October 3, 1998
Department of Electrical and Communications Engineering
Helsinki University of Technology
Department of Mechanical Engineering
Helsinki University of Technology
A firewall is a security mechanism that allows limited acess to your site from the internet, allowing approved traffic in and out according to a thought-out plan. This lets you select the services appropriate to your business needs, while barring others which may have significant security holes. A firewall can significantly improve the level of site security while at the same time permitting access to vital Internet services.
A firewall is a system or group of systems that enforces an access control policy between two networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the other which exists to permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy.[7, 9]
The firewall is designed specifically as a security system for preventing unauthorized communications between one computer network and another computer network and more specifically for preventing unauthorized access to a private computer network from a public computer network such as the Internet.
Firewalls are also important since they can provide a single "check point" where security and audit can be imposed. Unlike in a situation where a computer system is being attacked by someone dialing in with a modem, the firewall can act as an effective "phone tap" and tracing tool. Firewalls provide an important logging and auditing function; often they provide summaries to the administrator about what kinds and amount of traffic passed through it, how many attempts there were to break into it, etc.
Firewalls are a secured gateway at the perimeter of security domains. They are a computer version of a customs checkpoint, enforcing the site's security policy to control traffic into and out of the internal networks. Safe internal connectivity is the major objective of firewalls. Some firewalls do work with protocols TCP/IP, known as TCP/IP firewalls .
There are a number of basic design issues that should be addressed while designing, specifying, and implementing or overseeing the installation of a firewall.[2,7,11]
The first and most important is reflects the policy of how your company or organization wants to operate the system: is the firewall in place to explicitly deny all services except those critical to the mission of connecting to the net, or is the firewall in place to provide a metered and audited method of "queuing" access in a non-threatening manner.
The second is: what level of monitoring, redundancy, and control do you want? Having established the acceptable risk level by resolving the first issue, you can form a checklist of what should be monitored, permitted, and denied. In other words, you start by figuring out your overall objectives, and then combine a needs analysis with a risk assessment, and sort the almost always conflicting requirements out into a laundry list that specifies what you plan to implement.
The third issue is financial: how much it will cost either to buy or to implement. For example, a complete firewall product may cost between $100,000 at the high end, and free at the low end. The free option, of doing some fancy configuring on a Cisco or similar router will cost nothing but staff time and cups of coffee. Implementing a high end firewall might cost several man-months, which may equate to $30,000 worth of staff salary and benefits. The systems management overhead is also a consideration. It's important, in other words, to evaluate firewalls not only in terms of what they cost now, but continuing costs such as support.
On the technical side, there are a couple of decisions to make, based on the fact that for all practical purposes what we are talking about is a static traffic routing service placed between the network service provider's router and your internal network. The traffic routing service may be implemented at an network level via something like screening rules in a router, or at an application level via proxy gateways and services.
The decision to make is whether to place an exposed stripped-down machine on the outside network to run proxy services for telnet, ftp, news, etc., or whether to set up a screening router as a filter, permitting communication with one or more internal machines. There are pluses and minuses to both approaches, with the proxy machine providing a greater level of audit and potentially security in return for increased cost in configuration and a decrease in the level of service that may be provided (since a proxy needs to be developed for each desired service).
There are two types of firewalls:Network level and Application level  .
Network level firewalls generally make their decisions based on the source, destination addresses and ports in individual IP packets. A simple router is the "traditional" network level firewall, since it is not able to make particularly sophisticated decisions about what a packet is actually talking to or where it actually came from. Modern network level firewalls have become increasingly sophisticated, and now maintain internal information about the state of connections passing through them, the contents of some of the data streams, and so on. One thing that's an important distinction about many network level firewalls is that they route traffic directly though them, so to use one you usually need to have a validly assigned IP address block. Network level firewalls tend to be very fast and tend to be very transparent to users.
Application level firewalls generally are hosts running proxy servers, which permit no traffic directly between networks, and which perform elaborate logging and auditing of traffic passing through them. Since the proxy applications are software components running on the firewall, it is a good place to do lots of logging and access control. Application level firewalls can be used as network address translators, since traffic goes in one "side" and out the other, after having passed through an application that effectively masks the origin of the initiating connection. Having an application in the way in some cases may impact performance and may make the firewall less transparent. Early application level firewalls such as those built using the TIS firewall toolkit , are not particularly transparent to end users and may require some training. Modern application level firewalls are often fully transparent. Application level firewalls tend to provide more detailed audit reports and tend to enforce more conservative security models than network level firewalls.
More and more companies are subscribing to online services that require Internet access, with this increasing access comes the challenge of securing previously isolated internet systems. To control this risk, most companies deploy some kind of firewall technology. Firewall implementation comes in different shapes and sizes, from network routers to expensive dedicated system using several computers and costing over $100K. Corporations place these firewalls between the internal trusted networks and the external public networks. All internal-external communications occur through these firewalls. There are many different kinds of firewalls which are used for different purposes[1,3]. Firewalls are an impediment to E-commerce via the web, they either block it or they allow it but do not improve security .
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