Table of Contents
Summary
Many organizations assume their firewall is enough to handle a DDoS attack, but firewalls and DDoS protection are built for different jobs. Firewalls enforce access policy; DDoS protection preserves service availability. During an attack, even a firewall making all the right decisions can be overwhelmed by traffic volume alone. The right approach isn’t one or the other. It’s both.
Introduction
It is a reasonable question. If your organization already has a firewall, why add dedicated DDoS protection?
The answer is simple: a firewall and a DDoS protection solution are built for different jobs.
A firewall controls access. It decides which traffic should be allowed, which traffic should be denied, and which parts of the network should be separated from each other.
DDoS protection preserves availability. It keeps internet-facing services reachable when attackers try to overwhelm bandwidth, devices, applications, or people with hostile traffic.
Those goals are related, but they are not the same. Treating them as the same problem is where many organizations get exposed.
Corero Network Security’s 2026 Threat Intelligence Report shows why this distinction matters. DDoS attacks are faster, larger, more repetitive, and more evasive than traditional perimeter defenses were designed to handle. Corero’s 2025 Threat Intelligence Report also makes an important point: many DDoS attacks are small enough to sit below broad volumetric thresholds, yet still capable of exhausting firewalls, disrupting applications, or creating expensive operational noise.
In plain English: a firewall can make the right security decision and still become the wrong place to absorb a DDoS attack. Cyber resilience depends on both strong security policy and the ability to keep critical services running when hostile traffic is already in motion.
What Firewalls Are For
NIST defines a firewall as “a device or program that controls the flow of network traffic between networks or hosts that employ differing security postures”. That is exactly the right way to think about it.
A firewall is a policy control. It helps answer questions such as:
- Is this source allowed to reach this destination?
- Is this port or application allowed?
- Should this traffic cross from one network zone to another?
- Does this connection match the rules we have approved?
That makes firewalls essential. They help enforce access control, segment networks, reduce exposure, and block traffic that clearly violates policy.
But a DDoS attack is not always trying to sneak through your policy. Often, it is trying to bury your infrastructure under more traffic, more sessions, or more requests than it can handle.
That is a different problem.
Key detail: Allowed traffic can still be harmful at scale.
For example, your firewall may allow web traffic to a public website because customers need to reach it. During a DDoS attack, the malicious traffic may also look like web traffic at first glance. The issue is not simply whether port 443 is allowed. The issue is whether your service, network, and security controls can stay available when that traffic arrives in abusive volume or pattern.
What DDoS Attacks Are Trying to Do
DDoS is an availability attack. The goal is to make a service slow, unstable, or unreachable.
That can happen in several ways:
- Fill the internet connection so legitimate users cannot get through
- Force a firewall, load balancer, or server to process too many packets
- Consume connection tables, memory, CPU, or buffers
- Create enough application requests to slow or break the service
- Distract operators with short, repeated incidents that are hard to investigate manually
This is why the “we already have a firewall” answer is risky. It assumes the firewall only needs to decide what is allowed. During a DDoS attack, the firewall also has to survive the traffic long enough to make those decisions.
That survival question is the part many firewall-only strategies miss.
Think of it this way: a security guard can check badges at a door. But if a crowd blocks the entrance, the problem is no longer just badge checking. The problem is keeping the entrance usable for the people who are supposed to get in.
Why Stateful Devices Can Become the Weak Point
Many firewalls are stateful. That means they track connection information so they can make better decisions about traffic. In normal conditions, statefulness is useful. It helps the firewall understand whether traffic belongs to an expected session and whether that session should continue.
During a DDoS attack, that same strength can become a weakness.
Key detail: Statefulness is useful for security, but risky during DDoS.
Attackers often try to create pressure by forcing devices to track too much at once. A flood of new connection attempts, half-open sessions, or constantly changing flows can consume connection tables, memory, or CPU. The firewall may still be doing its job from a policy perspective, but the device itself becomes overloaded.
RFC 4732 describes denial-of-service as a resource-exhaustion problem. That is the practical point for business and IT leaders: an attacker does not always need to break into anything. Sometimes they only need to make a critical device spend too much effort on unwanted traffic.
Here are three common examples.
Example 1: The Firewall Drops the Attack, But Still Gets Exhausted
An attacker sends a large flood toward an internet-facing service. The firewall identifies much of the traffic as unwanted and drops it.
That sounds like success, but the firewall still had to receive the packets, inspect them, match them against policy, log or classify them, and then drop them. Under enough pressure, the work required to reject bad traffic can overload the device.
Key detail: Dropping attack traffic still costs firewall resources.
Example 2: Connection Tracking Becomes the Target
An attacker sends a large number of connection attempts that force the firewall to create or evaluate session state. The goal is not to pass traffic through the firewall. The goal is to fill the firewall’s tables or exhaust its processing capacity.
When that happens, legitimate sessions can slow down, fail, or get dropped. Operators may also lose management access at the moment they need it most.
This is especially risky for software-based firewalls running on shared or general-purpose compute. Packet handling, state tracking, logging, management, and the firewall engine may all compete for the same CPU and memory. Under attack, the whole system can become unstable long before the internet link is fully saturated.
Example 3: The Link Fills Before the Firewall Can Help
If the attack saturates the upstream connection, the firewall may never get a fair chance to protect the service. The traffic has already consumed the path into the network.
In that scenario, saying “the firewall would have dropped it” does not help the users who cannot reach the service. The problem needs to be handled before, at, or alongside the congestion point, not only after the traffic arrives at the firewall.
This is why NIST SP 800-189 discusses DDoS mitigation as part of a broader resilience model, including upstream filtering, source address validation, BGP-based response methods, and routing security practices.
What DDoS Protection Adds
Dedicated DDoS protection is designed around the availability problem. It is not trying to replace the firewall. It is trying to keep the firewall, network, and application from becoming the casualty of the attack.
In practical terms, DDoS protection helps by adding:
- Faster detection of abnormal traffic patterns
- Automated mitigation before manual response becomes the bottleneck
- Filtering designed for floods, reflection attacks, protocol abuse, and multi-vector events
- Protection that reduces pressure on stateful devices and downstream infrastructure
- Better visibility into attack type, size, duration, and impact
- Deployment flexibility, including inline, out-of-band, scrubbing, and hybrid models
The key difference is the operating goal.
A firewall asks, “Should this traffic be allowed?”
DDoS protection asks, “How do we keep the service reachable while the attack is happening?”
Both questions matter. But if your public services are under attack, the second question becomes urgent very quickly.
Why the Right Answer Is Firewall Plus DDoS Protection
The point is not that firewalls are weak or unnecessary. The point is that firewalls should not be asked to do every job.
Use the firewall for what it does well:
- Enforce access policy
- Segment the network
- Control which services are exposed
- Reduce general security risk
- Support compliance and audit requirements
Use DDoS protection for what it does well:
- Detect attack traffic quickly
- Mitigate floods and multi-vector attacks
- Reduce pressure on firewalls and other stateful devices
- Preserve service availability during active attacks
- Improve response speed, visibility, and operational confidence
This is a stronger and more realistic cyber resilience posture. The firewall remains an important security control, but it is no longer the only device standing between a DDoS attack and the services your business depends on.
For many organizations, the practical question is not “Do we have a firewall?” It is:
- Can our firewall stay responsive during a flood?
- Can we detect short, repeated attacks without manual investigation?
- Can we mitigate before customers feel the impact?
- Can we protect the upstream path, not just the local policy point?
- Can we keep legitimate users connected while we filter attack traffic?
If the answer to those questions is uncertain, firewall-only protection is not a strategy. It is a risk acceptance decision.
How Corero Helps
At Corero Network Security, we help organizations close the gap between security policy, real-time DDoS protection, and cyber resilience.
SmartWall ONE™ is built to protect service availability with flexible deployment models that can align with different network architectures, risk profiles, and operating models. It helps teams detect and mitigate DDoS attacks quickly while reducing the burden on firewalls and other downstream infrastructure.
For organizations that need additional operational support, SecureWatch services provide monitoring, analytics, and expert assistance around DDoS detection and mitigation. That gives teams a more practical way to maintain visibility, respond faster, and reduce the pressure of managing attacks manually.
The goal is straightforward: keep firewall policy where it belongs, and put dedicated DDoS protection where availability is on the line.
FAQ
Partially. A firewall can drop traffic that violates policy, but the work of receiving, inspecting, and rejecting attack traffic still consumes resources. Under enough pressure, a firewall can become overloaded even while doing its job correctly.
A firewall controls access, deciding what traffic is allowed or denied. DDoS protection preserves availability, keeping services reachable when attackers try to overwhelm them with hostile traffic. Both are necessary; neither replaces the other.
Yes. Attack traffic can mimic legitimate traffic, such as standard web requests, and still cause harm at abusive volume or frequency. The issue isn’t always whether traffic is allowed. It’s whether your infrastructure can stay available when that traffic arrives at scale.
Stateful firewalls track connection information to make better decisions. Attackers exploit this by flooding devices with connection attempts or half-open sessions, filling connection tables and exhausting processing capacity without ever breaching policy.
It detects abnormal traffic patterns faster, automates mitigation before the attack escalates, filters floods and multi-vector attacks, and reduces pressure on firewalls and downstream infrastructure, all while keeping services available to legitimate users.
Corero’s SmartWall ONE provides always-on, real-time detection and mitigation with flexible deployment options. For teams that need additional support, SecureWatch services offer monitoring, analytics, and expert assistance to reduce the operational burden of managing attacks manually.
