A firewall is defined as a system which is designed to prevent unauthorized access to or from a private network. Claimed to be implemented in both hardware and software, or a combination of both, firewalls are frequently used in order to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets.
It is necessary for all messages entering or leaving the intranet to pass through the firewall, where each message is examined and those that do not meet the specified security criteria are blocked.
Types of firewall techniques:
Packet filter: Each packet entering or leaving the network is checked and based on user-defined rules it is either accepted or rejected. It is said to be fairly effective and transparent to users, but is difficult to configure and is susceptible to IP spoofing.
Application gateway: Security mechanisms are applied to specific applications, such as FTP and Telnet servers. Although this is very effective, performance degradation can be imposed.
Circuit-level gateway: Security mechanisms are applied when a TCP or UDP connection is established. Uponestablishing the connection, packets can flow between the hosts without further checking.
Proxy server: All messages are intercepted while entering and leaving the network, while the true network addresses are kept effectively hidden by the proxy server
Principle of a Firewall:
A set of predefined rules constitute a firewall system wherein the system is allowed to:
Authorise the connection (allow)
Block the connection (deny)
Reject the connection request without informing the issuer (drop)