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Introduction of Firewall in Computer Network

Last Updated : 04 Mar, 2024
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A firewall is a network security device that prevents unauthorized access to a network. It monitors both incoming and outgoing traffic using a predefined set of security to detect and prevent threats.

What is Firewall?

A firewall is a network security device, either hardware or software-based, which monitors all incoming and outgoing traffic and based on a defined set of security rules accepts, rejects, or drops that specific traffic.

  • Accept: allow the traffic
  • Reject: block the traffic but reply with an “unreachable error”
  • Drop : block the traffic with no reply

A firewall is a type of network security device that filters incoming and outgoing network traffic with security policies that have previously been set up inside an organization. A firewall is essentially the wall that separates a private internal network from the open Internet at its very basic level.


History and Need for Firewall

Before Firewalls, network security was performed by Access Control Lists (ACLs) residing on routers. ACLs are rules that determine whether network access should be granted or denied to specific IP address. But ACLs cannot determine the nature of the packet it is blocking. Also, ACL alone does not have the capacity to keep threats out of the network. Hence, the Firewall was introduced. Connectivity to the Internet is no longer optional for organizations. However, accessing the Internet provides benefits to the organization; it also enables the outside world to interact with the internal network of the organization. This creates a threat to the organization. In order to secure the internal network from unauthorized traffic, we need a Firewall.

Working of Firewall

Firewall match the network traffic against the rule set defined in its table. Once the rule is matched, associate action is applied to the network traffic. For example, Rules are defined as any employee from Human Resources department cannot access the data from code server and at the same time another rule is defined like system administrator can access the data from both Human Resource and technical department. Rules can be defined on the firewall based on the necessity and security policies of the organization. From the perspective of a server, network traffic can be either outgoing or incoming.

Firewall maintains a distinct set of rules for both the cases. Mostly the outgoing traffic, originated from the server itself, allowed to pass. Still, setting a rule on outgoing traffic is always better in order to achieve more security and prevent unwanted communication. Incoming traffic is treated differently. Most traffic which reaches on the firewall is one of these three major Transport Layer protocols- TCP, UDP or ICMP. All these types have a source address and destination address. Also, TCP and UDP have port numbers. ICMP uses type code instead of port number which identifies purpose of that packet.

Default policy: It is very difficult to explicitly cover every possible rule on the firewall. For this reason, the firewall must always have a default policy. Default policy only consists of action (accept, reject or drop). Suppose no rule is defined about SSH connection to the server on the firewall. So, it will follow the default policy. If default policy on the firewall is set to accept, then any computer outside of your office can establish an SSH connection to the server. Therefore, setting default policy as drop (or reject) is always a good practice.

Types of Firewall

Firewalls can be categorized based on their generation.

1. Packet Filtering Firewall

Packet filtering firewall is used to control network access by monitoring outgoing and incoming packets and allowing them to pass or stop based on source and destination IP address, protocols, and ports. It analyses traffic at the transport protocol layer (but mainly uses first 3 layers). Packet firewalls treat each packet in isolation. They have no ability to tell whether a packet is part of an existing stream of traffic. Only It can allow or deny the packets based on unique packet headers. Packet filtering firewall maintains a filtering table that decides whether the packet will be forwarded or discarded. From the given filtering table, the packets will be filtered according to the following rules: 

Packet Filter Firewall

  • Incoming packets from network are blocked.
  • Incoming packets destined for the internal TELNET server (port 23) are blocked.
  • Incoming packets destined for host are blocked.
  • All well-known services to the network are allowed.

2. Stateful Inspection Firewall

Stateful firewalls (performs Stateful Packet Inspection) are able to determine the connection state of packet, unlike Packet filtering firewall, which makes it more efficient. It keeps track of the state of networks connection travelling across it, such as TCP streams. So the filtering decisions would not only be based on defined rules, but also on packet’s history in the state table.

3.  Software Firewall

A software firewall is any firewall that is set up locally or on a cloud server. When it comes to controlling the inflow and outflow of data packets and limiting the number of networks that can be linked to a single device, they may be the most advantageous. But the problem with software firewall is they are time-consuming.

4. Hardware Firewall

They also go by the name “firewalls based on physical appliances.” It guarantees that the malicious data is halted before it reaches the network endpoint that is in danger.

5. Application Layer Firewall

Application layer firewall can inspect and filter the packets on any OSI layer, up to the application layer. It has the ability to block specific content, also recognize when certain application and protocols (like HTTP, FTP) are being misused. In other words, Application layer firewalls are hosts that run proxy servers. A proxy firewall prevents the direct connection between either side of the firewall, each packet has to pass through the proxy.

6. Next Generation Firewalls (NGFW)

NGFW consists of Deep Packet Inspection, Application Inspection, SSL/SSH inspection and many functionalities to protect the network from these modern threats.

7. Proxy Service Firewall

This kind of firewall filters communications at the application layer, and protects the network. A proxy firewall acts as a gateway between two networks for a particular application.

8. Circuit Level Gateway Firewall

This works as the Sessions layer of the OSI Model’s . This allows for the simultaneous setup of two Transmission Control Protocol (TCP) connections. It can effortlessly allow data packets to flow without using quite a lot of computing power. These firewalls are ineffective because they do not inspect data packets; if malware is found in a data packet, they will permit it to pass provided that TCP connections are established properly.

Functions of Firewall

  • Every piece of data that enters or leaves a computer network must go via the firewall.
  • If the data packets are safely routed via the firewall, all of the important data remains intact.
  • A firewall logs each data packet that passes through it, enabling the user to keep track of all network activities.
  • Since the data is stored safely inside the data packets, it cannot be altered.
  • Every attempt for access to our operating system is examined by our firewall, which also blocks traffic from unidentified or undesired sources.

Advantages of using Firewall

  • Protection from unauthorized access: Firewalls can be set up to restrict incoming traffic from particular IP addresses or networks, preventing hackers or other malicious actors from easily accessing a network or system. Protection from unwanted access.
  • Prevention of malware and other threats: Malware and other threat prevention: Firewalls can be set up to block traffic linked to known malware or other security concerns, assisting in the defense against these kinds of attacks.
  • Control of network access: By limiting access to specified individuals or groups for particular servers or applications, firewalls can be used to restrict access to particular network resources or services. 
  • Monitoring of network activity: Firewalls can be set up to record and keep track of all network activity.
  • Regulation compliance: Many industries are bound by rules that demand the usage of firewalls or other security measures.
  • Network segmentation: By using firewalls to split up a bigger network into smaller subnets, the attack surface is reduced and the security level is raised. 

Disadvantages of using Firewall

  • Complexity: Setting up and keeping up a firewall can be time-consuming and difficult, especially for bigger networks or companies with a wide variety of users and devices.
  • Limited Visibility: Firewalls may not be able to identify or stop security risks that operate at other levels, such as the application or endpoint level, because they can only observe and manage traffic at the network level.
  • False sense of security: Some businesses may place an excessive amount of reliance on their firewall and disregard other crucial security measures like endpoint security or intrusion detection systems.
  • Limited adaptability: Because firewalls are frequently rule-based, they might not be able to respond to fresh security threats. 
  • Performance impact: Network performance can be significantly impacted by firewalls, particularly if they are set up to analyze or manage a lot of traffic.
  • Limited scalability: Because firewalls are only able to secure one network, businesses that have several networks must deploy many firewalls, which can be expensive.
  • Limited VPN support: Some firewalls might not allow complex VPN features like split tunneling, which could restrict the experience of a remote worker.
  • Cost: Purchasing many devices or add-on features for a firewall system can be expensive, especially for businesses.

Practice Question

Question: A packet filtering firewall can [ISRO CS 2013]

(A) Deny certain users from accessing a service

(B) Block worms and viruses from entering the network

(C) Disallow some files from being accessed through FTP

(D) Block some hosts from accessing the network

Answer: Option (D)

For more details you can refer ISRO | ISRO CS 2013 | Question 44 published quiz.

Frequently Asked Question on Firewalls – FAQs

Can Network Speeds Be Slowed Down by a Firewall?

Yes, Network speeds can be slowed down by a firewall.

How do firewall stop traffic?

The firewall acts as a constant filter, analyzing incoming data and blocking anything that appears suspicious from entering your network to protect system.

Can Firewalls Stop Worms?

Yes, Installing a firewall helps prevent worms and malicious software from infecting a computer in addition to blocking unwanted traffic.

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