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Prevent route replacement overwrites in ASG Lambda #169

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44 changes: 31 additions & 13 deletions functions/replace-route/app.py
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@
LIFECYCLE_HOOK_NAME_KEY = "LifecycleHookName"
AUTO_SCALING_GROUP_NAME_KEY = "AutoScalingGroupName"
LIFECYCLE_ACTION_TOKEN_KEY = "LifecycleActionToken"
EC2_INSTANCE_ID_KEY = "EC2InstanceId"

# Checks every CONNECTIVITY_CHECK_INTERVAL seconds, exits after 1 minute
DEFAULT_CONNECTIVITY_CHECK_INTERVAL = "5"
Expand Down Expand Up @@ -209,15 +210,25 @@ def is_source_dest_check_enabled(instance_id):
logger.error(f"Error checking source/dest check: {e}")
return None


def find_default_routes(route_table_ids):
routes = [None for _ in route_table_ids]
response = ec2_client.describe_route_tables(RouteTableIds=route_table_ids)
for route_table in response["RouteTables"]:
for route in route_table["Routes"]:
if route.get("DestinationCidrBlock") == "0.0.0.0/0":
idx = route_table_ids.index(route_table["RouteTableId"])
routes[idx] = route
break
return routes


def are_any_routes_pointing_to_nat_gateway(route_table_ids):
ec2 = boto3.client('ec2')
try:
response = ec2.describe_route_tables(RouteTableIds=route_table_ids)
for rtb in response.get('RouteTables', []):
for route in rtb.get('Routes', []):
if route.get('DestinationCidrBlock') == "0.0.0.0/0" and 'NatGatewayId' in route and route.get('State') == 'active':
return True
return False
default_routes = find_default_routes(route_table_ids)
return any(
route is not None and "NatGatewayId" in route for route in default_routes
)
except Exception as e:
logger.error(f"Error checking NAT Gateway routes: {e}")
return False
Expand Down Expand Up @@ -414,10 +425,12 @@ def handler(event, _):
if (
LIFECYCLE_HOOK_NAME_KEY in message
and AUTO_SCALING_GROUP_NAME_KEY in message
and EC2_INSTANCE_ID_KEY in message
):
asg = message[AUTO_SCALING_GROUP_NAME_KEY]
lifecycle_hook_name = message[LIFECYCLE_HOOK_NAME_KEY]
lifecycle_action_token = message[LIFECYCLE_ACTION_TOKEN_KEY]
instance_id = message[EC2_INSTANCE_ID_KEY]
else:
logger.error("Failed to find lifecycle message to parse")
raise LifecycleMessageError
Expand All @@ -428,16 +441,21 @@ def handler(event, _):
availability_zone, vpc_zone_identifier = get_az_and_vpc_zone_identifier(asg)
public_subnet_id = vpc_zone_identifier.split(",")[0]
az = availability_zone.upper().replace("-", "_")
route_tables = az in os.environ and os.getenv(az).split(",")
if not route_tables:
route_table_ids = az in os.environ and os.getenv(az).split(",")
if not route_table_ids:
raise MissingEnvironmentVariableError
vpc_id = get_vpc_id(route_tables[0])

vpc_id = get_vpc_id(route_table_ids[0])

nat_gateway_id = get_nat_gateway_id(vpc_id, public_subnet_id)

for rtb in route_tables:
replace_route(rtb, nat_gateway_id)
logger.info("Route replacement succeeded")
default_routes = find_default_routes(route_table_ids)
for route_table_id, route in zip(route_table_ids, default_routes):
if route is not None and route.get("InstanceId") == instance_id:
replace_route(route_table_id, nat_gateway_id)
logger.info("Route replacement succeeded")
else:
logger.info("Skipping route replacement in table %s", route_table_id)

complete_asg_lifecycle_action(
asg, lifecycle_hook_name, lifecycle_action_token, "CONTINUE"
Expand Down
64 changes: 34 additions & 30 deletions functions/replace-route/tests/test_replace_route.py
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Original file line number Diff line number Diff line change
Expand Up @@ -39,20 +39,17 @@ def setup_networking():
CidrBlock="10.1.1.0/24",
AvailabilityZone=f"{az}"
)

private_subnet = ec2.create_subnet(
VpcId=vpc.id,
CidrBlock="10.1.2.0/24",
AvailabilityZone=f"{az}",
)

private_subnet_two = ec2.create_subnet(
VpcId=vpc.id,
CidrBlock="10.1.3.0/24",
AvailabilityZone=f"{az}",
)


route_table = ec2.create_route_table(VpcId=vpc.id)
route_table_two = ec2.create_route_table(VpcId=vpc.id)
sg = ec2.create_security_group(GroupName="test-sg", Description="test-sg")
Expand All @@ -64,16 +61,33 @@ def setup_networking():
AllocationId=allocation_id
)["NatGateway"]["NatGatewayId"]

eni = ec2_client.create_network_interface(
SubnetId=public_subnet.id, PrivateIpAddress="10.1.1.5"
launch_template = ec2_client.create_launch_template(
LaunchTemplateName="test_launch_template",
LaunchTemplateData={"ImageId": EXAMPLE_AMI_ID, "InstanceType": "t2.micro"},
)["LaunchTemplate"]

autoscaling_client = boto3.client("autoscaling")
autoscaling_client.create_auto_scaling_group(
AutoScalingGroupName="alternat-asg",
VPCZoneIdentifier=public_subnet.id,
MinSize=1,
MaxSize=1,
LaunchTemplate={
"LaunchTemplateId": launch_template["LaunchTemplateId"],
"Version": str(launch_template["LatestVersionNumber"]),
},
)

reservations = ec2_client.describe_instances()["Reservations"]
instance_id = reservations[0]["Instances"][0]["InstanceId"]

ec2_client.associate_route_table(
RouteTableId=route_table.id,
SubnetId=private_subnet.id
)
ec2_client.create_route(
DestinationCidrBlock="0.0.0.0/0",
NetworkInterfaceId=eni["NetworkInterface"]["NetworkInterfaceId"],
InstanceId=instance_id,
RouteTableId=route_table.id
)
ec2_client.associate_route_table(
Expand All @@ -82,19 +96,16 @@ def setup_networking():
)
ec2_client.create_route(
DestinationCidrBlock="0.0.0.0/0",
NetworkInterfaceId=eni["NetworkInterface"]["NetworkInterfaceId"],
InstanceId=instance_id,
RouteTableId=route_table_two.id
)

return {
"vpc": vpc.id,
"public_subnet": public_subnet.id,
"private_subnet": private_subnet.id,
"private_subnet_two": private_subnet_two.id,
"nat_gw": nat_gw_id,
"route_table": route_table.id,
"route_table_two": route_table_two.id,
"sg": sg.id,
"instance": instance_id,
}


Expand Down Expand Up @@ -135,29 +146,12 @@ def verify_nat_instance_route(mocked_networking, instance_id):
@mock_aws
def test_handler(monkeypatch):
mocked_networking = setup_networking()
ec2_client = boto3.client("ec2")
template = ec2_client.create_launch_template(
LaunchTemplateName="test_launch_template",
LaunchTemplateData={"ImageId": EXAMPLE_AMI_ID, "InstanceType": "t2.micro"},
)["LaunchTemplate"]

autoscaling_client = boto3.client("autoscaling")
autoscaling_client.create_auto_scaling_group(
AutoScalingGroupName="alternat-asg",
VPCZoneIdentifier=mocked_networking["public_subnet"],
MinSize=1,
MaxSize=1,
LaunchTemplate={
"LaunchTemplateId": template["LaunchTemplateId"],
"Version": str(template["LatestVersionNumber"]),
},
)

from app import handler

script_dir = os.path.dirname(__file__)
with open(os.path.join(script_dir, "../sns-event.json"), "r") as file:
asg_termination_event = file.read()
asg_termination_event = json.loads(file.read())

az = f"{os.environ['AWS_DEFAULT_REGION']}a".upper().replace("-", "_")
monkeypatch.setenv(az, ",".join([mocked_networking["route_table"],mocked_networking["route_table_two"]]))
Expand All @@ -169,10 +163,20 @@ def mock_make_api_call(self, operation_name, kwarg):
if operation_name == "CompleteLifecycleAction":
return mock_complete_lifecycle_action(self, operation_name, kwarg)
return orig_make_api_call(self, operation_name, kwarg)

with mock.patch("botocore.client.BaseClient._make_api_call", new=mock_make_api_call):
handler(json.loads(asg_termination_event), {})
handler(asg_termination_event, {})
mock_complete_lifecycle_action.assert_called_once()
verify_nat_instance_route(mocked_networking, mocked_networking["instance"])

sns_message = json.loads(asg_termination_event["Records"][0]["Sns"]["Message"])
sns_message["EC2InstanceId"] = mocked_networking["instance"]
asg_termination_event["Records"][0]["Sns"]["Message"] = json.dumps(sns_message)

mock_complete_lifecycle_action.reset_mock()
with mock.patch("botocore.client.BaseClient._make_api_call", new=mock_make_api_call):
handler(asg_termination_event, {})
mock_complete_lifecycle_action.assert_called_once()
verify_nat_gateway_route(mocked_networking)


Expand Down