A host-based intrusion detection system (HIDS) is an intrusion detection system that is capable of monitoring and analyzing the internals of a computing system as well as the network packets on its network interfaces, similar to the way a network-based intrusion detection system (NIDS) operates. This was the first type of intrusion detection software to have been designed, with the original target system being the mainframe computer where outside interaction was infrequent. Overview A host-based IDS is capable of monitoring all or parts of the dynamic behavior and the state of a computer system, based on how it is configured. Besides such activities as dynamically inspecting network packets targeted at this specific host (optional component with most software solutions commercially available), a HIDS might detect which program accesses what resources and discover that, for example, a word-processor has suddenly and inexplicably started modifying the system password database. Similarly a HIDS might look at the state of a system, its stored information, whether in RAM, in the file system, log files or elsewhere; and check that the contents of these appear as expected, e.g. have not been changed by intruders. One can think of a HIDS as an agent that monitors whether anything or anyone, whether internal or external, has circumvented the system's security policy. Monitoring dynamic behavior Many computer users have encountered tools that monitor dynamic system behavior in the form of anti-virus (AV) packages. While AV programs often also monitor system state, they do spend a lot of their time looking at who is doing what inside a computer – and whether a given program should or should not have access to particular system resources. The lines become blurred here, as many of the tools overlap in functionality. Some intrusion prevention systems protect against buffer overflow attacks on system memory and can enforce security policy. Monitoring state The principle operation of a HIDS depends on the fact that successful intruders (hackers) will generally leave a trace of their activities. In fact, such intruders often want to own the computer they have attacked, and will establish their "ownership" by installing software that will grant the intruders future access to carry out whatever activity (keystroke logging, identity theft, spamming, botnet activity, spyware-usage etc.) they envisage. In theory, a computer user has the ability to detect any such modifications, and the HIDS attempts to do just that and reports its findings. Ideally