Enabling Internet Access for an EC2 Application Server

Correct Answer: A.

The AWS Documentation mentions the following.

You can use a network address translation (NAT) gateway to enable instances in a private subnet to connect to the internet or other AWS services.

But you prevent the internet from initiating a connection with those instances.

All other options are implementation schemes for allowing access to the Internet via a public subnet.

And here we have a private subnet.

For more information on the NAT gateway, please visit the below URL-

https://docs.aws.amazon.com/vpc/latest/userguide/vpc-nat-gateway.html

To allow an application server running on an Amazon EC2 instance in a private subnet in a VPC to connect to the internet and download software updates, you need to create a network address translation (NAT) gateway and update the route table accordingly. Option A is the correct answer.

Explanation:

A private subnet is a subnet that does not have direct access to the internet. An EC2 instance in a private subnet can access the internet only if there is a way to route the traffic from the instance to the internet. In this case, you need to create a NAT gateway to route the traffic.

A NAT gateway is a managed service that allows instances in a private subnet to connect to the internet or other AWS services, but prevents the internet from initiating connections with the instances. It translates the source IP address of the instances in the private subnet to its own IP address, and then forwards the traffic to the internet. When the response returns, the NAT gateway translates the destination IP address to the instance's private IP address and forwards the response back to the instance.

To create a NAT gateway, you need to do the following:

1. Create an Elastic IP address: A NAT gateway requires an Elastic IP address to associate with it. An Elastic IP address is a static, public IP address that you can allocate to your AWS account and associate with your NAT gateway.

2. Create a NAT gateway: After you have an Elastic IP address, you can create a NAT gateway in the same Availability Zone as your private subnet.

3. Update the route table: After you have created the NAT gateway, you need to update the route table for your private subnet to route traffic to the NAT gateway. To do this, you need to add a route to the route table that specifies the destination as 0.0.0.0/0 (which represents all IP addresses) and the target as the NAT gateway.

Option B is incorrect because assigning an Elastic IP address to the EC2 instance does not provide a route to the internet. An Elastic IP address provides a static, public IP address for an EC2 instance, but it does not allow the instance to connect to the internet on its own.

Option C is incorrect because updating the security group for the EC2 instance to allow Port 80 from 0.0.0.0/0 allows incoming traffic to the instance, but does not allow outgoing traffic to the internet. Port 80 is typically used for HTTP traffic, which is incoming traffic to a web server.

Option D is incorrect because updating the rule for the inbound network access control list for the subnet containing the EC2 instance controls incoming traffic to the subnet, but does not allow outgoing traffic to the internet. A network access control list (ACL) is an optional layer of security for your VPC that acts as a firewall for controlling traffic in and out of one or more subnets.