What is firewall?

What is firewall?

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A firewall is a program or hardware device that prevents data packets from coming in or going out to a specified network or computer. It is used to separate an internal network from the Internet. A firewall is configured as shown in the image below:



A firewall restricts inbound and outbound access, and can analyze all the traffic between a network and the Internet. It checks the protocol through which the packet is being delivered, and the port number to which it is being sent. If a firewall is configured to accept the specified protocol through the targeted port, the packet is allowed through. The main function of a firewall is to provide centralized access control on how users use the network. A firewall can also perform the following functions:

• Block unwanted traffic.
• Direct incoming traffic to more trustworthy internal computers.
• Hide vulnerable computers that are exposed to the Internet.
• Log traffic to and from the private network.
• Hide information, such as computer names, network topology, network device types, and internal user IDs from external users.

A firewall enhances security through various methods, including packet filtering, circuit-level filtering, and application filtering.

In packet filtering, data packets are analyzed against a set of filters. Packets that successfully pass through the filters are sent to the requesting computer and all others are discarded.

In circuit-level filtering, a firewall does not simply allow or disallow packets but also determines whether or not the connection between both ends is valid according to configurable rules. It then opens a session and permits traffic only from the allowed source and possibly for a limited period of time.

In application filtering, a firewall analyzes a data stream for a particular application and provides application-specific processing including inspecting, blocking, redirecting, or modifying data as it passes through the firewall. This mechanism is used to protect data from insecure SMTP commands or attacks against internal Domain Name Servers (DNSs).

A firewall can be customized according to user requirements by adding or removing filters. A filter can be added or removed on the basis of IP addresses, protocols, and domain names. When using firewalls, certain domain names can be blocked or allowed access only to specific domain names.

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