Allow Health Checks for GCP Network Load Balancers

Google Cloud provides health checks to determine if backends respond to traffic.

See https://cloud.google.com/load-balancing/docs/health-checks to create firewall rules for network load balancers. Then in the management center, create access rules to allow the health check traffic. See https://cloud.google.com/load-balancing/docs/health-check-concepts for the required network ranges. See Access Control Rules.

You also need to configure dynamic manual NAT rules to redirect the health check traffic to the Google metadata server at 169.254.169.254. See Configure Dynamic Manual NAT.

nat (inside,outside) source dynamic GCP-HC ILB-SOUTH destination static ILB-SOUTH METADATA
nat (outside,outside) source dynamic GCP-HC ELB-NORTH destination static ELB-NORTH METADATA

nat (outside,inside) source static any interface destination static ELB-NORTH Ubuntu-App-VM
nat (inside,outside) source dynamic any interface destination static obj-any obj-any

object network Metadata
 host 169.254.169.254

object network ILB-SOUTH
host <ILB_IP>
object network ELB-NORTH
host <ELB_IP>

object-group network GCP-HC
 network-object 35.191.0.0 255.255.0.0
 network-object 130.211.0.0 255.255.252.0
 network-object 209.85.204.0 255.255.252.0
 network-object 209.85.152.0 255.255.252.0

nat (inside,outside) source dynamic GCP-HC ILB-East destination static ILB-East Metadata
nat (outside,outside) source dynamic GCP-HC ILB-West destination static ILB-West Metadata

object network Metadata
 host 169.254.169.254

object network ILB-East
host <ILB_East_IP>
object network ILB-West
host <ILB_West_IP>

object-group network GCP-HC
 network-object 35.191.0.0 255.255.0.0
 network-object 130.211.0.0 255.255.252.0
 network-object 209.85.204.0 255.255.252.0
 network-object 209.85.152.0 255.255.252.0