't a CEH exam objective, understanding the tools used in step 6 of the CEH scanning methodology—drawing a network diagram of vulnerable hosts—is a must. A number of network management tools can assist you with this step. Such tools are generally used to manage network devices but can be turned against security administrators by enterprising hackers.

SolarWinds Toolset, Queso, Harris Stat, and Cheops are network management tools that can be used for detecting operating systems, mapping network diagrams, listing services running on a network, performing generalized port scanning, and so on.

These tools diagram entire networks in a GUI interface, including routers, servers, hosts, and firewalls. Most of these tools can discover IP addresses, hostnames, services, operating systems, and version information.

Netcraft and HTTrack are tools that fingerprint an operating system. Both are used to determine the OS and web server software version numbers.

Netcraft is a website that periodically polls web servers to determine the operating system version and the web server software version. Netcraft can provide useful information the hacker can use in identifying vulnerabilities in the web server software. In addition, Netcraft has an antiphishing toolbar and web server verification tool you can use to make sure you're using the actual web server rather than a spoofed web server. Exercise 1 shows how to use Netcraft to identify the OS or a web server.

Exercise 1: Use Netcraft to Identify the OS of a Web Server

---

1. Open a web browser to the Netcraft website, www.netcraft.com.
2. Type a website name in the What's That Site Running? field in the upper-left corner of the screen.
3. Scroll down to Hosting History to see what OS and web server software are running on the server.

---

HTTrack arranges the original site's relative link structure. You open a page of the mirrored website in your browser, and then you can browse the site from link to link as if you were viewing it online. HTTrack can also update an existing mirrored site and resume interrupted downloads.