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A '''host-based intrusion detection system''' ('''HIDS''') is an [[intrusion detection system]] that monitors and analyzes the internals of a computing system rather than the network packets on its external interfaces (as a [[Network_intrusion_detection_system|network-based intrusion detection system]] (NIDS) would do).

== Overview ==
A host-based IDS monitors all or parts of the dynamic behavior and the state of a computer system. Much as a NIDS will dynamically inspect network packets, 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 [[Random Access Memory|RAM]], in the file system, log files or elsewhere; and check that the contents of these appear as expected.

One can think of a HIDS as an [[software agent|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 behaviour in the form of [[anti-virus software|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 very blurred here, as many of the tools overlap in functionality.

=== Monitoring state ===
The principle operation of a HIDS depends on the fact that successful intruders ([[cracking|crackers]]) 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|botnet activity]], [[Spyware|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 a HIDS works in conjunction with a NIDS, such that a HIDS finds anything that slips past the NIDS.

Ironically, most successful intruders, on entering a target machine, immediately apply best-practice security techniques to secure the system which they have infiltrated, leaving only their own [[Backdoor (computing)|backdoor]] open, so that other intruders can not take over ''their'' computers.

==== Technique ====
In general a HIDS uses a [[database]] (object-database) of system objects it should monitor - usually (but not necessarily) file system objects. A HIDS could also check that appropriate regions of memory have not been modified, for example - the system call table comes to mind for [[Linux]], and various [[virtual method table|vtable]] structures in [[Microsoft Windows]].

For each object in question a HIDS will usually remember its attributes (permissions, size, modifications dates) and create a [[checksum]] of some kind (an [[MD5]], [[SHA1]] hash or similar) for the contents, if any. This information gets stored in a secure database for later comparison (checksum database).

An alternate method to HIDS would be to provide NIDS type functionality at the network interface (NIC) level of an end-point (either server, workstation or other end device). Providing HIDS at the network layer has the advantage of providing more detailed logging of the source (IP address) of the attack and attack details, such as packet data), neither of which a dynamic behavioral monitoring approach could see.

==== Operation ====
At installation time - and whenever any of the monitored objects change legitimately - a HIDS must initialize its checksum-database by scanning the relevant objects. Persons in charge of computer security need to control this process tightly in order to prevent intruders making un-authorized changes to the database(s). Such initialization thus generally takes a long time and involves [[cryptography|cryptographically]] locking each monitored object and the checksum databases or worse. Because of this, manufacturers of HIDS usually construct the object-database in such a way that makes frequent updates to the checksum database unnecessary.

Computer systems generally have many dynamic (frequently changing) objects which intruders want to modify - and which a HIDS thus should monitor - but their dynamic nature makes them unsuitable for the checksum technique. To overcome this problem, HIDS employ various other detection techniques: monitoring changing file-attributes, log-files that decreased in size since last checked, and numerous other means to detect unusual events.

Once a system administrator has constructed a suitable object-database - ideally with help and advice from the HIDS installation tools - and initialized the checksum-database, the HIDS has all it requires to scan the monitored objects regularly and to report on anything that may appear to have gone wrong. Reports can take the form of logs, e-mails or similar.

=== Protecting the HIDS ===
A HIDS will usually go to great lengths to prevent the object-database, checksum-database and its reports from any form of tampering. After all, if intruders succeed in modifying any of the objects the HIDS monitors, nothing can stop such intruders from modifying the HIDS itself - unless security administrators take appropriate precautions. Many [[Computer worm|worms]] and [[Computer virus|viruses]] will try to disable anti-virus tools, for example.

Apart from crypto-techniques, HIDS might allow administrators to store the databases on a [[CD-ROM]] or on other read-only memory devices (another factor militating for infrequent updates...) or storing them in some off-system memory. Similarly, a HIDS will often send its logs off-system immediately - in some instances via one-way communications channels, such as a serial port which only has "Transmit" connected, for example.

One could argue that the [[trusted platform module]] comprises a type of HIDS. Although its scope differs in many ways from that of a HIDS, fundamentally it provides a means to identify whether anything/anyone has tampered with a portion of a computer. Architecturally this provides the ultimate (at least {{As of|2005|alt=at this point in time}}) host-based intrusion detection, as depends on hardware external to the [[central processing unit|CPU]] itself, thus making it that much harder for an intruder to corrupt its object and checksum databases.

==See also==
* [[Intrusion detection system]] (IDS)
* [[Network intrusion detection system]] (NIDS)
* [[Verisys]] - commercial HIDS
* [[Tripwire (software)]] - commercial HIDS
* [[OSSEC]] - a multi-platform open source HIDS
* [[Trusted Computing Group]]
* [[Trusted platform module]]

==External links==
[https://mosaicsecurity.com/categories/74-endpoint-security-hostbased-intrusion-detection-system Complete List of Commercial HIDS, Mosaic Security Research]
[[Category:Intrusion detection systems]]

Host-based intrusion detection system - Revision history

wikademia>Eme: from and by http://en.wikipedia.org/w/index.php?title=Host-based_intrusion_detection_system&action=history