AWS VPC Security – Security Group vs NACLs

~ Last updated on : ~ jayendrapatil
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AWS VPC Security Group vs NACLs
Security Groups
Network Access Control Lists – NACLs
Security Group vs NACLs
AWS Certification Exam Practice Questions

AWS VPC Security Group vs NACLs

  • In a VPC, both Security Groups and Network ACLs (NACLS) together help to build a layered network defence.
  • Security groups – Act as a virtual firewall for associated instances, controlling both inbound and outbound traffic at the instance level
  • Network access control lists (NACLs) – Act as a firewall for associated subnets, controlling both inbound and outbound traffic at the subnet level

Security Groups vs NACLs

Security Groups

  • Acts at an Instance level and not at the subnet level.
  • Each instance within a subnet can be assigned a different set of Security groups
  • An instance can be assigned 5 security groups with each security group having 50 60 rules.
  • allows separate rules for inbound and outbound traffic.
  • allows adding or removing rules (authorizing or revoking access) for both Inbound (ingress) and Outbound (egress) traffic to the instance
    • Default Security group allows no external inbound traffic but allows inbound traffic from instances with the same security group
    • Default Security group allows all outbound traffic
    • New Security groups start with only an outbound rule that allows all traffic to leave the instances.
  • can specify only Allow rules, but not deny rules
  • can grant access to a specific IP, CIDR range, or to another security group in the VPC or in a peer VPC (requires a VPC peering connection)
  • are evaluated as a Whole or Cumulative bunch of rules with the most permissive rule taking precedence for e.g. if you have a rule that allows access to TCP port 22 (SSH) from IP address 203.0.113.1 and another rule that allows access to TCP port 22 from everyone, everyone has access to TCP port 22.
  • are Stateful – responses to allowed inbound traffic are allowed to flow outbound regardless of outbound rules, and vice versa. Hence an Outbound rule for the response is not needed
  • Instances associated with a security group can’t talk to each other unless rules allowing the traffic are added.
  • are associated with ENI (network interfaces).
  • are associated with the instance and can be changed, which changes the security groups associated with the primary network interface (eth0) and the changes would be applicable immediately to all the instances associated with the Security Group.

Connection Tracking

  • Security groups are Stateful as they use Connection tracking to track information about traffic to and from the instance.
  • Responses to inbound traffic are allowed to flow out of the instance regardless of outbound security group rules, and vice versa.
  • Connection Tracking is maintained only if there is no explicit Outbound rule for an Inbound request (and vice versa)
  • However, if there is an explicit Outbound rule for an Inbound request, the response traffic is allowed on the basis of the Outbound rule and not on the Tracking information
  • Tracking flow e.g.
    • If an instance (host A) initiates traffic to host B and uses a protocol other than TCP, UDP, or ICMP, the instance’s firewall only tracks the IP address & protocol number for the purpose of allowing response traffic from host B.
    • If host B initiates traffic to the instance in a separate request within 600 seconds of the original request or response, the instance accepts it regardless of inbound security group rules, because it’s regarded as response traffic.
  • This can be controlled by modifying the security group’s outbound rules to permit only certain types of outbound traffic. Alternatively, Network ACLs (NACLs) can be used for the subnet, network ACLs are stateless and therefore do not automatically allow response traffic.

Network Access Control Lists – NACLs

  • A Network ACLs (NACLs) is an optional layer of security for the VPC that acts as a firewall for controlling traffic in and out of one or more subnets.
  • are not for granular control and are assigned at a Subnet level and are applicable to all the instances in that Subnet
  • has separate inbound and outbound rules, and each rule can either allow or deny traffic
    • Default ACL allows all inbound and outbound traffic.
    • The newly created ACL denies all inbound and outbound traffic.
  • A Subnet can be assigned only 1 NACL and if not associated explicitly would be associated implicitly with the default NACL
  • can associate a network ACL with multiple subnets
  • is a numbered list of rules that are evaluated in order starting with the lowest numbered rule, to determine whether traffic is allowed in or out of any subnet associated with the network ACL e.g. if you have a Rule No. 100 with Allow All and 110 with Deny All, the Allow All would take precedence and all the traffic will be allowed.
  • are Stateless; responses to allowed inbound traffic are subject to the rules for outbound traffic (and vice versa) for e.g. if you enable Inbound SSH on port 22 from the specific IP address, you would need to add an Outbound rule for the response as well.

Security Group vs NACLs

Security Groups vs NACLs

AWS Certification Exam Practice Questions

  • Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).
  • AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.
  • AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated
  • Open to further feedback, discussion and correction.
  1. Instance A and instance B are running in two different subnets A and B of a VPC. Instance A is not able to ping instance B. What are two possible reasons for this? (Pick 2 correct answers)
    1. The routing table of subnet A has no target route to subnet B
    2. The security group attached to instance B does not allow inbound ICMP traffic
    3. The policy linked to the IAM role on instance A is not configured correctly
    4. The NACL on subnet B does not allow outbound ICMP traffic
  2. An instance is launched into a VPC subnet with the network ACL configured to allow all inbound traffic and deny all outbound traffic. The instance’s security group is configured to allow SSH from any IP address and deny all outbound traffic. What changes need to be made to allow SSH access to the instance?
    1. The outbound security group needs to be modified to allow outbound traffic.
    2. The outbound network ACL needs to be modified to allow outbound traffic.
    3. Nothing, it can be accessed from any IP address using SSH.
    4. Both the outbound security group and outbound network ACL need to be modified to allow outbound traffic.
  3. From what services I can block incoming/outgoing IPs?
    1. Security Groups
    2. DNS
    3. ELB
    4. VPC subnet
    5. IGW
    6. NACL
  4. What is the difference between a security group in VPC and a network ACL in VPC (chose 3 correct answers)
    1. Security group restricts access to a Subnet while ACL restricts traffic to EC2
    2. Security group restricts access to EC2 while ACL restricts traffic to a subnet
    3. Security group can work outside the VPC also while ACL only works within a VPC
    4. Network ACL performs stateless filtering and Security group provides stateful filtering
    5. Security group can only set Allow rule, while ACL can set Deny rule also
  5. You are currently hosting multiple applications in a VPC and have logged numerous port scans coming in from a specific IP address block. Your security team has requested that all access from the offending IP address block be denied for the next 24 hours. Which of the following is the best method to quickly and temporarily deny access from the specified IP address block?
    1. Create an AD policy to modify Windows Firewall settings on all hosts in the VPC to deny access from the IP address block
    2. Modify the Network ACLs associated with all public subnets in the VPC to deny access from the IP address block
    3. Add a rule to all of the VPC 5 Security Groups to deny access from the IP address block
    4. Modify the Windows Firewall settings on all Amazon Machine Images (AMIs) that your organization uses in that VPC to deny access from the IP address block
  6. You have two Elastic Compute Cloud (EC2) instances inside a Virtual Private Cloud (VPC) in the same Availability Zone (AZ) but in different subnets. One instance is running a database and the other instance an application that will interface with the database. You want to confirm that they can talk to each other for your application to work properly. Which two things do we need to confirm in the VPC settings so that these EC2 instances can communicate inside the VPC? Choose 2 answers
    1. A network ACL that allows communication between the two subnets.
    2. Both instances are the same instance class and using the same Key-pair.
    3. That the default route is set to a NAT instance or Internet Gateway (IGW) for them to communicate.
    4. Security groups are set to allow the application host to talk to the database on the right port/protocol
  7. A benefits enrollment company is hosting a 3-tier web application running in a VPC on AWS, which includes a NAT (Network Address Translation) instance in the public Web tier. There is enough provisioned capacity for the expected workload tor the new fiscal year benefit enrollment period plus some extra overhead Enrollment proceeds nicely for two days and then the web tier becomes unresponsive, upon investigation using CloudWatch and other monitoring tools it is discovered that there is an extremely large and unanticipated amount of inbound traffic coming from a set of 15 specific IP addresses over port 80 from a country where the benefits company has no customers. The web tier instances are so overloaded that benefit enrollment administrators cannot even SSH into them. Which activity would be useful in defending against this attack?
    1. Create a custom route table associated with the web tier and block the attacking IP addresses from the IGW (internet Gateway)
    2. Change the EIP (Elastic IP Address) of the NAT instance in the web tier subnet and update the Main Route Table with the new EIP
    3. Create 15 Security Group rules to block the attacking IP addresses over port 80
    4. Create an inbound NACL (Network Access control list) associated with the web tier subnet with deny rules to block the attacking IP addresses
  8. Which of the following statements describes network ACLs? (Choose 2 answers)
    1. Responses to allowed inbound traffic are allowed to flow outbound regardless of outbound rules, and vice versa (are stateless)
    2. Using network ACLs, you can deny access from a specific IP range
    3. Keep network ACL rules simple and use a security group to restrict application level access
    4. NACLs are associated with a single Availability Zone (associated with Subnet)
  9. You are designing security inside your VPC. You are considering the options for establishing separate security zones and enforcing network traffic rules across different zone to limit Instances can communications.  How would you accomplish these requirements? Choose 2 answers
    1. Configure a security group for every zone. Configure a default allow all rule. Configure explicit deny rules for the zones that shouldn’t be able to communicate with one another (Security group does not allow deny rules)
    2. Configure you instances to use pre-set IP addresses with an IP address range every security zone. Configure NACL to explicitly allow or deny communication between the different IP address ranges, as required for interzone communication
    3. Configure a security group for every zone. Configure allow rules only between zone that need to be able to communicate with one another. Use implicit deny all rule to block any other traffic
    4. Configure multiple subnets in your VPC, one for each zone. Configure routing within your VPC in such a way that each subnet only has routes to other subnets with which it needs to communicate, and doesn’t have routes to subnets with which it shouldn’t be able to communicate. (default routes are unmodifiable)
  10. Your entire AWS infrastructure lives inside of one Amazon VPC. You have an Infrastructure monitoring application running on an Amazon instance in Availability Zone (AZ) A of the region, and another application instance running in AZ B. The monitoring application needs to make use of ICMP ping to confirm network reachability of the instance hosting the application. Can you configure the security groups for these instances to only allow the ICMP ping to pass from the monitoring instance to the application instance and nothing else” If so how?
    1. No Two instances in two different AZ’s can’t talk directly to each other via ICMP ping as that protocol is not allowed across subnet (i.e. broadcast) boundaries (Can communicate)
    2. Yes Both the monitoring instance and the application instance have to be a part of the same security group, and that security group needs to allow inbound ICMP (Need not have to be part of same security group)
    3. Yes, The security group for the monitoring instance needs to allow outbound ICMP and the application instance’s security group needs to allow Inbound ICMP (is stateful, so just allow outbound ICMP from monitoring and inbound ICMP on monitored instance)
    4. Yes, Both the monitoring instance’s security group and the application instance’s security group need to allow both inbound and outbound ICMP ping packets since ICMP is not a connection-oriented protocol (Security groups are stateful)
  11. A user has configured a VPC with a new subnet. The user has created a security group. The user wants to configure that instances of the same subnet communicate with each other. How can the user configure this with the security group?
    1. There is no need for a security group modification as all the instances can communicate with each other inside the same subnet
    2. Configure the subnet as the source in the security group and allow traffic on all the protocols and ports
    3. Configure the security group itself as the source and allow traffic on all the protocols and ports
    4. The user has to use VPC peering to configure this
  12. You are designing a data leak prevention solution for your VPC environment. You want your VPC Instances to be able to access software depots and distributions on the Internet for product updates. The depots and distributions are accessible via third party CDNs by their URLs. You want to explicitly deny any other outbound connections from your VPC instances to hosts on the Internet. Which of the following options would you consider?
    1. Configure a web proxy server in your VPC and enforce URL-based rules for outbound access Remove default routes. (Security group and NACL cannot have URLs in the rules nor does the route)
    2. Implement security groups and configure outbound rules to only permit traffic to software depots.
    3. Move all your instances into private VPC subnets remove default routes from all routing tables and add specific routes to the software depots and distributions only.
    4. Implement network access control lists to all specific destinations, with an Implicit deny as a rule.
  13. You have an EC2 Security Group with several running EC2 instances. You change the Security Group rules to allow inbound traffic on a new port and protocol, and launch several new instances in the same Security Group. The new rules apply:
    1. Immediately to all instances in the security group.
    2. Immediately to the new instances only.
    3. Immediately to the new instances, but old instances must be stopped and restarted before the new rules apply.
    4. To all instances, but it may take several minutes for old instances to see the changes.

References

45 thoughts on “AWS VPC Security – Security Group vs NACLs”

  1. Hello, I am not sure about the answer to question 11.

    I think both B and C will work but B is more accurate.

    C is used in the default security group.(Default Security Group allow inbound traffic from instances assigned to the same security group.) Instances associated with a security group can’t talk to each other unless you add rules allowing it (exception: the default security group has these rules by default).

    B is allow instances from same subnet communicate with each other.

    Your thoughts?

    1. Only CIDR block and security group can be defined as the source in the rules, so the subnet would not work

        1. Ok, I guess, the question does not state that the security group was configured to allow any access. By default I know it denies all traffic.

          1. thats right, By default, new security groups start with only an outbound rule that allows all traffic to leave the instances. You must add rules to enable any inbound traffic or to restrict the outbound traffic.

          2. I found this on https://medium.com/@mda590/aws-routing-101-67879d23014d
            “Routing between subnets inside of a local VPC
            By default, each VPC comes with 1 route table pre-configured with a “local” route. The scope of the “local” route is only within the subnet defined for the entire VPC. For example, if your VPC was set up to have the address space of 172.16.0.0/16, your “local” route would be defined as “172.16.0.0/16”. This allows all of the resources created within the VPC to talk to each other without any additional configuration. You cannot delete the “local” route from your route table, and anytime a new route table is created within a VPC, the “local” route is included by default.”

            This confused me more. Is this author correct?

        1. The route table allows communication within the subnets without any IGW or VGW, thats what local provides.
          However, this doesn’t mean the instances are free to interact with each other as well. The communication within the instances are guided through NACLs at subnet level and Security Groups at Instance level.

          1. Jayendra, but in security groups you can not create implicit deny rules (question 9 about restrict subnets)

  3. hey update in ec2-classic ” you can have up to 500 security groups in each region for each account.
    You can associate an instance with up to 500 security groups a
    nd
    add up to 100 rules to a security group.”

  4. For question number 9, could you point to any reference that says default route is unmodifiable .

    Thanks
    Naseer

    1. Refer VPC Route Tables

      Every route table contains a local route for communication within the VPC over IPv4. If you've associated an IPv6 CIDR block with your VPC, every route table also contains a local route for communication within the VPC over IPv6. You cannot modify or delete these routes.

    1. Security groups are stateful and so outbound even if deny would allow outbound from an incoming request. ACLs are stateless and need to allow outbound for every incoming request.

      1. To access the instance we need to consider only inbound traffic which can be accessible here then why we are caring about outbound traffic? And also another question sir if security group allows a particular traffic and acl denies same traffic then the taffic is allowed or denied to an instance?

        1. Outbound needs to be considered for ACL, as it is stateless and the traffic should go back through as well as a response.
          You don’t need to consider outbound response for inbound request with Security Groups as they are Stateful

  6. Regarding Q2, I think C is the right answer. since to connect to the instance using SSH , the port is open to anywhere in inbound and NACL also allows all inbound traffic.
    I think Outbound traffic doesn’t play any role in taking SSH. Please explain Sir.

    1. I have checked by Denying all outbound traffic in NACL and i was unable to take SSH. After allowing all outbound traffic for NACL and with no outbound rules in security groups, i could take SSH.

      Your answers are very reliable. Great work. Thanks a lot 🙂

  7. Dear Jayendrapatil,
    “Security groups are evaluated as a Whole or Cumulative bunch of rules for e.g. if there is an Allow Rule for SSH from all IP addresses and an Allow Rule for SSH from a specific IP address the specific IP address would take precedence”

    Shouldn’t it be the ‘more permissive rule would take precedence’.

  8. You are designing a data leak prevention solution for your VPC environment. You want your VPC Instances to be able to access software depots and distributions on the Internet for product updates. The depots and distributions are accessible via third party CDNs by their URLs. You want to explicitly deny any other outbound connections from your VPC instances to hosts on the Internet. Which of the following options would you consider?
    a. Configure a web proxy server in your VPC and enforce URL-based rules for outbound access Remove default routes. (Security group and NACL cannot have URLs in the rules nor does the route)
    b. Implement security groups and configure outbound rules to only permit traffic to software depots.
    c. Move all your instances into private VPC subnets remove default routes from all routing tables and add specific routes to the software depots and distributions only.
    d.Implement network access control lists to all specific destinations, with an Implicit deny as a rule.

    Please explain why the answer is a.
    Is the question “Which of the following options would NOT you consider?”
    Because we cant enforce URL-based rules

    1. As you can enforce url based rules in Routing tables, Security Groups and NACLs you need a custom solution to deny the outbound connections which can done using a web proxy.

  9. As you CAN’T enforce url based rules in Routing tables, Security Groups and NACLs you need a custom solution to deny the outbound connections which can done using a web proxy.
    Thanks for the explanation.

  10. Jayendra…your site and question bank is great…can u do a favour to everyone….can u not highlight the answers to every q instead at the end of q u can give the list of answers e.g.1)a 2)b 3)c etc.

    1. Pretty much agreed Anil, and i have had multiple requests for the same. I am trying to figure out a way but the backlog is huge. Expect some changes soon.

    1. Security group allows either security groups or ips to be mentioned as source. Subnet would not work.

  13. Hi. on Q8 (“Which of the following statements describes network ACLs? (Choose 2 answers)”),

    Why answer D was not chosen? (“NACLs are associated with a single Availability Zone (associated with Subnet)”)

    Since one NACL is associated with one Subnet. And one subnet is associated with one AZ. Although one AZ can have many subnets. Is it correct?
    Thanks!

    1. However a Single AZ can have multiple Subnets and each of them can have different NACLs attached to them.

  14. Hi Sir, I don’t understand why Question 2 is b instead of c. The description states all inbound traffics are allowed in NACL level which means all internet can access this subnet, and then it states on the Security Group level, any ip can SSH into the instances behind it but thoese instances can’t access to outside of their subnet. According to the question, outside world can already SSH the instance in this subnet but the instances can’t connect to outside. So why the answer is b?

    1. NACLs are stateless while Security groups are Stateful.
      Security groups allows outgoing traffic for any incoming traffic i.e. request needs to be allowed however response need not be.
      NACLs groups do not remember the traffic. Hence you need to enable traffic for both the request and response and hence option B.

  15. Hi. My understanding is that in question 9 answer c is wrong and d is correct. In d, nothing is written about that the default route has to be used. And c says “Configure a security group for every zone” but security groups can’t be created in/for a zone.

    Correct me if I’m wrong.

  16. Hello,

    I think C is the correct option in the question 2, because security groups is set us as a deny all outbound traffic so how it works if we modify allow outbound at ACL level?

    Thanks
    Malav

    1. Security groups are stateful and do not need rules for response traffic. However, ACLs are stateless and need both request and response traffic to be enabled.

  17. An organization has tens of applications deployed on thousands of Amazon EC2 instances. During testing, the Application team needs information to let them know whether the network access control lists (network ACLs) and security groups are working as expected.
    How can the Application team’s requirements be met?

    A. Turn on VPC Flow Logs, send the logs to Amazon S3, and use Amazon Athena to query the logs.
    B. Install an Amazon Inspector agent on each EC2 instance, send the logs to Amazon S3, and use Amazon EMR to query the logs.b
    C. Create an AWS Config rule for each network ACL and security group configuration, send the logs to Amazon S3, and use Amazon Athena to query the logs.
    D. Turn on AWS CloudTrail, send the trails to Amazon S3, and use AWS Lambda to query the trails.

    Would it be fair to say A ?

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