initial attempt

builtin/providers/aws/resource_aws_instance.go

@@ -128,11 +128,20 @@ func resourceAwsInstance() *schema.Resource {
 				Computed: true,
 			},
 
+			// TODO: Deprecate me
 			"network_interface_id": {
 				Type:     schema.TypeString,
 				Computed: true,
 			},
 
+			"primary_network_interface": {
+				ConflictsWith: []string{"associate_public_ip_address", "subnet_id", "private_ip", "vpc_security_group_ids", "security_groups", "ipv6_addresses", "ipv6_address_count"},
+				Type:          schema.TypeString,
+				Optional:      true,
+				ForceNew:      true,
+				Computed:      true,
+			},
+
 			"public_ip": {
 				Type:     schema.TypeString,
 				Computed: true,
@@ -533,7 +542,8 @@ func resourceAwsInstanceRead(d *schema.ResourceData, meta interface{}) error {
 		for _, ni := range instance.NetworkInterfaces {
 			if *ni.Attachment.DeviceIndex == 0 {
 				d.Set("subnet_id", ni.SubnetId)
-				d.Set("network_interface_id", ni.NetworkInterfaceId)
+				d.Set("network_interface_id", ni.NetworkInterfaceId) // TODO: Deprecate me
+				d.Set("primary_network_interface", ni.NetworkInterfaceId)
 				d.Set("associate_public_ip_address", ni.Association != nil)
 				d.Set("ipv6_address_count", len(ni.Ipv6Addresses))
 
@@ -544,7 +554,8 @@ func resourceAwsInstanceRead(d *schema.ResourceData, meta interface{}) error {
 		}
 	} else {
 		d.Set("subnet_id", instance.SubnetId)
-		d.Set("network_interface_id", "")
+		d.Set("network_interface_id", "") // TODO: Deprecate me
+		d.Set("primary_network_interface", "")
 	}
 
 	if err := d.Set("ipv6_addresses", ipv6Addresses); err != nil {
@@ -1260,6 +1271,9 @@ func buildAwsInstanceOpts(
 		}
 	}
 
+	// Check if using non-defaullt primary network interface
+	interface_id, interfaceOk := d.GetOk("primary_network_interface")
+
 	if hasSubnet && associatePublicIPAddress {
 		// If we have a non-default VPC / Subnet specified, we can flag
 		// AssociatePublicIpAddress to get a Public IP assigned. By default these are not provided.
@@ -1285,6 +1299,13 @@ func buildAwsInstanceOpts(
 			}
 		}
 
+		opts.NetworkInterfaces = []*ec2.InstanceNetworkInterfaceSpecification{ni}
+	} else if interfaceOk {
+		ni := &ec2.InstanceNetworkInterfaceSpecification{
+			DeviceIndex:        aws.Int64(int64(0)),
+			NetworkInterfaceId: aws.String(interface_id.(string)),
+		}
+
 		opts.NetworkInterfaces = []*ec2.InstanceNetworkInterfaceSpecification{ni}
 	} else {
 		if subnetID != "" {

builtin/providers/aws/resource_aws_instance_test.go

@@ -877,6 +877,27 @@ func TestAccAWSInstance_changeInstanceType(t *testing.T) {
 	})
 }
 
+func TestAccAWSInstance_primaryNetworkInterface(t *testing.T) {
+	var instance ec2.Instance
+	var ini ec2.NetworkInterface
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckInstanceDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccInstanceConfigPrimaryNetworkInterface,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckInstanceExists("aws_instance.foo", &instance),
+					testAccCheckAWSENIExists("aws_network_interface.bar", &ini),
+					resource.TestCheckResourceAttrSet("aws_instance.foo", "primary_network_interface"),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckInstanceNotRecreated(t *testing.T,
 	before, after *ec2.Instance) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
@@ -1536,3 +1557,35 @@ resource "aws_instance" "foo" {
 	subnet_id = "${aws_subnet.foo.id}"
 }
 `
+
+const testAccInstanceConfigPrimaryNetworkInterface = `
+resource "aws_vpc" "foo" {
+  cidr_block = "172.16.0.0/16"
+  tags {
+    Name = "tf-instance-test"
+  }
+}
+
+resource "aws_subnet" "foo" {
+  vpc_id = "${aws_vpc.foo.id}"
+  cidr_block = "172.16.10.0/24"
+  availability_zone = "us-west-2a"
+  tags {
+    Name = "tf-instance-test"
+  }
+}
+
+resource "aws_network_interface" "bar" {
+  subnet_id = "${aws_subnet.foo.id}"
+  private_ips = ["172.16.10.100"]
+  tags {
+    Name = "primary_network_interface"
+  }
+}
+
+resource "aws_instance" "foo" {
+	ami = "ami-22b9a343"
+	instance_type = "t2.micro"
+	primary_network_interface = "${aws_network_interface.bar.id}"
+}
+`