Types of Firewalls in Computer Networks

A firewall is a network security device; it is a protective layer for the server that monitors and filters all the incoming and outgoing network traffic. It uses a set of rules to determine whether to allow or block a specific network traffic. Firewalls can prevent unauthorized use before reaching the servers. Firewalls can be hardware or software-based.

Firewall Policies

To protect private networks and individual machines, a firewall can be employed to filter incoming or outgoing traffic based on a predefined set of rules known as firewall policies.

Packet flowing through a firewall can have one of the following three outcomes −

• Accepted − Permitted through the firewall.
• Dropped − Not allowed through with no indication of failure
• Rejected − Not allowed through accompanied by an attempt to inform the source that the packet was rejected.

Properties of the packets and the protocols are −

• TCP or UDP
• The source and destination IP address
• The source and destination ports
• The application-level payload of the packet

Types of Firewall

• Packet Filters (Stateless Firewall) − In the packet filters, if a packet matches then the packet filters set of rules and filters will drop or accept it.
• Stateful firewall filters − It is also known as a network firewall; this filter maintains a record of all the connections passing through. It can determine if a packet is either the start of a new connection or a part of an existing connection or is an invalid packet.
• Application firewall − A web application firewall is used for HTTP applications. There are sets of rules that are applied to monitor or block data packets from HTTP network traffic. For example, these rules can help block cross-site scripting (XSS) and SQL injections.

Packet Filter Firewall

A packet filter firewall can forward or block packets based on the information in the network layer and transport layer headers source and destination IP addresses, source, and destination port address, and type of protocol (TCP and UDP).

A packet filter firewall is a router that uses a filtering table to decide which packet must be discarded or not to forward. It filters at the network or transport layer.

Proxy Based Firewall

A proxy-based firewall acts as an intermediary between the requested data by the end-users and the source servers. The proxy filters all the network traffic and will block or allow the traffic based on its rules and policies.

The proxy can also examine the entire network packet besides the network address and the port number. This type of firewall is labeled as the most secured, as it prevents direct network contact between the systems.