Nginx - how to restrict outbound traffic

Question

Setup

AWS load balancer forward requests from example.com/(internalIP) to a pool of nginx. Nginx will have to grab that url (example.com/internalIP) and forward the request to that specific "internalIP".

The nginx pool need to have internet access (security tool installed on the nginx ec2 needs it) so I can't restrict the aws security group of the nginx pool to a specific subnet of private ip's.

PROBLEM

Given that setup I need to find an nginx configuration that will only forward to a specific subnet of private ips. This will avoid access to our infrastructure using any private ip