Users Unable to Access NGINX Web Server on Port 80

Answer - B.

Options A and C are incorrect because the Security Group should only allow inbound traffic on port 80.

Option C is incorrect because it should be the outbound traffic for ephemeral ports.

When a connection is established on a client, you need to ensure that outbound traffic is enabled on any ephemeral ports for the client.

This is also given in the AWS Documentation.

Ephemeral Ports.

The example network ACL in the preceding section uses an ephemeral port range of 32768-65535

However, you might want to use a different range for your network ACLs depending on the type of client that you're using or with which you're communicating.

The client that initiates the request chooses the ephemeral port range.

The range varies depending on the client's operating system.

Many Linux kernels (including the Amazon Linux kernel) use ports 32768-61000

Requests originating from Elastic Load Balancing use ports 1024-65535

Windows operating systems through Windows Server 2003 use ports 1025-5000

Windows Server 2008 and later versions use ports 49152-65535

A NAT gateway uses ports 1024-65535

For example, if a request comes into a web server in your VPC from a Windows XP client on the Internet, your network ACL must have an outbound rule to enable traffic destined for ports 1025-5000.

For more information on NACLs, please refer to the below URL.

https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_ACLs.html

The issue described in the question is that users are not able to access an NGINX web server running on port 80 on an EC2 instance. This can happen due to a variety of reasons related to networking and security configurations.

Let's examine each answer option and see if it could be the cause of the issue:

A. The Security Group does not allow outbound traffic on port 80. If the Security Group associated with the EC2 instance does not allow outbound traffic on port 80, then the NGINX web server on the EC2 instance won't be able to respond to requests from clients. However, this is not the most likely cause of the issue, as the Security Group rules usually control inbound traffic to an EC2 instance, not outbound traffic.

B. The NACL doesn't allow outbound traffic on ephemeral ports. Network Access Control Lists (NACLs) are stateless and control inbound and outbound traffic at the subnet level. If the NACLs do not allow outbound traffic on ephemeral ports, which are the randomly allocated ports used by the client to initiate connections to the server, then the clients will not be able to establish a connection to the NGINX server running on port 80. Therefore, this option could be a possible cause of the issue.

C. The NACL doesn't allow inbound traffic on ephemeral ports. If the NACLs do not allow inbound traffic on ephemeral ports, the clients won't be able to receive responses from the NGINX server running on port 80, even if the server is configured to listen on port 80 and the Security Group allows inbound traffic to that port. Therefore, this option could also be a possible cause of the issue.

D. The Security Group does not allow inbound traffic on ephemeral ports. If the Security Group associated with the EC2 instance does not allow inbound traffic on ephemeral ports, clients won't be able to establish a connection to the NGINX server running on port 80, as the server will be unable to respond to requests sent on ephemeral ports. This option is similar to option B, but it specifically mentions the Security Group instead of NACLs.

Therefore, the most likely causes of the issue are options B, C, and D. To troubleshoot the problem, you can check the NACLs and Security Group rules associated with the EC2 instance and ensure that they allow inbound and outbound traffic on ephemeral ports and port 80.