Hardware: The Appliance Model

The hardware appliance model is exactly that: hardware with built-in software to perform the desired vulnerability scans. The devices are typically placed throughout a network and report back to a central server. The scanning appliances are usually complete but simple computer systems. A typical design has an operating system (OS), supporting software modules, and the specialized code written by the developers to perform scans and communicate results. Some vendors use open-source tools and others will use a commercial OS and components.

One major advantage of a hardware-based system is that the vendor will have in-depth knowledge about the configuration of the host. The vendor takes responsibility for the maintenance and stability of that configuration. Any failure of the software to perform as advertised should be addressed in the client–vendor relationship.

In deployment, the hardware approach has the disadvantage of having to be shipped to the location and installed by someone who may not be qualified to do so. In most cases, however, deployment is not so complex. If the local technologist can configure a typical host computer, he or she can configure a vulnerability scanner. If you are uncertain about the capabilities of local personnel, then you may be well-advised to preconfigure the device and provide simple installation instructions.

In most designs, each scanner will report back to a central server. The vulnerability and compliance information collected will be transmitted back to the server for analysis and reporting. Devices will also receive assessment instructions over the network. Those instructions may be delivered by polling, on-demand connection, or through reverse polling. The impact of these strategies will be minimal but important, depending on your network security architecture.

Polling is the process of taking a poll of the vulnerability scanners associated with a central server. Each scanner is typically contacted through a TCP port with special authentication methods that keep the entire conversation encrypted. The devices that are polled may be only those for which the server has a job prepared or in progress. The server checks the status to see if any data is available or if the unit is ready to accept a job. This approach can be cumbersome but has the advantage of only requiring a connection originating from the server. In some cases, not all scanners are polled unless there is scheduled work that can result in not knowing the status of a scanner until that time. Most vendors that poll will poll all scanners. Figure 1 illustrates the simple polling approach.

 
Figure 1: The simple polling approach.

Reverse polling is the process whereby each scanner contacts the server on a regular basis. Should there be a job scheduled for the scanner, it would then be provided. The same strong authentication and encryption methods apply. The scanner will send the results of the scan back to the central server either during the scan or at the conclusion, depending on the software designer’s choice. This approach has the added advantage of allowing the scanner to complete a local job even if the connection with the server is lost. The scan results may simply be cached until a connection can be re-established.

Reverse polling also has an advantage when deployed in a secure zone where in-bound communications to the scanner may be undesirable in order to limit possible external connections. This is also a disadvantage should the scanner be deployed outside the organization’s boundaries because accommodations must be made in the security infrastructure for connections from the scanner.