Network ACLs overview in AWS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Network ACL in AWS using Terraform.

AWS

resource "aws_network_acl" "example" {
  vpc_id = "[1]"
}

Drag options to blanks, or click blank then click option'

Avpc-123abc

Bsubnet-456def

Csg-789ghi

Deni-012jkl

Attempts:

3 left

💡 Hint

Common Mistakes

Using a subnet ID instead of a VPC ID

Using a security group ID instead of a VPC ID

The vpc_id must reference the VPC where the Network ACL will be created. Using a subnet ID or security group ID is incorrect.

2fill in blank

medium

Complete the code to add an inbound rule allowing HTTP traffic on port 80.

AWS

resource "aws_network_acl_rule" "allow_http_inbound" {
  network_acl_id = aws_network_acl.example.id
  rule_number    = 100
  protocol       = "[1]"
  rule_action   = "allow"
  egress        = false
  cidr_block    = "0.0.0.0/0"
  from_port     = 80
  to_port       = 80
}

Drag options to blanks, or click blank then click option'

Aicmp

Budp

Ctcp

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Setting protocol to UDP or ICMP instead of TCP

Using 'all' which allows all protocols, not specific to HTTP

HTTP uses TCP protocol on port 80, so the protocol must be set to "tcp".

3fill in blank

hard

Fix the error in the rule that blocks all outbound traffic.

AWS

resource "aws_network_acl_rule" "deny_all_outbound" {
  network_acl_id = aws_network_acl.example.id
  rule_number    = 200
  protocol       = "[1]"
  rule_action   = "deny"
  egress        = true
  cidr_block    = "0.0.0.0/0"
  from_port     = 0
  to_port       = 0
}

Drag options to blanks, or click blank then click option'

Aany

B0

Call

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'all' or 'any' instead of '-1' for all protocols

Using '0' which is invalid for protocol

To block all protocols, the protocol must be set to "-1" in AWS Network ACL rules.

4fill in blank

hard

Fill both blanks to create a rule allowing inbound SSH traffic from a specific IP.

AWS

resource "aws_network_acl_rule" "allow_ssh_inbound" {
  network_acl_id = aws_network_acl.example.id
  rule_number    = 110
  protocol       = "[1]"
  rule_action   = "allow"
  egress        = false
  cidr_block    = "[2]"
  from_port     = 22
  to_port       = 22
}

Drag options to blanks, or click blank then click option'

Atcp

Budp

C203.0.113.0/24

D0.0.0.0/0

Attempts:

3 left

💡 Hint

Common Mistakes

Using UDP instead of TCP for SSH

Using 0.0.0.0/0 which allows all IPs instead of a specific range

SSH uses TCP protocol on port 22, and the CIDR block must specify the allowed IP range, here a specific subnet.

5fill in blank

hard

Fill all three blanks to create a Network ACL rule that denies outbound ICMP traffic to a specific subnet.

AWS

resource "aws_network_acl_rule" "deny_icmp_outbound" {
  network_acl_id = aws_network_acl.example.id
  rule_number    = 150
  protocol       = "[1]"
  rule_action   = "[2]"
  egress        = [3]
  cidr_block    = "192.168.1.0/24"
  from_port     = -1
  to_port       = -1
}

Drag options to blanks, or click blank then click option'

Aicmp

Bdeny

Ctrue

Dallow

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'allow' instead of 'deny' for blocking

Setting egress to false for outbound rules

Using wrong protocol name

ICMP protocol is specified as "icmp". To block traffic, the action is "deny". Outbound rules have egress set to true.

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