Lesson 059 · AWS Learning Path

AWS 059: Network ACLs

Anil Jalela · Published Aug 23, 2025 · 7 min read

The problem

A team uses Network ACLs as a label or Console setting without proving the identity, scope, behavior, failure boundary, cost, or operational result. The configuration appears complete but the real requirement remains untested.

Final outcome

The learner will inspect, explain, test, and document Network ACLs. The submission must connect the Console and CLI view to the same AWS control, state what the evidence proves, diagnose one likely failure, and make a requirement-based architecture decision.

Learning outcomes

You will be able to:

1. explain a network acl is a stateless subnet filter;
2. explain lowest rule number wins;
3. explain rules can allow or deny;
4. explain default and custom acl behavior differs;
5. explain one subnet uses one acl;

1. distinguish successful configuration from successful workload behavior;
2. preserve redacted evidence and complete the stated cleanup or retention decision.

Mental model

requirement
   -> identity and permission
   -> account, Region, and resource scope
   -> configuration or request
   -> observable state
   -> workload result
   -> failure evidence
   -> cleanup or controlled retention

Never begin with a create button or a copied command. Start with the result that must be proven and the boundary that must remain protected.

Core facts

How to reason about it

Use security groups for resource relationships and NACLs for coarse subnet guardrails or explicit network denies. When troubleshooting, examine inbound and outbound ACL rules on every traversed subnet, including the return port and any NAT or load-balancer subnet.

Use the following decision table as a starting point, then change the answer when the scenario changes.

Prerequisites, permissions, Region, and cost

Run this lesson as the approved non-root course identity. Begin with aws sts get-caller-identity, confirm the private account record, and set the fixed project Region before any regional query. IAM resources are global within an account, while STS endpoints and the services reached by an identity can be regional.

Use only the read or change actions required for this lesson. An AccessDenied result is evidence to analyse, not permission to switch to root or attach AdministratorAccess. Record the action, resource, principal type, request context, and smallest justified correction.

The listed cost tier is T0 - no resource creation. Free Tier eligibility and credits are account-specific. Before a mutating lab, identify every resource that can charge, estimate its duration, start a timer, and write the reverse cleanup order. A budget reports cost after billing data arrives and is not a real-time stop control.

AWS Management Console method

1. Open VPC Network ACLs and select the ACL associated with the source and destination subnets.
2. Read inbound and outbound rules from the lowest rule number and stop at the first matching entry.
3. Verify both the application port and return ephemeral-port range before changing any rule.

For every step record the service page, selected account and Region, exact object, visible state, and why that state matters. A Console label or green status is not enough unless it is tied to the final workload outcome.

AWS CLI or API evidence

Find the ACL associated with a subnet and read its numbered ingress and egress entries.

aws ec2 describe-network-acls --filters Name=association.subnet-id,Values=subnet-0123456789abcdef0 --query 'NetworkAcls[0].Entries' --output table

Egress true marks outbound rules. Evaluate from the lowest rule number and include the wildcard final deny when no earlier rule matches.

Before running the command, replace every placeholder, explain each option, and decide whether the operation is read-only or mutating. Capture the exit code immediately. Redact account IDs, ARNs, public addresses, request identifiers, and personal data before sharing.

The CLI and Console are clients of AWS APIs. Matching state across them increases confidence, but neither substitutes for data-plane or application verification.

Practical work

Create p04-nacl-plan.md for nw-p04-nacl. Write numbered inbound and outbound rules, including return ephemeral ports for approved outbound HTTPS, and finish with the implicit deny. Simulate one allowed flow and three denied flows in both directions. Do not associate the custom NACL until the rule review proves that the later Session Manager path remains possible.

The evidence package must include:

• non-root principal type, account verified privately, and Region;
• exact intended and observed state;
• one Console observation and the matching CLI or API result;
• one successful result and one denied, failed, or counterexample result;
• what each result does not prove;
• cost state and elapsed lab time;
• cleanup evidence or a named retained-state owner and next lesson.

Verification standard

Use three levels of proof:

1. Control plane: the object or policy exists with the intended configuration.
2. Data plane or behavior: the request, packet, session, storage path, or application does what the requirement states.
3. Operations: monitoring, failure diagnosis, cost, ownership, and cleanup are known.

A control-plane response can precede final readiness. Use waiters or state polling where supported, then test the actual behavior. If a request times out, do not assume it failed. Inspect state and use documented idempotency before retrying a mutation.

Troubleshooting method

Keep the original symptom and timestamp. State one hypothesis, make one reversible change, repeat the original test, and record rollback. Never open a management port to the world, expose credentials, disable TLS verification, format an unknown disk, or add broad permissions as a generic fix.

Architecture and certification decisions

Professional-level questions provide competing valid features. Identify the requirement that decides between them: human or workload identity, same-account or cross-account access, regional or zonal scope, stateful or stateless filtering, durable or ephemeral data, latency, RTO/RPO, cost, or operational ownership.

Explain why the selected option fits and why each plausible alternative fails one stated requirement. Do not rely on feature memorization or reproduce protected certification questions.

Knowledge check

1. Which NACL rule wins, allow 100 or deny 200 for the same packet?

Expected direction: Allow 100 because the lowest matching rule is applied first.

1. Does an allowed inbound TCP request guarantee its response passes?

Expected direction: No. The outbound ACL must allow the response, normally to the client ephemeral port.

1. Can a NACL reference an application security group?

Expected direction: No. Use CIDRs, protocols, and ports.

1. What does a new custom NACL allow by default?

Expected direction: Nothing. Its default wildcard entries deny inbound and outbound traffic.

Cost, cleanup, and retained state

Retain only the state explicitly required by the next P04 lesson and record it in the private resource ledger.

Cleanup evidence includes the final state query, not only a successful delete response. Search related resources, other Regions used by the lab, retained storage, public IPv4 addresses, logging destinations, and service-managed dependencies. Schedule a later billing review because cost data can lag.

Completion gate

Pass when the practical artifact explains the real problem, matches Console and CLI evidence, answers every knowledge check, diagnoses one failure without broadening access, records cost, and proves cleanup or approved retention. The learner must defend one decision orally and name the requirement that would change it.

Official sources

• Network ACLs
• NACL rules