Firewalls are basically a barrier between your computer (or a network) and the Internet (outside world). A firewall can be simply compared to a security guard who stands at the entrance of your house and filters the visitors coming to your place. He may allow some visitors to enter while deny others whom he suspects of being intruders. Similarly a firewall is a software program or a hardware device that filters the information (packets) coming through the Internet to your personal computer or a computer network.
How Firewall Works?
Firewalls may decide to allow or block network traffic between devices based on the rules that are pre-configured or set by the firewall administrator. Most personal firewalls such as Windows firewall operate on a set of pre-configured rules which are most suitable under normal circumstances, so that the user need not worry much about configuring the firewall. The operation of firewall is illustrated in the below figure 2.5.
Personal firewalls are easy to install and use and hence preferred by end-users to secure their personal computers. However, in order to meet customized needs large networks and companies prefer those firewalls that have plenty of options to configure. For example, a company may set up different firewall rules for FTP servers, telnet servers and web servers. In addition, the company can even control how the employees connect to the Internet by blocking access to certain websites and restricting the transfer of files to other networks. Thus, in addition to security, a firewall can give the company a tremendous control over how people use their network. Firewalls use one or more of the following methods to control the incoming and outgoing traffic in a network:
1. Packet Filtering: In this method, packets (small chunks of data) are analyzed against a set of filters. Packet filters has a set of rules that come with accept and deny actions which are pre-configured or can be configured manually by the firewall administrator. If the packet manages to make it through these filters then it is allowed to reach the destination; otherwise it is discarded. 2. Stateful Inspection: This is a newer method that doesn’t analyze the contents of the
packets. Instead, it compares certain key aspects of each packet to a database of trusted source. Both incoming and outgoing packets are compared against this database and if the comparison yields a reasonable match, then the packets are allowed to travel further. Otherwise they are discarded.
Firewall Configuration:
Firewalls can be configured by adding one or more filters based on several conditions as mentioned below:
1. IP addresses: In any case, if an IP address outside the network is said to be unfavourable, then it is possible to set filter to block all the traffic to and from that IP address. For example, if a certain IP address is found to be making too many connections to a server, the administrator may decide to block traffic from this IP using the firewall. 2. Domain names: Since it is difficult to remember the IP addresses, it is an easier and smarter way to configure the firewalls by adding filters based on domain names. By setting up a domain filter, a company may decide to block all access to certain domain names, or may provide access only to a list of selected domain names. 3. Ports/Protocols: If the services running on a given port is intended for the public or network users, they are usually kept open. Otherwise they are blocked using the firewall so as to prevent intruders from using the open ports for making unauthorized connections. 4. Specific words or phrases: A firewall can be configured to filter one or more specific words or phrases so that both the incoming and outgoing packets are scanned for the words in the filter. For example, you may set up a firewall rule to filter any packet that contains an offensive term or a phrase that you may decide to block from entering or leaving your network.
Hardware vs. Software Firewall:
Hardware firewalls provide higher level of security and hence preferred for servers where security has the top most priority. The software firewalls on the other hand are less expensive and hence preferred in home computers and laptops. Hardware firewalls usually come as an in-built unit of a router and provide maximum security as it filters each packet at the hardware level itself even before it manages to enter your computer. A good example is the Linksys Cable/DSL router.
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