ec2: enable primary IPv6 on ENI for EC2 instances

AWS requires that when registering targets by instance ID for an IPv6
target group, the targets must have an assigned primary IPv6 address.

Note: The default subnets managed by CAPA are already set up to assign
IPv6 addresses to newly created ENIs.

Author: tthvo

pkg/cloud/services/ec2/instances.go

@@ -577,14 +577,21 @@ func (s *Service) runInstance(role string, i *infrav1.Instance) (*infrav1.Instan
 
 		input.NetworkInterfaces = netInterfaces
 	} else {
-		input.NetworkInterfaces = []types.InstanceNetworkInterfaceSpecification{
-			{
-				DeviceIndex:              aws.Int32(0),
-				SubnetId:                 aws.String(i.SubnetID),
-				Groups:                   i.SecurityGroupIDs,
-				AssociatePublicIpAddress: i.PublicIPOnLaunch,
-			},
+		netInterface := types.InstanceNetworkInterfaceSpecification{
+			DeviceIndex:              aws.Int32(0),
+			SubnetId:                 aws.String(i.SubnetID),
+			Groups:                   i.SecurityGroupIDs,
+			AssociatePublicIpAddress: i.PublicIPOnLaunch,
 		}
+
+		// When registering targets by instance ID for an IPv6 target group,
+		// the targets must have an assigned primary IPv6 address.
+		if s.scope.VPC().IsIPv6Enabled() {
+			netInterface.Ipv6AddressCount = aws.Int32(1)
+			netInterface.PrimaryIpv6 = aws.Bool(true)
+		}
+
+		input.NetworkInterfaces = []types.InstanceNetworkInterfaceSpecification{netInterface}
 	}
 
 	if i.NetworkInterfaceType != "" {