Firewall Configuration

Before continue recollect the basics Firewall Introduction and Firewall Techniques.

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. In any case if an IP address outside the network is said to be unfavorable, 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 outside the company access too many files from a server, the administrator we block all the traffic from this IP using the firewall.

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.ihacc.co.cc than it is to remember 72.14.203.121. A company might block all access to certain domain names, or allow access only to a list of selected domain names.

Ports / Protocols: Every service running on a server is made available to the Internet using numbered ports, one for each service. In simple words, ports can be compared to virtual doors of the server through which services are made available. For example, if a server is running a Web (HTTP) service then it will be typically available on port 80. In order to avail this service, the client needs to connect to the server via port 80. Similarly different services such as Telnet (Port 23), FTP (port 21) and SMTP (port 25) services may be running on the server. If the services are intended for the public, 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.

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 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.

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Firewall Introduction, Firewall Techniques, Firewall Configuration