RangerIDS - Intrusion Detection System (IDS)
Heuristic-based signatures use some kind of algorithmic logic on which to base their alarm decisions. These algorithms are often statistical evaluations of the type of traffic being presented.
A good example of this type of signature is one that would be used to detect a port sweep. This signature looks for the presence of a threshold number of unique ports being touched on a particular machine. The signature may further restrict itself through the specification of the types of packets that it is interested in (that is, SYN packets). Additionally, there may be a requirement that all the probes must originate from a single source, and even that valid SYN ACK packets must be seen to be returned by the host being probed.
Signatures of this type will react differently on different networks, and can be a significant source of false positives if not tuned correctly, requiring some threshold manipulations to make them conform to the utilisation patterns on the network they are monitoring. This type of signature may be used to look for very complex relationships as well as the simple statistical example given.