There are less noisy alternatives to nmap that fall into the passive fingerprinting category. Instead of actively sending packets to a host and service, they passively analyze network traffic to identify unique characteristics for particular operating systems, client applications, and network services. Their strength is based on the breadth of their signature database, which is not always great. To date, the majority of free and open-source passive fingerprinting tools have focused on OS fingerprinting
Passive fingerprinting can go way beyond just identifying the operating system. Web and mail servers often give up more information than necessary when communicating with clients, and in general, as long as that traffic is unique and can be sniffed, a fingerprint can be created to identify it. The same goes for Web browsers, email clients, and any other application that communicates over the network. Often their version numbers are passed as part of their communications with servers. Tie that version information with a vulnerability database and vulnerable systems can start being identified without ever interacting with the system.
To date, PRADS is the one of the few open-source tools I've found that currently includes fingerprinting additional things like services; however, Michal Zalewski announced on Jan. 10 the availability of p0f v3, which includes the ability to fingerprint TCP services. This is the first release in about six years and is a complete rewrite. This new version currently supports HTTP response and request signatures, but additional protocols are expected to be added in the future.
The obvious difficulty with passive fingerprinting is the need to sniff the traffic, but if you're just monitoring traffic between yourself and another system, that's not an issue. In larger environments, you'll want to leverage a mirror port on a network switch or a network tap in order to see as much traffic as possible. The cool thing about p0f is that it can help find NAT devices in large environments where you might have users plugging in wireless routers.
There's a lot of value and fun that can be had through passive fingerprinting. I'll be discussing it more in the future as I start working on a project specific to mobile devices and application fingerprinting. For a quick preview and review on passive fingerprinting, check out my FireTalk, "Passive Aggressive Pwnage," at ShmooCon 2012.
John Sawyer is a Senior Security Analyst with InGuardians. The views and opinions expressed in this blog are his own and do not represent the views and opinions of his employer. He can be reached at [email protected] and found on Twitter @johnhsawyer.