Default Allow vs. Default Deny

Every firewall faces a question: what happens to traffic that doesn't match any rule?

The answer is your default policy. And this seemingly technical choice reveals something fundamental about how you think about security.

The Core Difference

Default allow permits all traffic except what you've explicitly blocked. Everything works unless you specifically disable it.

Default deny blocks all traffic except what you've explicitly allowed. Nothing works unless you specifically enable it.

Here's the insight that matters: Default allow protects you from what you know about. Default deny protects you from what you don't.

Why Default Allow Fails

Default allow seems friendly. New services work immediately. Users don't complain. You only write rules when something needs blocking.

But consider what this actually means. Every new vulnerability discovered tomorrow? Permitted. Every attack technique you haven't heard of? Permitted. Every service accidentally left running on your server? Exposed to the Internet.

You can't block what you don't know about. And there will always be more you don't know than you do.

Picture a server where you intend to run only a web server on port 80. But SSH is also running on port 22—you forgot to disable it. And there's a database listening on port 3306 that was only meant for local connections.

With default allow, all three are accessible from the Internet. Not because you decided they should be. Because you didn't explicitly decide they shouldn't be. An attacker scanning your server finds three open doors instead of one.

This is the horror of default allow: you're not just responsible for what you intentionally permit. You're responsible for everything you forgot to block.

Why Default Deny Works

With default deny, you flip the model. You create a rule allowing port 80 for your web server. SSH and the database? Still running, but invisible to the Internet. Not because you remembered to block them—because you never allowed them.

An attacker sees one open port. The attack surface is exactly what you intended.

This follows the principle of least privilege: grant only the minimum access necessary. But it's more than a principle. It's a fundamentally different relationship with the unknown.

Default deny means new vulnerabilities aren't automatically exploitable. Forgotten services aren't automatically exposed. Your security posture doesn't degrade every time you fail to learn about a new threat.

The Real Cost of Each Approach

With default allow, mistakes are silent. Forget to block something? It's exposed. Misconfigure a rule? Traffic flows. You might never know until the breach.

With default deny, mistakes are loud. Forget to allow something? It doesn't work. Someone notices. You fix it. The failure mode is availability, not security—an inconvenience, not a catastrophe.

The common objection is that default deny is less convenient. Users can't just start using new services. They have to wait for firewall rules.

This objection is real but misses the point. How often do you genuinely need new firewall rules? For most organizations, rarely. And when you do, the process of requesting and reviewing them ensures the access is actually necessary.

The temporary inconvenience of waiting for a rule is a feature, not a bug. It's the friction that prevents accidental exposure.

Implementing Default Deny

Default deny requires upfront work. You must identify what traffic is legitimate before you can allow it. This forces you to understand what your systems actually need—valuable knowledge you might not have had.

A practical approach:

1. Audit first. Discover what traffic currently flows. You'll find services you didn't know existed.
2. Document requirements. What actually needs to communicate with what? Write it down.
3. Enable logging mode. Set default deny but log instead of block. See what would be denied.
4. Create allow rules. For each legitimate traffic pattern, create a specific rule.
5. Enforce. Once you're confident, switch from logging to blocking.

The transition takes effort. But you emerge with something valuable: actual understanding of your network, and actual protection from everything else.

Different Zones, Different Policies

In complex environments, you might vary the default by zone.

Internet-facing interfaces should always use default deny. Nothing from the Internet should reach your systems unless explicitly permitted. This isn't debatable.

Internal networks might use different policies depending on trust levels. But even internally, default deny for access to sensitive systems is wise. Breaches often involve lateral movement—an attacker who compromises one system looking for paths to others.

The Industry Position

Security professionals overwhelmingly recommend default deny. Every major compliance framework requires it. This isn't theoretical preference—it's based on decades of breaches that began with services unintentionally exposed because they weren't explicitly blocked.

Default allow isn't a security policy. It's the absence of one.