Host Firewall

If it acts with the hardware, on which the Firewall software runs, not over specific equipment (separate around for example a PC with Linux, Windows or also a Sun workstation), then rather to the special software is referred and calls it then software Firewall. Thus for example personnel Firewalls are called software Firewalls, because they normally run on (also for other purposes used) the PC.

The software component of the Firewall works on the layers 2 to 7 of the OSI reference model, and therefore the implementation level can fail very differently. Therefore a Firewall often consists of different software components. The different parts are to be described here briefly:

Package filter

A package filter makes only the decisions, which can be made completely on package basis, thus particularly on basis of the source/destination addresses of the packages.

For further information see article package filter

Content filter

A Firewall cannot however only work on the low level of the package filter, but also take over more complex tasks. A content filter examines for example contents of the packages and not only the meta data of the packages such as pouring and/or destination address. Such tasks can be for example the following:

• Filter HTML sides requested by ActiveX and/or Javascript from
• Filters/characteristics of Spam Mails
• Delete from virus Mails
• Filter from confidential firm information (e.g. Balance)

Most systems permit only the definition of very simple rules; the problem is however very in principle complex and the concept is possibly technically not completely convertible (e.g. really surely and completely confidential information is to be filtered from the data traffic to not authorized systems, then the technical problem would have to be solved among other things, be recognized and filtered like confidential steganografische or coded information is).

Despite the rules arranged quite simply in current Firewall systems their execution can become very complex: Frequently individual packages must be built up, thus the regarded data traffic (e.g. HTML side) to be as a whole recognized, scanned and possibly changed can. Subsequently, the data traffic must again into individual packages be divided and can then be further-sent. Note: I) Usually the deletion of virus Mails is the task of virus scanners. Virus scanners scan among other things the entire outgoing and detailed data stream for viruses and delete these with positive findings. Typically this is not task of a Firewall. II) Spam Mails are marked by Spam filters. Typically this is not task of a Firewall.

Proxy

Major item: Proxy

Many Firewallsysteme possess one or more integrated transparent Proxys, which are to a large extent unbemerkbar for Client and servers and are applied automatically of the Firewall to appropriate connections. Purpose of these Proxys is (simplified) the implementing of the minutes validating and adjustment (ith S. of a normalization or defined restriction) of transferred minutes communication for the reduction of the attack region on application level or purposeful closing of certain minutes transactions (e.g. purposeful prevention from haven mode ftp). Firewallsysteme differ strongly in the number and kind of minutes supported by Proxys (e.g. ) As well as configuration options which is available if necessary for this Proxys.Ohne pro XY concept the possibilities of the minutes normalization are very limited for ftp, DNS, HTTP, smtp, SQL*Net, POP3, ms RPC etc., since an active intervention in the data stream is limited to break/Blacklisting. Many Firewalls with Proxy cannot adapt beyond that minutes options, approximately in a smtp transaction BDAT, VRFY permit.

Application Layer Firewall

Is a Firewall, which on layer 7 (application layer) of the ISO OSI model works. A Firewall, which examines contents of requested HTML sides before the distribution e.g. for viruses, is an example of a Application Layer Firewall.

Condition-steered filtering

The condition-steered filtering (English stateful inspection ) a method is for the extension of the function of a package filter. The weakness of a simple package filter is it that each package is regarded individually and only on the basis the information in this packet is decided, whether it is valid or not. The condition-steered filtering notes against it the status of a connection (identify by suitable characteristic data, for example IP addresses and haven) and can a new packet to a coherent logical data stream assign. This information can be consulted as the further filter criterion. Contrary to a Proxy however the connection is not affected. The company Check POINT software Technologies Ltd. this technology takes up to have invented and patent for itself, (U.S. patent # 5.606.668).

Advantage

The advantage of the condition-steered filtering on the basis an example:

If a computer A with a computer B communicates over a simple package filter, then must permit these two connections (NAT and the like omitted):

• A pours after a goal B
• B pours after a goal A (for the answer packages)

That means that both computers can take up communication, there it no possibility give to clarify, who may begin.

With the condition-steered filtering only one rule is needed (and/or second is implicitly added):

• A pours after a goal B

The package filter notes the fact that computer A with computer B communicated and permitted also from computer B at computer A. Rechner B cannot begin answers to it however. Under normal conditions also on pouring and goal haven is tested (this to be allowed any longer not change, so that they belong to the same connection) and thus communication limits to exactly possible communication.

Still large systems examine additionally whether a package is at all permitted at a certain time in communication send (for example further packages, although the other participant already locked communication).

Personal Firewalls

Major item: Personal Firewall

Personal Firewalls or also Desktop Firewalls are programs, which are installed on the computer which can be protected locally. Thus this kind is meant from Firewall not for controlling traffic between several nets for in-permitting or for not out-permitting but certain traffic into the local computer. The installation on the computer which can be protected makes it possible also to filter userally-specific. Many products put their emphasis on simple configuration. The protective effect of personnel Firewalls is however rather small.

Example

A simple concept is to clarify this dry subject: A company would like their personal computers in Internet to bring. One decides for a Firewall, and due to the virus/worm danger one would like to develop only the connections to a Mail server. So that also a search is possible in the Internet, a PC is to receive access over a Proxy to web pages. The Surf computer is protected additionally by the fact that ActiveX is filtered from the requested HTML sides from safety reasons.

Other accesses from the outside to the firm net are to be simply blocked. It is important that in this constellation the personal computers themselves cannot develop any direct connection to the Internet. Thus once transferred harming programs can spread only further or reload further parasits from the Internet, if they find first times the Proxy or the Mailserver - fortunately (at present) the parasits cannot usually.

DSL-modem/DSL-rout

DSL-rout normally take over Routing functionality and can accesses from the Internet to the local area network block (haven filter functionality). By NAT it is possible to operate several computers at a DSL modem. Such products do not contain a content filter mostly.

Products

• Endian Firewall is open a SOURCE Linux distribution for Firewall systems.
• Astaro Security Linux is a commercial Linux distribution for Firewall systems.
• pf a OpenSource Firewall is originally for OpenBSD (Berkeley Software Distribution) was developed and on other BSD operating systems portiert later.
• Smoothwall is one for Firewall systems optimized Linux distribution.
• Netfilter - package filters within the Linux Kernels.
• That a-disketterout fli4l is apart from the CD variant Gibraltar a project, which permits the use of old PCs in the sense of a lasting use as Firewall.
• IPCop is a Linux distribution which can be served simply, which has the goal to be a thoroughly safe Firewall.
• M0n0wall is a BSD based Firewall, which partly approaches with its functions at professional Firewalls and nevertheless very simply to be configured is.
• BrazilFW is one on older PCs executable Linux Firewall, which can be content also with a disk.
• mGuard a hardware Firewall is for the security of individual servers of the company Innominate
• .vtFW are Firewalls on basis of OpenBSD pf the company .vantronix.
• Microsoft Internet Security and Acceleration server is a commercial Firewall of Microsoft.

See also

• Computer security
• Intrusion Detection system
• Air Gap

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