If you have been using the Internet for any length of time, and especially if you work at a larger company and browse the Web while you are at work, you have probably heard the term Firewall used. For example, you often hear people in companies say things like, "I can't use that site because they won't let it through the firewall."

If you have a fast Internet connection into your home, you may have found yourself hearing about firewalls for your home network as well. It turns out that a small home network has many of the same security issues that a large corporate network does. You can use a firewall to protect your home network and family from offensive Web sites and potential hackers.

Basically, a firewall is a barrier to keep destructive forces away from your property. In fact, that's why it’s called a firewall. Its job is similar to a physical firewall that keeps a fire from spreading from one area to the next.

A security device built into many Internet servers. Firewalls help prevent unauthorized activity on the server. Programs that charge a fee generally have a firewall that prevents access unless you enter an authorized username and password. On networks, firewalls are used to prevent Internet users from accessing files other than public internet material. Firewall also refers to a class of software that can be installed on computers that use cable modems or DSL. These communication systems are "always on" so it is possible that someone else on the Internet can maliciously access your computer.

Therefore, firewall is a device that interfaces the network to the outside world and shields the network from unauthorized users. The firewall does this by blocking certain types of traffic. For example, some firewalls permit only electronic mail traffic to enter the network from elsewhere. This helps protect the network against attacks made to other network resources, such as sensitive files, databases, and applications.

A Firewall is a set of related programs, located at a network gateway server that protects the resources of a private network from users from other networks. Basically, a firewall, working closely with a router program, filters all network packets to determine whether to forward them toward their destination. A firewall is often installed away from the rest of the network so that no incoming request can get directly at private network resources. There are a number of firewall screening methods. A simple one is to screen requests to make sure they come from acceptable (previously identified) domain names and IP addresses. For mobile users, firewalls allow remote access in to the private network by the use of secure logon procedures and authentication certificates.
A combination hardware and software buffer that many companies or organizations have in place between their internal networks and the Internet. A firewall allows only specific kinds of messages from the Internet to flow in and out of the internal network. This protects the internal network from intruders or hackers who might try to use the Internet to break into those systems.
 A combination of hardware and software that secures access to and from the LAN. There are three main types of firewall architecture; Tasteful Inspection, Proxy based and Packet Filtering, whereby the former provides the highest level of access control. Firewalls can also be used to secure internal network resources from internal network users too.

A Firewall is simply a program or hardware device that filters the information coming through the Internet connection into your private network or computer system. If an incoming packet of information is flagged by the filters, it is not allowed through.

Let's say that you work at a company with 500 employees. The company will therefore have hundreds of computers that all have network cards connecting them together. In addition, the company will have one or more connections to the Internet through something like T1 or T3 lines. Without a firewall in place, all of those hundreds of computers are directly accessible to anyone on the Internet. A person who knows what he or she is doing can probe those computers, try to make FTP connections to them, try to make Internet connections to them and so on. If one employee makes a mistake and leaves a security hole, hackers can get to the machine and exploit the hole.

With a firewall in place, the landscape is much different. A company will place a firewall at every connection to the Internet (for example, at every T1 line coming into the company). The firewall can implement security rules.

Firewalls use one or more of three methods to control traffic flowing in and out of the network:

• Packet filtering - Packets (small chunks of data) are analyzed against a set of filters. Packets that make it through the filters are sent to the requesting system and all others are discarded.
• Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa.
• State inspection - A newer method that doesn't examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to the outside is monitored for specific defining characteristics, then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.

Firewalls are customizable. This means that you can add or remove filters based on

several conditions. Some of these are:

• IP addresses - Each machine on the Internet is assigned a unique address called an IP address. IP addresses are 32-bit numbers, normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this: 216.27.61.137. For example, if a certain IP address outside the company is reading too many files from a server, the firewall can block all traffic to or from that IP address.
• Domain names - Because it is hard to remember the string of numbers that make up an IP address, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. For example, it is easier for most of us to remember www.howstuffworks.com than it is to remember 216.27.61.137. A company might block all access to certain domain names, or allow access only to specific domain names.
• Protocols - The protocol is the pre-defined way that someone who wants to use a service talks with that service. The "someone" could be a person, but more often it is a computer program like a Web browser. Protocols are often text, and simply describe how the client and server will have their conversation. The http in the Web's protocol. Some common protocols that you can set firewall filters for include: IP, TCP, HTTP, FTP, UDP, ICMP, SMTP, SNMP, Telnet.

A company might set up only one or two machines to handle a specific protocol and ban that protocol on all other machines.

• Ports - Any server machine makes its services available to the Internet using numbered ports, one for each service that is available on the server. For example, if a server machine is running a Web (HTTP) server and an FTP server, the Web server would typically be available on port 80, and the FTP server would be available on port 21. A company might block port 21 accesses on all machines but one inside the company.
• Specific words and phrases - This can be anything. The firewall will sniff (search through) each packet of information for an exact match of the text listed in the filter. For example, you could instruct the firewall to block any packet with the word "X-rated" in it. The key here is that it has to be an exact match. The "X-rated" filter would not catch "X rated" (no hyphen). But you can include as many words, phrases and variations of them as you need.

Designing a Firewall requires that you understand and identify the boundaries between security domains in your network. A network security domain is a contiguous region of a network that operates under a single, uniform security policy. Wherever these domains intersect, there is a potential need for a policy conflict resolution mechanism at that boundary. This is where firewall technology can help.

The most common boundary where firewalls are applied today is between an organization’s internal networks and the Internet. When establishing an Internet firewall, the first thing you must decide is its basic architecture (assuming you have previously established your firewall requirements and the security policy it is intended to implement). In this context, architecture refers to the inventory of components (hardware and software), and the connectivity and distribution of functions among them. There are two classes of firewall architectures, which we refer to as the single layer and the multiple layer architectures.

In single layer architecture, one network host is allocated all firewall functions and is connected to each network for which it is to control access. This approach is usually chosen when containing cost is a primary factor or when there are only two networks to interconnect. It has the advantage that everything there is to know about the firewall resides on that one host. In cases where the policy to be implemented is simple and there are few networks being interconnected, this approach can also be very cost-effective to operate and maintain over time. The greatest disadvantage of the single layer approach is its susceptibility to implementation flaws or configuration errors — depending on the type, a single flaw or error might allow firewall penetration.

In multiple layer architecture, the firewall functions are distributed among a small number of hosts typically connected in series, with DMZ networks between them. This approach is more difficult to design and operate, but can provide substantially greater security by diversifying the defenses you are implementing. Although more costly, we advise using different technology in each of these firewall hosts. This reduces the risk that the same implementation flaws or configuration errors will exist in every layer. The most common design approach for this type of architecture is an Internet firewall composed of two hosts interconnected with one DMZ network.

Having chosen the basic architecture (i.e., the number of hosts, the method in which they are connected, the tasks that each will perform), the next step is to select the firewall functions to be implemented in these hosts. The two most basic categories of firewall function are packet filtering and application proxies. These functions can be used separately or jointly and can be implemented on the same or on different firewall hosts. Recently, packet filtering firewall products have gained some of the features of application proxies and are generally referred to as stateful inspection packet filters.

There are good reasons to use both packet filtering and application proxies. Certain services (e.g., SMTP, HTTP, or NTP) are usually safe to control via packet filters while others (e.g., DNS, FTP) may require the more complex features available only in proxies. Packet filtering is fast, while application proxies are generally slower. In cases where greater access control is required and the poorer performance of proxies cannot be tolerated, stateful inspection packet filters may be an acceptable compromise. In any case, one should plan to have as many of these different functions (i.e., packet filters, proxies, and stateful inspection) available as possible, applying each where appropriate.

It is necessary for us to approach the task of implementing a Firewall by going through the following steps:

1. Determine the access denial methodology to use.

It is recommended you begin with the methodology that denies all access by default. In other words, start with a gateway that routes no traffic and is effectively a brick wall with no doors in it.

2. Determine inbound access policy.

If all of your Internet traffic originates on the LAN this may be quite simple. A straightforward NAT router will block all inbound traffic that is not in response to requests originating from within the LAN. The true IP addresses of hosts behind the firewall are never revealed to the outside world, making intrusion extremely difficult. Indeed, local host IP addresses in this type of configuration are usually non-public addresses, making it impossible to route traffic to them from the Internet. Packets coming in from the Internet in response to requests from local hosts are addressed to dynamically allocated port numbers on the public side of the NAT router. These change rapidly making it difficult or impossible for an intruder to make assumptions about which port numbers to use.

If your requirements involve secure access to LAN based services from Internet based hosts, then you will need to determine the criteria to be used in deciding when a packet originating from the Internet may be allowed into the LAN. The stricter the criteria, the more secure your network will be. Ideally you will know which public IP addresses on the Internet may originate inbound traffic. By limiting inbound traffic to packets originating from these hosts, you decrease the likelihood of hostile intrusion. You may also want to limit inbound traffic to certain protocol sets such as ftp or http. All of these techniques can be achieved with packet filtering on a NAT router. If you cannot know the IP addresses that may originate inbound traffic, and you cannot use protocol filtering then you will need more a more complex rule-based model and this will involve a stateful multilayer inspection firewall.

3. Determine outbound access policy.

If your users only need access to the web, a proxy server may give a high level of security with access granted selectively to appropriate users. As mentioned, however, this type of firewall requires manual configuration of each web browser on each machine. Outbound protocol filtering can also be transparently achieved with packet filtering and no sacrifice in security. If you are using a NAT router with no inbound mapping of traffic originating from the Internet, then you may allow LAN users to freely access all services on the Internet with no security compromise. Naturally, the risk of employees behaving irresponsibly with email or with external hosts is a management issue and must be dealt with as such.

4. Determine if dial-in or dial-out access is required.

Dial-in requires a secure remote access PPP server that should be placed outside the firewall. If dial-out access is required by certain users, individual dial-out computers must be made secure in such a way that hostile access to the LAN through the dial-out connection becomes impossible. The surest way to do this is to physically isolate the computer from the LAN. Alternatively, personal firewall software may be used to isolate the LAN network interface from the remote access interface.

5. Decide whether to buy a complete firewall product, have one implemented by a systems integrator or implement one yourself.

Once the above questions have been answered, it may be decided whether to buy a complete firewall product or to configure one from multipurpose routing or proxy software. This decision will depend as much on the availability of in-house expertise as on the complexity of the need. A satisfactory firewall may be built with little expertise if the requirements are straightforward. However, complex requirements will not necessarily entail recourse to external resources if the system administrator has sufficient grasp of the elements. Indeed, as the complexity of the security model increases, so does the need for in-house expertise and autonomy.

Personal Firewall is a technology that helps prevent intruders from accessing data on your PC via the Internet or another network, by keeping unauthorized data from entering or exiting your system.

Hackers don't just target national security organizations for cyber attacks: They want your tax returns, network passwords, or bank account numbers. And you don't want the FBI kicking in your door because someone hijacked your PC to participate in the latest denial-of-service attack on the Internet. Now that "always-on" broadband connections such as cable modems and digital subscriber line are becoming more popular, home users are at risk. Fortunately, you can protect your data with a kind of security utility--firewalls.

Firewalls can block malicious attacks and protect your PC from outside threats. A firewall can prevent an unauthorized user from accessing your PC, either from the Internet or from within your local network. It blocks some Trojan horse programs and many hostile applications that seek to take over your computer.

When you're connected to the Internet, you're sending and receiving information in small units called packets. A packet contains the addresses of the sender and the recipient along with a piece of data, a request, a command, or almost anything having to do with your connection to the Internet. But just as with postal mail, not every package that arrives at your computer is one you want to open.

A firewall examines each data packet sent to or from your computer to see if it meets a set of criteria. The firewall then selectively passes or blocks the packet.

The criterion a firewall uses for passing packets along depends on the kind of firewall you use. The most common type you'll find for home and small business use is called an application gateway firewall.

An application gateway, often called a proxy, acts like a customs officer for data: Anything you send or receive stops first at the firewall, which filters packets based on IP addresses and content, as well as the specific functions of an application. For instance, if you're running an FTP program, the proxy could permit file uploads while blocking other FTP functions, such as viewing or deleting files. You can also set the firewall to ignore all traffic for FTP services but allow all packets generated during Web browsing.

Other kinds of firewalls include packet filters, which examine every packet for an approved IP address; circuit-level firewalls, which allow communication only with approved computers and Internet service providers; and the newest type, stateful inspection firewalls, which note the configuration of approved packets and then pass or block traffic based on those characteristics.

Packet-filter, circuit-level, and stateful inspection firewalls are mostly found in corporate network setups. They require major upkeep, so they aren't suitable for smaller companies and home users.

If you work at a large corporation, odds are good that a Firewall sits between you and the outside world. But the increased availability of cable and DSL service means you could spend more time connected to the Internet from home--and more time as a potential target for hackers. You're somewhat vulnerable even on short dial-up connections. Unfortunately, most people become aware of the danger only after they become victims. With cyber attacks increasing, it is predicted that firewalls will be ubiquitous in five or six years.

But you don't have to buy an expensive, hard-to-maintain security system for your PC. Personal firewalls, usually based on the application gateway model, can keep you safe. These products don't require you to program complex restrictions. They'll guide you through a setup that asks you what you want to allow or block. They can also help you monitor intrusion attempts and protect you from most Trojan horse or spy ware programs that let a hacker control your computer over the Internet. They can hide your identity while you surf, too. On the basis of their iniquitousness, firewalls Move into the Mainstream

While most personal firewalls are available now as software that you install on your PC, some experts predict that firewalls will be integrated into hardware in the next few years. That means the next DSL or cable modem you buy or lease may have a firewall already installed. To make maintaining a firewall easy, they say, companies will offer subscription services. Maybe You just need to pay $50 a month and the company will make sure your firewall is up-to-date. That maintenance is key to keeping your data safe: As soon as hackers hear about a weakness in a firewall, they hunt for people who haven't upgraded to the latest version and break in.

As our dependence on the Internet and computers grows, so will the personal consequences of a security breach? Whether to protect your personal information from theft or to keep your PC from being hijacked by a hacker, installing a personal firewall makes sense.

Surfing the Web seems similar to watching television, listening to the radio, or reading a magazine. The difference is that you joining the Internet and connecting to it makes your computer as accessible to others as any Web site that you visit.

The two-way nature of the Internet can be misused by people who want to take control of your computer, look at your financial data, or delete your personal files. These intruders probably aren’t targeting you personally. Attacks are often launched by automated attack tools. Everyone who connects to the Internet using a broadband connection will be probed several times a day. I have an always-on cable modem at home, and get attacked about two dozen times a day.

Notwithstanding some happening, the Internet doesn’t have to be a scary place. Just as you lock the front door to your home, it’s important to protect your PC. One of the best ways to protect your PC or small business network from malicious hackers is to use a Firewall. Consumer-level firewalls provide good security without requiring that you be a computer security expert.

What is a Firewall?

A firewall is a security system designed to prevent unauthorized access from the Internet to or from your network. A firewall works by screening out many types of malicious traffic. In addition, firewalls can help keep your computer from participating in attacks on others without your knowledge. Firewalls take the form of hardware, software, or both and I will do some explanation about various kinds of firewalls and will help you choose the right one for your network.

Hardware firewall

Hardware firewall products protect your computer and home network by guarding your Internet connection and filtering any requests that you haven’t specifically allowed. Software firewalls are installed directly on your PC, and filter requests after they reach your computer.

For maximum security, the most reliable way for home users to protect a network is to purchase a router with firewall capabilities. These routers do more than act as a firewall—they network multiple computers, allow them to share a single Internet connection, and may even support wireless networking. If you have more than one computer and an always-on broadband connection, a router-firewall gives you the benefits of a home network and connects every computer to the Internet. If you bring a laptop home from work, it may even be a requirement of your company’s security policy.

The router is generally a separate device from the cable or DSL modem—it’s important to understand that most cable and DSL modems offer your home network no protection whatsoever. If you didn’t choose to pay extra for security features, you probably don’t have any. If you’re unsure about your modem, ask your Internet service provider (ISP) what level of protection your modem provides.

Software Firewalls

Software firewalls are often less expensive and easier to configure than hardware firewalls. Software firewalls also don’t require you to move any cables around. Depending on the software you choose, a software firewall can offer features beyond those of router firewalls, such as protecting your computer from spy ware (a component of some free software that tracks your Web browsing habits) and Trojan horses ( a program that claims to do one thing, but does another, malicious thing, such as recording your passwords. If you travel with a laptop, a software firewall is a necessity—you need protection wherever you connect to the Internet, and your hardware firewall can protect you only at home.

To Choose Hardware or Software Firewall?

You wouldn’t park your car and leave your keys in the ignition, and you shouldn’t connect to the Internet without a personal firewall. No matter what type of computer or network you have, there’s a firewall to meet your needs.

If you have a stand-alone computer or connect to the Internet with a dial-up connection, a software firewall is the right choice. A hardware firewall is more complex to configure, but once you get it set up; it runs on its own to protect your network.

Can firewall replace anti-virus software?

Note that firewall software does not replace anti-virus software so you should use both to protect your network.

So combined with a software-based firewall, you’ve got maximum security for your network. And whichever you choose, you’ll find using the Internet much more enjoyable when you’re nestled safely behind a firewall.

The Internet has made large amounts of information available to the average computer user at home, in business and in education. For many people, having access to this information is no longer just an advantage, it is essential. Yet connecting a private network to the Internet can expose critical or confidential data to malicious attack from anywhere in the world. Users who connect their computers to the Internet must be aware of these dangers, their implications and how to protect their data and their critical systems. Firewalls can protect both individual computers and corporate networks from hostile intrusion from the Internet, but must be understood to be used correctly.

A firewall can protect networked computers from intentional hostile intrusion that could compromise confidentiality or result in data corruption or denial of service. It may be a hardware device or a software program running on a secure host computer. In either case, it must have at least two network interfaces, one for the network it is intended to protect, and one for the network it is exposed to. A firewall sits at the junction point or gateway between the two networks, usually a private network and a public network such as the Internet. The earliest firewalls were simply routers. The term firewall comes from the fact that by segmenting a network into different physical subnet works, they limited the damage that could spread from one subnet to another just like fire doors or firewalls.

A firewall can examine all traffic routed between the two networks to see if it meets certain criteria. If it does, it is routed between the networks, otherwise it is stopped. A firewall filters both inbound and outbound traffic. It can also manage public access to private networked resources such as host applications. It can be used to log all attempts to enter the private network and trigger alarms when hostile or unauthorized entry is attempted. Firewalls can filter packets based on their source and destination addresses and port numbers. This is known as address filtering. Firewalls can also filter specific types of network traffic. This is also known as protocol filtering because the decision to forward or reject traffic is dependant upon the protocol used, for example HTTP, ftp or telnet. Firewalls can also filter traffic by packet attribute or state.

Therefore, anyone who is responsible for a private network that is connected to a public network needs firewall protection. Furthermore, anyone who connects so much as a single computer to the Internet via modem should have personal firewall software. Many dial-up Internet users believe that anonymity will protect them. They feel that no malicious intruder would be motivated to break into their computer. Dial up users who have been victims of malicious attacks and who have lost entire days of work, perhaps having to reinstall their operating system, know that this is not true. Irresponsible pranksters can use automated robots to scan random IP addresses and attack whenever the opportunity presents itself.

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