The question is should you use one firewall for perimeter and internal? Some would argue if an attacker can hack the first firewall then he/she can definitely attack the internal firewall then why should I spent for 2 firewalls. Another argument single firewall proponents bring to the table is if there is any configuration mismatch between internal and perimeter then there is no use for 2 firewalls. For example, if by human error if anything is bypassed then the doors for attackers are wide opened. In addition to this, you have to protect your environment from worms and malware as well which are in the form of bots and keep on looking for loopholes to exploit. So, don’t think someone in China or US is waiting for you to make mistake 🙂

If you read CISSP, one of the very basic design principles is to have the defence in-depth. This means you have layers security which would make the job hectic for the attackers, viz. IPS at the firewall, Anti-virus at the firewall, stateful inspection at the firewall, anomalies detection at the firewall, then security modules are routers, internal firewall, the WAF then firewall of the servers, then if possible host-level Anti-virus and IPS. See how many such checkpoints are there? Many right?

Likewise, government agencies suggest to use physical layer wherever possible, like using physical IPS, 2 firewalls etc but nowadays since virtualization came and everyone is looking to save cost the regulatory bodies are making such requirement mandatory because a converged setup with one physical hardware imposes risk if the hardware itself has the bug and been compromised. The risk and business impact would be huge including the brand image if any such incident occurs. Thus, one should be very careful in saving cost and not at the cost of brand image and lifeline of any company. Attackers are learning and no software is full proof save. Let throw some light on Defence-in-depth strategy.

A good Defense-in-Depth strategy involves many different technologies, such as Intrusion Detection, Content Filtering, and Transport Layer Security. The single most important element, however, is a system of internal firewalls. Proper deployment of these devices can address concerns that we have from security:

§ Employees will not have unrestricted access to the entire network, and their activity can be monitored.

§ Partners, customers, and suppliers can be given limited access to whatever resources they require, while maintaining isolation of critical servers.

§ Critical servers can be closely monitored when they are isolated behind an internal firewall. Any malicious activity would be much easier to detect, since the firewall has a limited amount of traffic passing through it.

§ Remote users can be restricted to certain portions of the network, and VPN traffic can be contained and easily monitored.

§ A security breach in one segment of the network will be limited to local machines, instead of compromising the security of the entire network. With a system of internal firewalls in place, we can come much closer to our ideal network. Instead of an all-or-nothing security posture, we can achieve Defense-in-Depth by forcing an attacker to penetrate multiple layers of security to reach mission-critical servers.


The best practice, and we have been engaged with medium to high-end customers including finance and insurance and have noticed dual firewall policy for DMZ and internal LAN and that is the most secure approach we recommend. It is also the most secure approach, according to Stuart Jacobs, is to use two firewalls to create a DMZ. The first firewall (also called the “front-end” or “perimeter” firewall) must be configured to allow traffic destined to the DMZ only. The second firewall (also called “back-end” or “internal” firewall) only allows traffic from the DMZ to the internal network.

This setup is considered more secure since two devices would need to be compromised. There is even more protection if the two firewalls are provided by two different vendors, because it makes it less likely that both devices suffer from the same security vulnerabilities. In a scenario when there is a bug on one firewall and that imposes threat on the whole infrastructure is minimized by having firewall from two different vendors. For example, accidental misconfiguration is less likely to occur the same way across the configuration interfaces of two different vendors, and a security hole found to exist in one vendor’s system is less likely to occur in the other one.


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