ELB: Allow secondary_ips.auto_assigned.per_subnet for modify_load_balancer_attributes

Author: nik-localstack

moto/elbv2/models.py

@@ -582,6 +582,7 @@ class FakeLoadBalancer(CloudFormationModel):
         "routing.http2.enabled",
         "waf.fail_open.enabled",
         "zonal_shift.config.enabled",
+        "secondary_ips.auto_assigned.per_subnet",
     }
 
     def __init__(

tests/test_elbv2/test_elbv2.py

@@ -1452,6 +1452,27 @@ def test_modify_load_balancer_attributes_routing_http_drop_invalid_header_fields
     assert routing_http_drop_invalid_header_fields_enabled["Value"] == "false"
 
 
+@mock_aws
+def test_modify_load_balancer_attributes_secondary_ips_auto_assigned_per_subnet():
+    response, _, _, _, _, client = create_load_balancer("network")
+    arn = response["LoadBalancers"][0]["LoadBalancerArn"]
+
+    client.modify_load_balancer_attributes(
+        LoadBalancerArn=arn,
+        Attributes=[
+            {"Key": "secondary_ips.auto_assigned.per_subnet", "Value": "3"}
+        ],
+    )
+    response = client.describe_load_balancer_attributes(LoadBalancerArn=arn)
+    routing_http_drop_invalid_header_fields_enabled = list(
+        filter(
+            lambda item: item["Key"] == "secondary_ips.auto_assigned.per_subnet",
+            response["Attributes"],
+        )
+    )[0]
+    assert routing_http_drop_invalid_header_fields_enabled["Value"] == "3"
+
+
 @mock_aws
 def test_describe_load_balancer_attributes_default():
     response, _, _, _, _, client = create_load_balancer()