A company introduced a security requirement to inspect all traffic from corporate AWS instances in its VPC for policy violations and to block specific IP addresses. To meet this, the company deployed EC2 instances running approved proxy software in private subnets as transparent proxies, updated subnet route tables to use the proxy instances as the default route, and attached security groups compliant with policy. Despite these changes, traffic from instances in private subnets is not being forwarded to the internet through the proxies. What should a solutions architect do to fix this?

The correct solution is to disable source/destination checks on the EC2 instances running the proxy software. By default, an EC2 instance performs source/destination checks, meaning it must be the source or destination of any traffic it sends or receives. When an instance acts as a proxy or NAT device, it needs to send and receive traffic on behalf of other instances. Disabling this check allows the proxy instance to forward traffic where it is neither the source nor the destination, which is essential for its function. Adding a security group rule to allow all traffic between instances using it and assigning it to every EC2 instance is too broad and doesn't address the routing issue. Changing the VPC DHCP options to set DNS servers to the proxy instances' IPs is incorrect; DNS resolution is separate from traffic forwarding. Attaching an additional ENI to each proxy instance is unnecessary; a single ENI can handle both inbound and outbound traffic if routing is configured correctly and source/destination checks are disabled.