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What is an Internal Firewall?

Not all threats to a corporation's network are perpetrated from the Internet by anonymous hackers, and firewalls are not a stand-alone, technology-based quick fix for network security.

Internal users, or those inside the firewall, are responsible for 60% of network attacks. Disgruntled employees, former employees, or acquaintances of employees are responsible for 568 of 600 occurrences of network hacking. Thirty percent of Internet sites that reported breaches had firewalls in place. Because most instances of computer crime happen within workplaces, a new category of software known as internal firewalls has begun to appear.

An internal firewall is a security solution that protects a network from attacks that have passed the perimeter. An internal firewall is a more complex application of the concept of a firewall, which is a device or software meant to monitor traffic and prevent unwanted access.

How Does an Internal Firewall Work?

The Internal Network Firewall (INFW) is not a novel technology, but rather a subset of the Next Generation Firewall (NGFW) architecture.

In most IT or hosted environments, traffic patterns can be classified as follows:

  • North to South: Traffic from LAN to the Internet and vice versa.
  • East to West: Intra-organization traffic that routes server-to-server, server-to-client, or client-to-client but does not leave the organization. East-west traffic may occur between IP subnets or routed VLAN interfaces (typical in many enterprises with L3 switches). The majority of traffic in an organization or virtualized environment flows east to west.

An internal firewall is located at crucial points throughout the internal network and employs a zero-trust strategy to isolate attacks and reduce potential damage. In other words, it believes that threats have already entered the network and prohibit them from traveling freely within it.


Figure 1. How Does an Internal Firewall Work?

Internal firewalls function utilizing two key strategies:

  1. Using micro-segmentation, which splits the network into granular zones that are secured individually, the attack surface is reduced.
  2. Intelligent automation is being used to apply and update security policies based on "known good" behavior.

How to Configure Internal Firewall?

The usage of VLANs and subnets makes it simple to integrate an internal firewall into the network environment. In general, the steps necessary are as follows:

  1. Trunk Mode: This is utilized when the aggregate traffic levels between VLANs (i.e. 1, 20, 30) do not surpass the trunk port's port speed. For example, if you simply need to transfer 100Mbps between VLANs, a 1GE trunk port will suffice. If you need to pass many Gbps of data on a regular basis, you'll require a 10GE port or interface mode.
  • VLAN 1/20/30, subnets, and necessary security policies are set up on a firewall with a trunk interface.
  • A firewall interface for the WAN provider has been added to the configuration.
  • The switch stack was connected to the firewall through a 1GE or 10GE interface.
  • Switchport configured as a trunk, but VLAN interfaces (e.g., SVI, IRB) disabled/removed from the switch.
  • Firewall-enabled WAN provider inserted into a switch port.
  1. Interface Mode: Firewall can be configured in interface mode by following the next steps.
  • Multiple firewall interfaces with subnets and security policies set.
  • A firewall interface for the WAN provider has been added to the configuration.
  • On the switch stack, several access ports are configured with their respective VLANs.
  • VLAN interfaces (such as SVI and IRB) have been disabled/removed from the switch.
  • Firewall switch ports are enabled.
  • The WAN provider has been connected to the Firewall.

What are Best Practices for an Internal Firewall?

Best practices for an internal firewall are outlined below:

  • Keep a record of your firewall rules and their purpose. It's easy to forget why a particular rule was put in place, especially if the IT employee who put it in place has since left the company. Documentation is essential for long-term maintenance because it allows you to reevaluate security policies and eliminate those that no longer serve a function.
  • Regularly review incident logs. This will assist you in determining which security rules are and aren't being used, allowing you to delete unused rules while adjusting others to improve security and avoid gaps.
  • Use automation to keep rules up to date. Long lists of firewall rules, if left unattended, can result in "rule bloat," increased overhead, and security holes. By leveraging automation to decrease the pressure on IT employees, you may avoid these issues and stay up with rapid development.
  • Use zero-trust security. This entails not trusting anyone by default, whether inside or outside the network and is critical for promptly containing attacks. With the increasing frequency of cyber-attacks, it is no longer safe to believe that attackers would not gain access to the network. A zero-trust security approach is critical for mitigating the effect of threats that breach the perimeter.

Do Enterprises Need an Internal Firewall?

An internal firewall is an essential component of network firewall security, especially as networks become more dispersed and it becomes more difficult to keep intruders outside the network perimeter. It works in conjunction with a perimeter firewall to provide defense-in-depth limiting east-west traffic and preventing lateral movement of threats within your company. With the frequency and sophistication of cyberattacks increasing, it's nearly certain that an organization's network perimeter will be breached. When this occurs, an internal firewall limits the amount of harm that attackers can cause.

In a large organization, internal firewalls make a lot of sense. After all, there's no reason to give your research experts privileged access to an accounting computer or to let those sitting in front of data-entry terminals try their hand at breaking into the R&D department's file servers. You can use an internal firewall to add extra security where it is needed.

The purpose of establishing independent internal networks should be to reduce the damage that will occur if one of your internal networks is hacked, either by an intruder or, more often, by an insider.

What is the Difference Between an Internal Firewall and a Perimeter Firewall?

A network perimeter firewall, also known as an external firewall, is a secure border that serves as the primary line of defense for a private network as well as other public networks such as the internet. The firewall monitors and protects the network from malicious traffic, potentially harmful programs, and intrusion attempts.

There are some fundamental differences between an internal firewall and a perimeter firewall. Unlike a standard perimeter firewall, an internal firewall must give visibility and protection from internal threats on a proactive basis, and it must be fast enough to keep up with internal traffic demands. Cyber-attacks are increasingly likely to breach the network perimeter, and internal firewalls mitigate the damage that such attacks might do.

An internal firewall, as opposed to a perimeter firewall, monitors and secures east-west (internal) network traffic. An external firewall monitors the network's perimeter and prevents illegal external access.

Internal firewalls are designed to protect virtual machines, as opposed to network perimeter firewalls. They are also intended for the agility that internal networks require, allowing managers to reallocate virtual resources as they see appropriate without violating firewall restrictions.