https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html#eni-basics Source/destination checking "You must disable source/destination checks if the instance runs services such as network address translation, routing, or firewalls."
The correct answer is C. Disable the Network Source/Destination check on the security appliance’s elastic network interface.
This answer is correct because disabling the Network Source/Destination check allows the virtual security appliance to route traffic that is not addressed to or from itself. By default, this check is enabled on all EC2 instances, and it prevents them from forwarding traffic that does not match their own IP or MAC addresses.However, for a virtual security appliance that acts as a router or a firewall, this check needs to be disabled, otherwise it will drop the traffic that it is supposed to route12.
The other options are incorrect because:
A. Disabling network ACLs is not a solution, because network ACLs are optional layers of security for the subnets in a VPC.They can be used to allow or deny traffic based on IP addresses and ports, but they donot affect the routing behavior of the virtual security appliance3.
B. Configuring the security appliance’s elastic network interface for promiscuous mode is not a solution, because promiscuous mode is a mode for a network interface that causes it to pass all traffic it receives to the CPU, rather than passing only the frames that it is programmed to receive.Promiscuous mode is normally used for packet sniffing or monitoring, but it does not enable the network interface to route traffic4.
D. Placing the security appliance in the public subnet with the internet gateway is not a solution, because it does not address the routing issue of the virtual security appliance.Thesecurity appliance can be placed in either a public or a private subnet, depending on the network design and security requirements, but it still needs to have the Network Source/Destination check disabled to route traffic properly5.
[References:, 1:Enabling or disablingsource/destination checks - Amazon Elastic Compute Cloud2:Virtual security appliance - Wikipedia3: Network ACLs - Amazon Virtual Private Cloud4:Promiscuous mode - Wikipedia5:NAT instances - Amazon Virtual Private Cloud, , , , , ]
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