Network ACLs overview in AWS - Step-by-Step Execution

Process Flow - Network ACLs overview

Incoming Traffic

↓

Network ACL Checks Rules

↓

Rule Match?

No→Default Deny

Yes↓

Allow or Deny Action

↓

Traffic Allowed or Blocked

↓

Outgoing Traffic

Traffic enters, Network ACL rules are checked in order. If a rule matches, its allow or deny action applies. If no rules match, traffic is denied by default.

Execution Sample

AWS

Rule 100: Allow TCP 80 inbound
Rule 110: Deny all inbound
Rule 100: Allow TCP 80 outbound
Rule 110: Deny all outbound

Network ACL rules are evaluated in order by number. Traffic matching an allow rule passes; otherwise, it is denied.

Process Table

Step	Traffic Direction	Packet Details	Rule Evaluated	Rule Action	Result
1	Inbound	TCP port 80	100	Allow	Allowed
2	Inbound	TCP port 22	100	No match	Continue
3	Inbound	TCP port 22	110	Deny	Denied
4	Outbound	TCP port 80	100	Allow	Allowed
5	Outbound	UDP port 53	100	No match	Continue
6	Outbound	UDP port 53	110	Deny	Denied

💡 Traffic is allowed or denied based on the first matching rule; if none match, default deny applies.

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6
Traffic Status	Pending	Allowed (Inbound TCP 80)	Denied (Inbound TCP 22)	Allowed (Outbound TCP 80)	Denied (Outbound UDP 53)

Key Moments - 3 Insights

Why does inbound TCP port 22 traffic get denied even though there is an allow rule for port 80?

Because rules are checked in order, and port 22 does not match the allow rule 100, so it continues to rule 110 which denies all other traffic (see execution_table rows 2 and 3).

What happens if no rule matches the traffic?

Traffic is denied by default if no rules match, as shown in the default deny action after rule evaluation (execution_table exit_note).

Are Network ACL rules stateful or stateless?

Network ACLs are stateless, so return traffic must be explicitly allowed by rules in both directions (inbound and outbound).

Visual Quiz - 3 Questions

Look at the execution table, what is the result for inbound TCP port 80 traffic at step 1?

A Denied

B Allowed

C No match

D Continues to next rule

💡 Hint

Check the 'Result' column in execution_table row 1.

At which step does inbound TCP port 22 traffic get denied?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Packet Details' and 'Result' columns in execution_table rows 2 and 3.

If rule 110 (deny all) was removed, what would happen to outbound UDP port 53 traffic?

A Allowed by rule 100

B Allowed by default

C Denied by default

D No change

💡 Hint

Recall that Network ACLs deny traffic by default if no rule matches (see exit_note).

Concept Snapshot

Network ACLs control traffic at subnet level.
Rules are evaluated in order by number.
First matching rule's allow or deny applies.
If no match, traffic is denied by default.
Network ACLs are stateless; rules needed for both directions.

Full Transcript

Network ACLs are like gatekeepers for your subnet's traffic. When a packet arrives, the ACL checks its rules in order. If the packet matches a rule, it is either allowed or denied based on that rule. If no rules match, the packet is denied by default. This process happens separately for inbound and outbound traffic because Network ACLs do not remember previous decisions (they are stateless). For example, if inbound TCP port 80 traffic arrives, it matches an allow rule and passes. But inbound TCP port 22 traffic does not match the allow rule and is denied by a later deny rule. Outbound traffic is checked similarly. Understanding this step-by-step helps you control your network traffic securely.

Practice

1. What is the primary purpose of a Network ACL in AWS?

easy

A. To monitor application performance

B. To manage user permissions for AWS services

C. To store data securely in the cloud

D. To control inbound and outbound traffic at the subnet level

Solution

1. Step 1: Understand Network ACL function

   Network ACLs act as a firewall controlling traffic entering and leaving subnets.

2. Step 2: Identify correct purpose

   They specifically control inbound and outbound traffic at the subnet level, not user permissions or data storage.

3. Final Answer:

   To control inbound and outbound traffic at the subnet level -> Option D

4. Quick Check:

   Network ACL = subnet traffic control [OK]

Hint: Network ACLs control subnet traffic, not users or data [OK]

Common Mistakes:

• Confusing Network ACLs with IAM permissions
• Thinking Network ACLs store data
• Assuming Network ACLs monitor performance

2. Which of the following is the correct way to define a rule in a Network ACL?

easy

A. User name, password, access level, allow or deny

B. Instance ID, security group, IP address, allow or deny

C. Rule number, protocol, port range, source/destination, allow or deny

D. Subnet ID, route table, gateway, allow or deny

Solution

1. Step 1: Recall Network ACL rule components

   Network ACL rules include a rule number, protocol, port range, source or destination IP, and action (allow or deny).

2. Step 2: Match correct option

   Rule number, protocol, port range, source/destination, allow or deny lists these components correctly; other options mention unrelated elements like user credentials or instance IDs.

3. Final Answer:

   Rule number, protocol, port range, source/destination, allow or deny -> Option C

4. Quick Check:

   Network ACL rule = numbered protocol and ports [OK]

Hint: Network ACL rules use numbers, protocols, ports, and allow/deny [OK]

Common Mistakes:

• Mixing user credentials with ACL rules
• Confusing security groups with ACL rules
• Using subnet or route info as rule components

3. Given a Network ACL with the following rules:
Rule 100: Allow TCP port 80 from 0.0.0.0/0
Rule 110: Deny all traffic
What happens to an incoming TCP request on port 80 from IP 192.168.1.1?

medium

A. The request is allowed because rule 100 permits it

B. The request is denied because rule 110 denies all traffic

C. The request is ignored due to missing rule for port 80

D. The request causes an error in the Network ACL

Solution

1. Step 1: Understand rule evaluation order

   Network ACLs evaluate rules by ascending rule number. Rule 100 is checked before 110.

2. Step 2: Apply rules to the request

   Rule 100 allows TCP port 80 from any IP, so the request from 192.168.1.1 is allowed before rule 110 denies all.

3. Final Answer:

   The request is allowed because rule 100 permits it -> Option A

4. Quick Check:

   Lower rule number allow overrides higher deny [OK]

Hint: Rules checked in order; first match decides allow or deny [OK]

Common Mistakes:

• Assuming deny all overrides allow rules
• Ignoring rule number order
• Thinking missing rules cause errors

4. You created a Network ACL with these rules:
Rule 100: Allow inbound TCP port 22 from 10.0.0.0/16
Rule 110: Deny all inbound traffic
But SSH connections from 10.0.1.5 are failing. What is the likely problem?

medium

A. The Network ACL is stateless and missing an outbound allow rule for port 22

B. The security group attached to the instance denies SSH

C. The subnet does not have a route to the internet

D. The IP 10.0.1.5 is outside the allowed range

Solution

1. Step 1: Recall Network ACL stateless behavior

   Network ACLs are stateless, so return traffic must be explicitly allowed by outbound rules.

2. Step 2: Analyze rules and failure cause

   Inbound SSH is allowed, but if outbound port 22 is denied, the response cannot return, causing failure.

3. Final Answer:

   The Network ACL is stateless and missing an outbound allow rule for port 22 -> Option A

4. Quick Check:

   Stateless ACLs need inbound and outbound rules [OK]

Hint: Stateless ACLs need both inbound and outbound rules [OK]

Common Mistakes:

• Assuming ACLs are stateful like security groups
• Ignoring outbound rules for return traffic
• Mistaking IP range or subnet routing as cause

5. You want to block all HTTP traffic (port 80) to a subnet except from a specific IP 203.0.113.5 using Network ACLs. Which rule set achieves this?

hard

A. Rule 100: Deny TCP port 80 from 0.0.0.0/0Rule 110: Allow TCP port 80 from 203.0.113.5Rule 120: Allow all other traffic

B. Rule 100: Allow TCP port 80 from 203.0.113.5Rule 110: Deny TCP port 80 from 0.0.0.0/0Rule 120: Allow all other traffic

C. Rule 100: Allow all trafficRule 110: Deny TCP port 80 from 0.0.0.0/0

D. Rule 100: Deny all trafficRule 110: Allow TCP port 80 from 203.0.113.5

Solution

1. Step 1: Understand rule evaluation order

   Network ACLs evaluate rules by ascending number; first matching rule applies.

2. Step 2: Analyze rules for desired effect

   Rule 100 allows port 80 only from 203.0.113.5. Rule 110 denies port 80 from all others. Rule 120 allows other traffic.

3. Step 3: Confirm correct blocking and allowing

   This setup blocks HTTP except from the specific IP, matching the requirement.

4. Final Answer:

   Rule 100: Allow TCP port 80 from 203.0.113.5; Rule 110: Deny TCP port 80 from 0.0.0.0/0; Rule 120: Allow all other traffic -> Option B

5. Quick Check:

   Allow specific IP first, then deny others [OK]

Hint: Allow specific IP first, then deny all others [OK]

Common Mistakes:

• Placing deny before allow for specific IP
• Not including allow for other traffic
• Assuming ACLs are stateful