IoT policy to restrict MQTT publish/subscribe by Thing Name

Question

Good morning.
I want to apply a MQTT policy that grants the IoT thing access to certain topic. For example, assuming that the thing name is 0R281, I would like a policy that allows the thing to publish only in a topic with the following name xgate/0R281/data

I created a policy structured like this

```
{
	"Version": "2012-10-17",
	"Statement": [
		{
			"Effect": "Allow",
			"Action": "iot:Connect",
			"Resource": "arn:aws:iot:eu-west-1::client/${iot:Connection.Thing.ThingName}"
		},
		{
			"Effect": "Allow",
			"Action": "iot:Subscribe",
			"Resource": "arn:aws:iot:eu-west-1::topicfilter/xgate/${iot:Connection.Thing.ThingName}/data"
		},
		{
			"Effect": "Allow",
			"Action": "iot:Publish",
			"Resource": "arn:aws:iot:eu-west-1::topic/xgate/${iot:Connection.Thing.ThingName}/data"
		}
	]
}

```

but it does not work. 
Trying with a policy made like this obviously works,

```
{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": "iot:", "Resource": "" } ] }
```

I don't understand where the mistake is

Greg_B · Accepted Answer

Hi. What MQTT client ID is your device using? For the `iot:Connection.Thing.ThingName` policy variable to work, the client ID must match the Thing name. And does your device use the same certificate as what's attached to the Thing in the registry?

https://docs.aws.amazon.com/iot/latest/developerguide/thing-policy-variables.html

> The thing name is obtained from the client ID in the MQTT Connect message sent when a thing connects to AWS IoT Core.

> AWS IoT Core uses the certificate the device presents when it authenticates to determine which thing to use to verify the connection.

UPDATE: The problem is that the client ID you're using in the Python code should just be `0R281` not `arn:aws:iot:eu-west-1::client/0R281`. I also recommend you swap to the V2 SDK (https://github.com/aws/aws-iot-device-sdk-python-v2), although that's not an immediate concern.

Margherita Cesetti · Answer

specifying the client with only OR 281 in the code works, thank you very much

Margherita Cesetti · Answer

I created the thing and then created and downloaded the certificates
![Enter image description here](/media/postImages/original/IMB6QZoTVpR5WMDWqnbG887g)
I associated the certificate with the policy written before
​![Enter image description here](/media/postImages/original/IMxBKJsDonRuqgC0pEVcoe7g)
Using the Python SDK I instantiated an MQTT client for sending messages with the certificates mentioned above
![Enter image description here](/media/postImages/original/IMlMYM-dfPQdyQ-c0bGFTnkQ)

The client that I instantiate uses the following class defined as follows
​
```
from AWSIoTPythonSDK import MQTTLib as AWSIoTPyMQTT
class AwsClient:
    def __init__(
        self,
        ca_file_path: str,
        key_path: str,
        certificate_path: str,
        client_id: str,
        endpoint: str,
        port: int = 8883,
    ):
        self.ca_file_path = ca_file_path
        self.key_path = key_path
        self.certificate_path = certificate_path
        self.client_id = client_id
        self.port = port
        self.endpoint = endpoint

def get_client(self):
        aws_client = AWSIoTPyMQTT.AWSIoTMQTTClient(self.client_id)
        aws_client.configureEndpoint(self.endpoint, self.port)
        aws_client.configureCredentials(
            CAFilePath=self.ca_file_path,
            KeyPath=self.key_path,
            CertificatePath=self.certificate_path,
        )
        return aws_client

```

when the policy has the first definition that I specified in the first post as its active version, the message is not sent and I get the following error

```
​AWSIoTPythonSDK.exception.AWSIoTExceptions.connectTimeoutException
```
If instead I use the second policy mentioned as the active version of the policy (the less restrictive one) the message is sent and I see it received as in the following image

![Enter image description here](/media/postImages/original/IMruzPEtccQ92Ms88PjKeFSg)

IoT policy to restrict MQTT publish/subscribe by Thing Name

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