Exam AWS-SysOps topic 1 question 253 discussion

Exam question from Amazon's AWS-SysOps

Question #: 253
Topic #: 1

A user has created a VPC with public and private subnets. The VPC has CIDR 20.0.0.0/16. The private subnet uses CIDR 20.0.1.0/24 and the public subnet uses
CIDR 20.0.0.0/24. The user is planning to host a web server in the public subnet (port 80. and a DB server in the private subnet (port 3306). The user is configuring a security group of the NAT instance. Which of the below mentioned entries is not required for the NAT security group?

A. For Inbound allow Source: 20.0.1.0/24 on port 80
B. For Outbound allow Destination: 0.0.0.0/0 on port 80
C. For Inbound allow Source: 20.0.0.0/24 on port 80
D. For Outbound allow Destination: 0.0.0.0/0 on port 443

Show Suggested Answer

Suggested Answer: C 🗳️
A user can create a subnet with VPC and launch instances inside that subnet. If the user has created a public private subnet to host the web server and DB server respectively, the user should configure that the instances in the private subnet can connect to the internet using the NAT instances. The user should first configure that NAT can receive traffic on ports 80 and 443 from the private subnet. Thus, allow ports 80 and 443 in Inbound for the private subnet 20.0.1.0/24. Now to route this traffic to the internet configure ports 80 and 443 in Outbound with destination 0.0.0.0/0. The NAT should not have an entry for the public subnet CIDR.

by karmaah at Dec. 19, 2019, 11:58 p.m.

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Comments

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karmaah

Highly Voted 2 years, 2 months ago

After some headache, I try to understand based on answers. DB connects to NAT from private subnet(20.0.1.0/24) for Internet connection thru http/https(ie,.80 or 443 ).So that , DB instance should be allowed as inbound in NAT SG. Hence A is required. Now NAT SG connects to internet(0.0.0.0) thru 80/443 which is Outbound.Hence B & D are Required. Remaining option C, which mentioned public subnet. it can be ignored.

upvoted 8 times

shimmy

2 years, 2 months ago

It is because 20.0.0.0/24 is in the public subnet so it does not need to go to the NAT instance. It already has public access.

upvoted 3 times

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albert_kuo

Most Recent 6 months, 1 week ago

Selected Answer: C

ince the NAT instance is not intended to receive inbound traffic from the public subnet, this inbound rule is not necessary for the NAT security group. The primary purpose of the NAT instance is to handle outbound traffic and provide internet connectivity to the resources in the private subnet.

upvoted 1 times

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sergioandreslq

2 years, 1 month ago

C correct: Reason: NAT instance is not used for inbound traffic from internet to reach public subnet, so, It is not required rule: "Inbound allow Source: 20.0.0.0/24 on port 80" because if there is a webserver on port 80, it will use another way for inbound traffic like EIP, ELB, CloudFront, API Gateway. A is required: DB instance in private subnet will send request to NAT instance from 20.0.0.0/24 to port 80 and port 443, so, the NAT instance should be allowed as inbound in NAT SG. B & D are Required: Now NAT SG connects to internet (0.0.0.0) using 80/443 which is Outbound. Outbound allow Destination: 0.0.0.0/0 on port 80 and Outbound allow Destination: 0.0.0.0/0 on port 443

upvoted 2 times

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Kt45

2 years, 2 months ago

This question is a little confusing. I understand that C is the more correct answer but taken within context of the question we also don't need port 443 rules or need to allow traffic to and from on port 80 with the private subnet. It seems like this question is testing for general best practices for NAT instances rather than this specific scenario.

upvoted 1 times

Golddust

2 years, 2 months ago

They asked what is NOT required. I agree with you that 443 should be included but answering the question would be C. Since public IP will route through the Internet Gateway.

upvoted 1 times

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