What is causing the issue in this scenario?
There is a network ACL on the private subnet set to deny all outbound traffic.
There is no NAT gateway deployed in the private subnet of the VPC.
The default security group for the VPC blocks all inbound traffic to the EC2 instances.
The default security group for the VPC blocks all outbound traffic from the EC2 instances.
Explanations:
If there is a network ACL on the private subnet that is configured to deny all outbound traffic, instances in that subnet will not be able to initiate connections to the internet, even if they have a route to a NAT gateway. Network ACLs operate at the subnet level and can block traffic regardless of the security group settings.
The NAT gateway is mentioned as being deployed and associated with the public subnet. It is not required to have a NAT gateway in the private subnet itself, as the private subnet uses the NAT gateway in the public subnet for outbound internet access.
The default security group for a VPC allows all outbound traffic by default, and unless specific inbound rules are applied, it does not block traffic. Therefore, this option does not explain why the instances cannot access the internet.
The default security group allows all outbound traffic by default, meaning it would not block outbound traffic from the EC2 instances in the private subnet. Thus, this cannot be the cause of the internet access issue.
I reckon the answer is:
There is a network ACL on the private subnet set to deny all outbound traffic.