A firewall is easier to secure when the default behavior is clear: traffic that has not been explicitly approved should not be allowed through.
This is commonly called a deny-by-default approach.
The idea sounds simple, but implementing it correctly in AWS Network Firewall requires more than adding one deny rule. You need to understand how stateless and stateful processing work, how rule ordering affects traffic, how return traffic is handled, and where legitimate application traffic needs an explicit exception.
The goal is not to block everything and then keep adding rules until the application works. A production deny-by-default policy should start from known application requirements and explicitly define the traffic that is allowed.
This approach is especially useful for environments where the network boundary needs to be predictable. Instead of asking whether a new connection happens to match an existing allow rule, the security team can ask a much simpler question:
"Has this traffic been explicitly approved?"
What Deny-by-Default Means
A deny-by-default firewall policy has two basic behaviors.
Known and approved traffic is allowed.
Everything else is denied.
A simplified model looks like this:
Incoming or outgoing traffic
|
v
Does it match an
approved rule?
/ \
Yes No
| |
v v
Allow Deny
This is different from an allow-by-default design.
With allow-by-default, a new destination can become reachable simply because no rule blocks it.
With deny-by-default, a new destination remains blocked until someone explicitly adds an approved rule.
That difference becomes important as an environment grows.
A development team may add a new external API, package repository, monitoring endpoint, or SaaS service. Under an allow-by-default design, that connection may start working without any security review.
Under deny-by-default, the connection fails until it is intentionally approved.
Why This Matters in Production
Applications rarely communicate with only one destination.
A typical production application might need access to:
Application
|
+-- Internal database
+-- Authentication service
+-- Object storage
+-- Payment API
+-- Email provider
+-- Monitoring service
+-- Package repository
+-- DNS
Each dependency creates a potential network path.
If the firewall allows broad outbound traffic, the application may also be able to reach destinations that were never part of its design.
For example:
Application subnet
|
+---- Approved API
|
+---- Unknown external server
|
+---- Unapproved service
|
+---- Newly introduced destination
A deny-by-default policy changes this behavior.
The application receives access only to destinations represented by the security policy.
This reduces the number of unexpected communication paths that need to be trusted.
Deny-by-Default Does Not Mean "Deny Everything"
One common misunderstanding is that deny-by-default means writing one giant deny rule.
That is not the real design.
The policy should instead have:
Explicit allow rules for required traffic.
Appropriate inspection rules for traffic that needs security analysis.
A default or explicit deny behavior for traffic that does not match an approved path.
The important part is that the allow rules describe legitimate application requirements.
For example:
Allow:
Application → Approved API → TCP 443
Allow:
Application → Internal database → TCP 5432
Allow:
Application → Approved DNS resolver → DNS
Deny:
Everything else
This gives the security team a clear understanding of what the application is expected to communicate with.
Start With an Application Traffic Inventory
Do not start by writing firewall rules.
Start by listing the traffic the application actually needs.
For each dependency, capture:
Source | Destination | Protocol | Port | Direction | Reason |
|---|
App subnet | Database subnet | TCP | 5432 | Outbound | Database access |
App subnet | Auth API | TCP | 443 | Outbound | Authentication |
App subnet | Payment API | TCP | 443 | Outbound | Payment processing |
App subnet | DNS resolver | UDP/TCP | 53 | Outbound | Name resolution |
Load balancer | App subnet | TCP | 443 | Inbound | Application traffic |
This inventory becomes the foundation of the firewall policy.
It also makes future reviews easier.
When someone asks why an outbound rule exists, the answer should be visible in the rule's documented purpose.
Understand the Traffic Direction
A firewall rule should not be evaluated only by destination.
You need to understand the direction of traffic.
For example, an application may initiate a connection:
Application
|
| TCP connection
v
External API
The external API then sends response traffic back:
External API
|
| Response
v
Application
The firewall must handle the traffic according to its stateful or stateless processing behavior.
This is one reason stateful rules are useful for application connections.
You should not design a policy by assuming that every response packet needs to be manually configured as a separate allow rule.
The actual behavior depends on the inspection mode and rule configuration.
Stateless Rules and Deny-by-Default
Stateless rules evaluate packets individually.
They are useful when you want to make an early decision based on packet attributes such as:
Source address
Destination address
Protocol
Port
A simplified example is:
Source: application subnet
Destination: approved API range
Protocol: TCP
Destination port: 443
Action: Forward to stateful inspection
Another packet can then be handled by a different stateless rule.
The important design question is what happens when no stateless rule matches.
The default action must be intentionally selected.
For a deny-by-default architecture, unmatched traffic should not accidentally bypass the stateful inspection path and become allowed.
This is why the stateless default action needs to be reviewed as carefully as the individual rules.
Stateful Rules Add Context
Stateful inspection can evaluate traffic using connection context and more detailed protocol information.
This makes stateful rules useful for application-level security requirements.
For example:
Application
|
| HTTPS
v
Approved API
A stateful rule can define the conditions under which that traffic is accepted.
Stateful rules can also be useful for:
The important point is that stateless and stateful rules are different layers of the policy.
Do not assume that an allow rule in one layer automatically produces the same result in another.
The Default Action Is Part of the Security Policy
A deny-by-default architecture is not complete until the default behavior has been defined.
Consider this simplified flow:
Packet
|
v
Stateless rules
|
+---- Match → Apply configured action
|
+---- No match
|
v
Default action
If the default action forwards unmatched traffic for further inspection, the stateful policy must then determine what happens.
If the default action drops unmatched traffic, traffic that does not match the expected stateless path is blocked earlier.
The correct choice depends on the overall Network Firewall architecture.
What matters is that the result is intentional and tested.
A security team should be able to answer:
"What happens to a packet that matches no explicit allow rule?"
If the answer is unclear, the policy is not yet ready for production.
Write Narrow Allow Rules
A common mistake is replacing an open policy with broad allow rules.
For example:
Allow:
Application subnet → 0.0.0.0/0 → TCP 443
This technically allows HTTPS, but it may not provide the level of restriction the organization expects.
The application can potentially establish HTTPS connections to a very large set of destinations.
A narrower rule might restrict the destination to approved address ranges or use an appropriate domain-based control where supported by the architecture.
The principle is:
Broad requirement
↓
Large allow rule
↓
Large attack surface
Specific requirement
↓
Narrow allow rule
↓
Smaller attack surface
The right rule is the smallest rule that satisfies the real application requirement.
Domain-Based Rules Need Special Care
Modern applications frequently use domain names rather than fixed IP addresses.
For example:
api.example.com
auth.example.com
payments.example.com
The corresponding IP addresses can change over time.
A rule based entirely on a fixed IP address may therefore become outdated.
Domain-based filtering can help when the application dependency is naturally represented by a domain.
However, domain-based controls should be tested carefully.
Verify:
How the application resolves DNS.
Which DNS resolver it uses.
Whether traffic is encrypted.
Whether the destination uses multiple domains.
Whether redirects occur.
Whether a CDN is involved.
Whether the application connects directly or through another service.
Do not assume that approving one domain automatically covers every network destination the application might use.
Do Not Forget DNS
Deny-by-default policies often break applications because DNS was forgotten.
An application may need to resolve:
api.example.com
before it can connect to the API.
If DNS traffic is blocked, the application may report:
Connection timeout
even though the actual problem is name resolution.
Make DNS an explicit dependency in the traffic inventory.
For example:
Application
|
+---- DNS resolver → UDP/TCP 53
|
+---- API endpoint → TCP 443
Also verify whether the application uses:
VPC-provided DNS
Route 53 Resolver
A custom DNS server
An internal DNS service
The correct firewall rule depends on the actual DNS architecture.
Do Not Treat IPS Rules as Simple Allow Rules
IPS rules have a different purpose.
An allow rule answers:
"Should this traffic be permitted?"
An IPS rule answers something closer to:
"Does this traffic match a known or suspicious attack pattern?"
That means the policy may contain both:
Access control
+
Threat inspection
For example:
Approved HTTPS traffic
|
v
Stateful inspection
|
+---- Normal traffic → Continue
|
+---- Matching threat signature → Alert/Drop
A deny-by-default policy should not be designed in a way that accidentally removes threat inspection from approved traffic.
Approved traffic can still be malicious.
That is an important security distinction.
Rule Order Matters
Suppose an application needs to reach a specific external destination.
You might have:
Rule 1:
Allow application → approved API
Rule 2:
Deny application → all external destinations
If the policy evaluates the rules in a way that causes the deny to take precedence, the intended API connection may fail.
The opposite problem is equally dangerous.
A broad allow rule placed before a narrow deny can unintentionally permit traffic that should have been blocked.
During design, review:
Rule priorities
Rule groups
Default actions
Stateless processing
Stateful processing
Pass rules
Drop rules
Alert rules
Do not assume that the order in the configuration file tells the entire story.
Use a Rule Naming Convention
Deny-by-default policies become difficult to maintain if rules have meaningless names.
Instead of:
rule-1
rule-2
rule-3
use names that explain their purpose.
For example:
Allow-App-To-Payment-API
Allow-App-To-Internal-DB
Allow-App-To-DNS
Deny-Unapproved-External-Traffic
A good name helps during:
Security reviews
Troubleshooting
Incident response
Change management
Policy migration
Audit reviews
The rule name should describe the business or security purpose, not merely its technical implementation.
Keep Rules Specific to Their Responsibility
Do not create one enormous rule group containing every application dependency in the organization.
As the environment grows, separating rules by responsibility makes policy management easier.
For example:
Firewall Policy
|
+-- Shared Network Rules
|
+-- Application Rules
|
+-- Database Rules
|
+-- Domain Rules
|
+-- IPS Rules
The exact grouping strategy depends on the environment, but the principle is useful:
Each rule group should have a clear reason for existing.
This makes reviews easier because engineers can understand what a group controls without reading hundreds of unrelated rules.
Test the Deny Path First
Most teams naturally test the traffic they expect to work.
For deny-by-default policies, testing blocked traffic is equally important.
Create negative test cases such as:
Application → Unknown external IP
Expected: Block
Application → Unapproved TCP port
Expected: Block
Application → Unapproved domain
Expected: Block
Application → Restricted internal subnet
Expected: Block
Then test the approved paths:
Application → Approved API
Expected: Allow
Application → Approved database
Expected: Allow
Application → Approved DNS
Expected: Allow
This gives you confidence that the policy has both halves of its security model.
Monitor What the Firewall Is Blocking
A deny-by-default policy will naturally generate blocked traffic.
Not every blocked packet represents an attack.
Some will be caused by:
This is why firewall logs need to be reviewed after deployment.
Look for repeated denied traffic from the same source.
For example:
Application subnet
|
+---- Approved API → Allow
|
+---- Unknown destination → Drop
|
+---- Unknown destination → Drop
|
+---- Unknown destination → Drop
Repeated drops may indicate either a security event or an undocumented dependency.
The response should be based on investigation, not immediately adding an allow rule.
Do Not Fix Every Block With an Allow Rule
This is one of the most common problems with deny-by-default implementations.
An application team sees:
Connection failed
The network team sees:
Firewall drop
Someone then adds:
Allow 0.0.0.0/0
The application works again, but the security model has been weakened.
Instead, investigate:
What destination does the application actually need?
Why does it need that destination?
Is the destination expected?
Which protocol and port are required?
Is the dependency documented?
Can the rule be limited to a specific destination?
Can the dependency use a stable domain or service endpoint?
Is the dependency actually required?
Only then should a new allow rule be considered.
Handling Temporary Access
Sometimes an application needs temporary access during troubleshooting.
Do not solve this by permanently weakening the firewall.
Instead, use a controlled change process where possible.
Document:
Reason
Source
Destination
Port
Duration
Owner
Approval
Removal date
Temporary access should have an explicit expiration plan.
Otherwise, temporary troubleshooting rules have a tendency to become permanent production dependencies.
Use Infrastructure as Code
A deny-by-default firewall policy should be reproducible.
Managing the configuration manually through a console can make it difficult to know:
Infrastructure as Code allows firewall configuration to be reviewed before deployment.
A simplified conceptual structure might look like:
network-firewall/
policy/
stateless-rules/
stateful-rules/
domains/
ips/
environments/
The exact implementation can use the organization's preferred tooling.
The important part is that security policy changes should be reviewable and reproducible.
Validate Changes Before Production
Before deploying a new allow rule, ask:
What traffic does this permit?
Why is it required?
Can the destination be narrowed?
Can the source be narrowed?
Can the port be narrowed?
What happens if this rule is removed?
How will we know if it is being abused?
These questions prevent rules from gradually becoming broader than the original requirement.
Example Production Policy
Consider a web application with these dependencies:
Internet users
|
v
Application Load Balancer
|
v
Application
|
+---- PostgreSQL database
|
+---- Authentication API
|
+---- Payment API
|
+---- DNS
A deny-by-default policy might conceptually allow:
Load balancer → Application
Application → Database
Application → Authentication API
Application → Payment API
Application → DNS
Everything else is denied unless another documented requirement exists.
This is much easier to review than:
Application → Internet → Allow
The difference is not simply the number of rules.
It is the level of control over the application's network behavior.
Common Mistakes
Allowing All HTTPS Traffic
HTTPS is encrypted, but encryption does not mean the destination is trusted.
Allowing TCP 443 to every destination can still provide broad outbound access.
Forgetting Return Traffic
Design the policy around the actual stateful behavior of the firewall rather than manually creating unnecessary rules for every response packet.
Blocking DNS
Applications often depend on DNS even when the application team does not consider DNS an explicit dependency.
Ignoring Background Jobs
Scheduled processes may use different external endpoints from the main request path.
Creating Permanent Temporary Rules
Temporary access should have an expiration process.
Making Rules Too Broad
A rule that solves today's problem by allowing an entire network range may create a larger security problem tomorrow.
Testing Only Allowed Traffic
A deny-by-default policy is incomplete until its deny behavior has been tested.
Production Checklist
Before deploying a deny-by-default AWS Network Firewall policy, verify:
All required application traffic is documented.
Source and destination are known.
Protocol and ports are known.
Traffic direction is documented.
DNS dependencies are documented.
Stateless behavior is understood.
Stateful behavior is understood.
Default actions are explicitly configured.
Rule ordering has been reviewed.
IPS rules are preserved where required.
Domain rules have been tested.
Positive traffic tests pass.
Negative traffic tests pass.
Firewall logging is enabled.
Repeated denied traffic is monitored.
Temporary rules have expiration controls.
Configuration is version-controlled.
Infrastructure changes are reviewed.
Rollback configuration is available.
Application teams have validated required dependencies.
Advantages and Disadvantages
Advantages
A deny-by-default design provides:
Clear network security boundaries
Smaller attack surface
Better visibility into application dependencies
Easier security review
More predictable network behavior
Reduced risk from unexpected outbound access
Stronger separation between approved and unapproved traffic
Disadvantages
It also requires more operational discipline:
Every required dependency must be identified
New application dependencies need firewall changes
Troubleshooting can initially take longer
DNS and third-party services require careful handling
Poorly designed rules can cause legitimate traffic failures
Rule management becomes important as the environment grows
The answer is not to avoid deny-by-default because it requires work. The answer is to make the rule-management process predictable.
Summary
A deny-by-default AWS Network Firewall policy starts with a simple security principle: traffic should be allowed because it is explicitly required, not because nobody has blocked it yet.
The implementation requires more thought than adding a final deny rule. You need to understand application dependencies, traffic direction, stateless and stateful processing, rule ordering, DNS, domain filtering, IPS inspection, logging, and default actions.
Start by documenting the traffic the application actually needs. Build narrow allow rules around those requirements. Keep threat inspection separate from basic access control. Test both allowed and denied traffic before production.
Most importantly, treat every new allow rule as a security decision.
When this model is maintained properly, the firewall becomes easier to reason about. Engineers know what traffic the application is expected to use, security teams can review those paths, and unexpected network connections have a clear default outcome: they are blocked until someone intentionally approves them.
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