Top 22 Interview Questions: Network Firewall [Updated 2024]

The network firewall acts as the first defence against cyber attacks.  It can protect different servers and other IT devices based on the recommended configuration. I believe questions and answers are the best way to understand any new topic. Here, we will discuss the commonly asked interview questions in interviews, which will also help you know more about firewall devices.

Top 22 Interview Questions and Answers of Network Firewall

Q1. What is Network Security?

Ans: 

Network security is securing IT infrastructure from unauthorized access, misuse, malfunction, modification, destruction, or improper disclosure. IT infrastructure includes firewalls, routers, switches, servers, an Intrusion Detection System (IDS), an Intrusion Prevention System (IPS), and other devices, that help host software applications and systems.

In simple terms, network security refers to all activities protecting the confidentiality, integrity, and availability of an organization's software and hardware assets.

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Q2. What is a Network Firewall?

Ans:

A network firewall protects your network from unauthorized access.

Network Firewall filters traffic based on the configuration set by the administrator.

The firewall performs two functions, block and permit traffic based on configuration.

Firewalls can be both hardware and software firewalls.

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Q3. How does a firewall work?

Ans:

The firewall filters network traffic based on the configuration set by the firewall administrator. 

It can permit or block any port number, web application, and network-layer protocols based on secure configuration.

A simple analogy of a firewall is the security guard to secure the house. If some person is identified as unknown to the security guard, he or she stops from entering the house.

Common ports:

• 80  HTTP
• 443  HTTPS
• 20 & 21  FTP
• 23  Telnet
• 22  SSH
• 25  SMTP
• 110 POP3
• 123 NTP
• 161 SNMP

Q4. How can a firewall protect the IT infrastructure inside your organization?

Ans:

Firewalls are configured to protect IT infrastructure from any unauthorized access.

It secures the network by implementing defined security policies, hiding and protecting your internal network addresses, and reporting threats and activities.

It also provides audit logs related to network traffic to the firewall administrator, identifying the root cause of a security breach. Click here if you are interested in knowing the Top Facts You Should Know About Network Firewall.

Firewall Function	Description
Monitors Incoming/Outgoing Traffic	Primary work of monitoring data moving in/out of the network, examining packets based on set rules.
Decision Making	Decides to allow or block packets as per rules; permitted packets continue, while block packets are dropped.
Types - Stateless/Stateful	Stateless inspects individual packets; Stateful tracks connection states for more refined decisions.
Packet Filtering	Assesses packets by examining specific attributes like source/destination IP, ports, and protocols against predetermined rules.
Application Layer Inspection	Scans packet contents at the application layer, detecting and blocking specific application-level threats
Logging and Reporting	Maintains logs of allowed/denied traffic for troubleshooting, analysis, and compliance requirements.
Regular Updates and Maintenance	Regular updates ensure firewall efficacy against new threats and vulnerabilities.
Network Address Translation (NAT)	Hides internal IP addresses from external networks, enhancing security by obscuring network structure.
Network Segmentation	Divides the network into zones, restricting traffic flow to enhance security and limit attack exposure.

Q5. Will IPSEC make firewalls obsolete?

Ans:

To discuss this question first, we need to understand what IPSEC (Internet Protocol Security) does. IPSEC provides host-to-host authentication and encryption.

In simple terms, it provides a solution for the integrity and confidentiality of data exchanged over the Internet to end customers.

While the firewall protects the network without doing encryption and host-to-host authentication, it monitors the traffic and permits or blocks based on configuration.

It means we need both IPSEC and firewalls, and we can think of combining firewalls with IPSEC-enabled hosts.

Q6. Where does a firewall fit in the security model?

Ans:

A security model is a scheme for specifying and enforcing security policies. Firewalls secure the network's perimeters by implementing defined security policies, hiding and protecting your internal network addresses, and reporting threats and activities.

Q7. What is a VPN?

Ans:

VPN stands for Virtual Private Network. It provides a secure tunnel that protects your data from any intrusion.

It is used to protect private web traffic from snooping, interference, and censorship. In simple terms, it established the connection between two private networks over the internet.

Types of VPN: Site-to-site VPN and Remote Access VPN.

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Q8. What are the types of firewalls?

Ans:

The National Institute of Standards and Technology (NIST), an organization from the US, divides firewalls into three basic types: Packet filters, Stateful inspection, and Proxy.

Packet filters permit or block packets based on port number, protocol source, and destination address.

Stateful inspection works on the principle of the state of active connections between client and server. It uses state information to allow or block network traffic.

Proxy firewall combines stateful inspection technology to enable deep packet inspection. Here, the firewall acts as a proxy; a client makes a connection with the firewall, and then the firewall makes a separate connection to the server on behalf of the client.

Q9. What is source-routed traffic and why is it a threat?

Ans:

Source routing is not very much used in practice. It allows the sender of a packet to partially or completely specify the route the packet takes through the network.

Generally, the router decides the route from destination to source. If source-routed traffic is allowed through the firewall, an attacker can generate traffic claiming to be from a system "inside'' the firewall.

In general, such traffic wouldn’t route to the firewall properly, but with the source routing option, all the routers between the attacker's machine and the target will return traffic along the source route's reverse path. Implementing such attacks is quite easy. Therefore it is a big threat to firewall devices.

Q10. What is IP spoofing and how can it be prevented?

Ans:

IP spoofing is a practice where an attacker illicitly impersonates another machine by manipulating IP packets. There are many tools available for IP Spoofing.

It can be prevented in the following ways:

• Invest in spoofing detection software
• Implement best security practices for IT assets
• Choose reliable ISP
• Implement Cryptographic protocols such as HTTP Secure (HTTPS), Secure, etc.
• Shell (SSH) and Transport Layer Security (TLS)
• Avoid Direct IP user authentication

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Q11. What is a Host-based Firewall?

Ans:

• These are personal firewalls running on your desktops and laptops as software.
• Firewall software is generally included in your operating system and is also available externally as a 3rd party solution.
• The main objective of the personal firewall is to stop unauthorized access to the network.
• These firewalls are generally "Stateful" firewalls and block connections based on port numbers.
• These firewalls are also used to block applications based on your configuration.
• The best example is the Windows Firewall, which works based on port number, application, and other attributes.

Q12. Whether a firewall block some specific pages in a web application?

Ans: The answer is big Yes

• With the firewall's help, you can allow or disallow applications such as MS SQL Server, Twitter, Facebook, and a subset of the application.
• Example: Suppose you can log in on Facebook but not post on Facebook because the firewall blocks the post feature on Facebook. Your firewall exactly knows what request you are sending to the Internet.

Q13. What are SOHO firewalls?

Ans:

• SOHO firewalls are abbreviated as Small Office/Home Office appliances. It usually provides multiple functions with many security features including a wireless access point, Router, Firewall, and Content filter.
• They are easy to set up, with basic features like filtering and protection against online threats.
• These compact devices are user-friendly, ensuring essential protection for smaller networks without complex configurations or high costs.
• It may not be able to provide advanced features of Dynamic Routing and Remote support.

Q14. What is Unified Threat Management (UTM)?

Ans: 

• It is also called the All-in-one security appliance and Web Security Gateway.
• These devices generally have a lot of security features such as URL filtering/content filtering, malware inspection (based on Malware signatures), spam filter, CSU/DSU built-in functionality, act as router/switch, firewall functionality built-in, IDS/IPS capability, Bandwidth shaper may act as a VPN endpoint.

Q15. What is the limitation of the network firewall?

Ans: 

• It acts as the first line of defence against any external attack. However, it is weaponless against any internal attack.
• The firewall acts as a gatekeeper, but inside the house, it can't stop any system harm. A firewall is designed to protect the network from other networks.

Q16. What is the packet filtering firewall?

Ans: 

Packet-filtering firewall filters traffic based on packet attributes such as source and destination addresses, source and destination port numbers, and protocol types.

Packet Filtering Firewall	Description
Type of Firewall	Examines individual data packets based on predefined rules.
Operates at	Network level (Layer 3) of the OSI model.
Evaluation Criteria	Considers packet attributes like source/destination IP, ports, and protocols.
Decision Making	Allows or blocks packets based on rule-defined criteria.
Efficiency	Efficient for processing high volumes of traffic due to quick evaluation.
Limitations	Limited in handling complex threats that require deeper application-level inspection.
Basic Security Level	Provides a fundamental level of security by filtering packets as they pass through the network.

Q17. One type of firewall is a circuit-level gateway, can you explain it?

Ans: 

Circuit-level gateway, as the name suggests, allows or drops connection based on creating a connection between destination and host.

It involves monitoring TCP/IP session requests between trusted LAN hosts and non-trusted Internet hosts. It verifies the TCP/IP connection procedure, also called handshaking, and the validity of the connection.

Q18. Which type of firewall is more secure, packet filtering firewall and circuit-level gateway, and Why?

Ans: 

Circuit Level Gateway is considered more secure because Packet-filtering solutions filter traffic based on packet attributes, as discussed in the previous question. Circuit Level Gateway filters are based on the communication pattern of TCP/IP packets.

Packet-filtering solutions open the system to denial-of-service (DoS) attacks (buffer overflow exploits in "allowed" applications on target machines, connections exhaustion).

However, Circuit Level Gateway filters can also not protect the system from DoS attacks completely.

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Q19. What is the application-level gateway in the context of a network firewall?

Ans:

• In this case, the firewall acts as a proxy between the internal client and the external server. The main purpose of this type of firewall is to monitor and sanitize external communications.
• Whenever a user requests something from the Internet, a firewall creates another similar request and checks whether the request resources do not have any malware or other security vulnerabilities.

Q20. What is a Stateful Inspection Firewall?

Ans:  

Stateful inspection is the most effective way to secure a network. It combines the features of the packet filtering firewall, Circuit Level Gateway, and Application Level Gateway.

Stateful Inspection Firewall	Description
Type of Firewall	Offers higher security by understanding connection contexts, and preventing certain types of attacks.
Operates at	Network level (Layer 3) and beyond, maintaining information on active connections.
Connection Awareness	Tracks attributes of connections (source/destination, ports, sequence) for context-aware decisions.
Decision Making	Makes informed decisions based on the state of connections, allowing or denying packets accordingly.
Enhanced Security	Offers higher security by understanding connection contexts, preventing certain types of attacks.

Q21. What are the attack methods on the network?

Ans: 

Some common attack methods are ping sweep, port scan, email reconnaissance, IP spoofing, DDoS attack, packet sniffing, DNS transfer, Malware (Trojan horses, backdoors, spyware), etc.

Attack Method	Description
DoS / DDoS	Overwhelm network resources with excessive traffic, making services unavailable.
Phishing / Social Engineering	Deceptive methods to trick individuals into revealing sensitive information or performing actions.
Man-in-the-Middle (MitM)	Intercept communication between parties, potentially eavesdropping or altering data.
Malware (Viruses, Worms, Trojans)	Malicious software infiltrates networks, causing damage or gaining unauthorized access.
Zero-Day Exploits	Exploit newly discovered vulnerabilities before patches are available.
DNS Spoofing / Cache Poisoning	Redirect users to fake websites or inject falsified data into DNS resolvers.
SQL Injection	Exploit web app vulnerabilities to access databases or inject malicious scripts.
Brute Force Attacks	Repeatedly attempt different passwords to gain unauthorized access.
ARP Spoofing	Manipulate Address Resolution Protocol to intercept traffic by linking an attacker's MAC address.
Eavesdropping	Unauthorized listening to network communications for gathering sensitive information.

Q22. Explain the concept of IP spoofing.

Ans:  

IP spoofing is a technique used by the attacker to hide the actual IP. Here, the attacker may send malicious traffic from fake IPs or spoof IPs. This is the challenge for security experts and law enforcement agencies to find the actual attacker. DDoS is the most popular attack using this technique.

Step	Description
1. Crafting Spoofed Packets	The attacker creates network packets with a falsified or spoofed source IP address.
2. Transmission	These spoofed packets are then sent across the network infrastructure towards the intended target or victim.
3. Deception	The target system receives these packets and assumes they are from the indicated spoofed IP address.
4. Exploitation	Depending on the attacker's intentions, these spoofed packets can be used for various malicious activities: