Implicit Deny means that any network traffic or connection not explicitly allowed by a security rule or policy is automatically denied. It is a fundamental principle that acts as a safety net, like saying, “if you’re not specifically given permission, you don’t get in.”

The principle of Implicit Deny is crucial because it minimizes the risk of unauthorized access. Say you have a network where several services are running, like email, web hosting, and a database. 

You will configure the rules to allow email traffic, web traffic, and database connections from specific sources. Any other type of traffic or connections from unknown or unverified sources are implicitly denied, ensuring no unexpected or malicious traffic gets through.

How the principle of Implicit Deny works

Imagine you have a company server that should only be accessed by the IT department. You set up a firewall rule that explicitly allows access only to the IP addresses used by the IT team. Any other IP address trying to connect to the server will be denied by default. 

That is Implicit Deny in action – you haven’t listed all the addresses that are denied, you’ve only listed the ones that are allowed, and everyone else is refused entry automatically.

By relying on Implicit Deny, you make your security posture more robust. It’s a proactive stance; instead of reacting to threats as they appear, you set up your defenses so that only known, safe entities are allowed through. This method significantly limits the attack surface and helps in maintaining a secure network environment.

In practice, this could be as simple as a firewall rule that allows only web traffic (HTTP/HTTPS) to a public-facing web server while Denying all other types of traffic. Or a more complex setup where you have layers of rules: allowing specific services, specific ports, and specific IP ranges, with everything else being denied by default. 

By adopting Implicit Deny, you ensure that your network is locked down tight, only accessible by those who have a legitimate reason to be there.

Implicit Deny vs. explicit Deny

Implicit Deny and explicit Deny are two sides of the same security coin, but they handle access control differently. With Implicit Deny, the idea is simple: if access isn’t explicitly granted, it’s denied. This approach helps minimize the risk of unauthorized access because anything not on the "yes" list is a hard "no.”

Explicit Deny is where you specifically state what you want to block. It's like putting up a sign that says, "No entry for anyone not in uniform." For example, say you have an internal database that should only be accessible by your finance team. 

You could create a rule that explicitly denies access to everyone except members of the finance team. This means you’re listing out specific user groups or IP addresses that are not allowed to connect to the database.

The major difference here is in approach. Implicit Deny is more about specifying what is allowed and letting everything else be denied by default. In contrast, explicit Deny involves listing out what should be blocked.

Consider a real-world example: you manage a building where only employees should enter. With Implicit Deny, you create a guest list with employees’ names. Only those on the list get in; everyone else is turned away. 

Explicit Deny, however, means you’d make a list of people who are not allowed in, perhaps ex-employees or unwanted visitors, and block just them.

Implicit Deny is generally easier to manage because you only need to focus on what you allow, and everything else is taken care of automatically. Explicit Deny requires more maintenance and vigilance to ensure your block list stays current - it's more labor-intensive.

Using Implicit Deny helps keep things tight and secure. You don't worry about keeping an exhaustive list of what to block. Instead, you focus on what we permit.

So, by using Implicit Deny, you minimize your attack surface and simplify your security management. Implicit Deny says, "If I don’t know you, you’re not getting in." Explicit Deny requires us to say, "I know you, and you are specifically blocked," which can be a never-ending list in a dynamic threat landscape.

Main use cases for Implicit Deny

Network policies

Implicit Deny acts like your security bouncer, but for your network. If someone isn't on the guest list, they don't get in. It’s that simple. Instead of keeping track of all the bad actors out there, you focus on who you trust and let everyone else fend for themselves.

For example, a company server should only be accessed by your IT department. To ensure this you set up a firewall rule that says, “Hey, these specific IP addresses from the IT team can come in.” Any other IP address trying to sneak in? Denied by default. 

This approach is crucial for keeping your network safe. Imagine you run several services like email, web hosting, and a database. You configure your network policies to allow only specific traffic types and connections from verified sources. Anything else is automatically denied access. This way, no unexpected or malicious traffic slips through the cracks.

Using Implicit Deny means you don’t sweat the small stuff. You lock down what you need and let the principle do the heavy lifting. This way, your network remains secure, and your workload stays manageable. Plus, you minimize your attack surface, making it tough for unauthorized entities to find a way in.

Firewall rules

When we talk about firewall rules in our network, Implicit Deny means you only permit explicitly trusted traffic, and everything else is stopped in its tracks.

Let’s visualize this with your company server. You want only the IT department to access it. So, you craft a firewall rule allowing access exclusively to the IP addresses from the IT team. If an unknown IP tries to connect, well, they hit an automatic brick wall. You don’t bother listing all the potential intruders; you just list who’s allowed. That’s Implicit Deny in action.

This strategy simplifies your life and boosts security. Imagine the headache of constantly updating a blacklist of every possible threat. With Implicit Deny, you focus on what you trust and let the firewall handle the unknowns. It’s cleaner, smarter, and way less work.

Ultimately, Implicit Deny in firewall rules means you set up your defenses once and let them do their job. You approve only the traffic you need and block everything else by default. This way, you maintain a tight security posture without endless hassle.

Access control lists (ACLs)

Implicit Deny plays a crucial role when setting up Access Control Lists (ACLs) on a network. It ensures that only approved traffic gets through. If something isn't explicitly allowed, it’s denied instantly – no questions asked.

Take your internal company server, for example. You want only the IT department to access it. So, you create an ACL that explicitly permits traffic from the IP addresses used by your IT team. 

Any other IP addresses trying to access the server are blocked by default because they’re not on your "approved" list. You don’t need to list all the addresses you want to Deny; you just list the ones you want to allow.

Even your Wi-Fi network can benefit from this principle. You can set ACLs to allow only traffic from devices registered with your IT department. Any unknown device trying to connect is denied access right away. You don’t need to chase down and block every possible device that shouldn’t connect; you just list the ones you trust, and Implicit Deny will handle the rest.

Therefore, using ACLs with Implicit Deny simplifies network management. You can focus on what you need and trust the Implicit Deny principle to handle the unknowns. This way, your network stays secure, and your workload stays manageable.