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Secure MQTT broker communication using BrokerListener

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Important

Azure IoT Operations Preview – enabled by Azure Arc is currently in preview. You shouldn't use this preview software in production environments.

You'll need to deploy a new Azure IoT Operations installation when a generally available release becomes available. You won't be able to upgrade a preview installation.

For legal terms that apply to Azure features that are in beta, in preview, or otherwise not yet released into general availability, see the Supplemental Terms of Use for Microsoft Azure Previews.

To customize the network access and security use the BrokerListener resource. A listener corresponds to a network endpoint that exposes the broker to the network. You can have one or more BrokerListener resources for each Broker resource, and thus multiple ports with different access control each.

Each listener port can have its own authentication and authorization rules that define who can connect to the listener and what actions they can perform on the broker. You can use BrokerAuthentication and BrokerAuthorization resources to specify the access control policies for each listener. This flexibility allows you to fine-tune the permissions and roles of your MQTT clients, based on their needs and use cases.

Tip

You can only access the default MQTT broker deployment by using the cluster IP, TLS, and a service account token. Clients connecting from outside the cluster need extra configuration before they can connect.

Listeners have the following characteristics:

  • You can have up to three listeners. One listener per service type of loadBalancer, clusterIp, or nodePort. The default BrokerListener named default is service type clusterIp.
  • Each listener supports multiple ports
  • BrokerAuthentication and BrokerAuthorization references are per port
  • TLS configuration is per port
  • Service names must be unique
  • Ports can't conflict over different listeners

For a list of the available settings, see the Broker Listener API reference.

Default BrokerListener

When you deploy Azure IoT Operations Preview, the deployment also creates a BrokerListener resource named default in the azure-iot-operations namespace. This listener is linked to the default Broker resource named default that's also created during deployment. The default listener exposes the broker on port 18883 with TLS and SAT authentication enabled. The TLS certificate is automatically managed by cert-manager. Authorization is disabled by default.

To view or edit the listener:

  1. In the Azure portal, navigate to your IoT Operations instance.

  2. Under Azure IoT Operations resources, select MQTT Broker.

    Screenshot using Azure portal to view Azure IoT Operations MQTT configuration.

  3. From the broker listener list, select the default listener.

    Screenshot using Azure portal to view or edit the default broker listener.

  4. Review the listener settings and make any changes as needed.

Create new broker listeners

This example shows how to create a new BrokerListener resource named loadbalancer-listener for a Broker resource. The BrokerListener resource defines a two ports that accept MQTT connections from clients.

  • The first port listens on port 1883 with no TLS and authentication off. Clients can connect to the broker without encryption or authentication.
  • The second port listens on port 18883 with TLS and authentication enabled. Only authenticated clients can connect to the broker with TLS encryption. TLS is set to automatic, which means that the listener uses cert-manager to get and renew its server certificate.

  1. In the Azure portal, navigate to your IoT Operations instance.

  2. Under Azure IoT Operations resources, select MQTT Broker.

  3. Select MQTT broker listener for LoadBalancer > Create. You can only create one listener per service type. If you already have a listener of the same service type, you can add more ports to the existing listener.

    Screenshot using Azure portal to create MQTT broker for load balancer listener.

    Enter the following settings:

    Setting Description
    Name Name of the BrokerListener resource.
    Service name Name of the Kubernetes service associated with the BrokerListener.
    Service type Type of broker service, such as LoadBalancer, NodePort, or ClusterIP.
    Port Port number on which the BrokerListener listens for MQTT connections.
    Authentication The authentication resource reference.
    Authorization The authorization resource reference.
    TLS Indicates whether TLS is enabled for secure communication. Can be set to automatic or manual.

  4. Select Create listener.

Configure TLS with automatic certificate management

To enable TLS with automatic certificate management, specify the TLS settings on a listener port.

Verify cert-manager installation

With automatic certificate management, you use cert-manager to manage the TLS server certificate. By default, cert-manager is installed alongside Azure IoT Operations Preview in the azure-iot-operations namespace already. Verify the installation before proceeding.

  1. Use kubectl to check for the pods matching the cert-manager app labels.

    kubectl get pods --namespace azure-iot-operations -l 'app in (cert-manager,cainjector,webhook)'

    NAME                                           READY   STATUS    RESTARTS       AGE
aio-cert-manager-64f9548744-5fwdd              1/1     Running   4 (145m ago)   4d20h
aio-cert-manager-cainjector-6c7c546578-p6vgv   1/1     Running   4 (145m ago)   4d20h
aio-cert-manager-webhook-7f676965dd-8xs28      1/1     Running   4 (145m ago)   4d20h

  2. If you see the pods shown as ready and running, cert-manager is installed and ready to use.

Tip

To further verify the installation, check the cert-manager documentation verify installation. Remember to use the azure-iot-operations namespace.

Create an Issuer for the TLS server certificate

The cert-manager Issuer resource defines how certificates are automatically issued. Cert-manager supports several Issuers types natively. It also supports an external issuer type for extending functionality beyond the natively supported issuers. MQTT broker can be used with any type of cert-manager issuer.

Important

During initial deployment, Azure IoT Operations is installed with a default Issuer for TLS server certificates. You can use this issuer for development and testing. For more information, see Default root CA and issuer with Azure IoT Operations. The steps below are only required if you want to use a different issuer.

The approach to create the issuer is different depending on your scenario. The following sections list examples to help you get started.

The CA issuer is useful for development and testing. It must be configured with a certificate and private key stored in a Kubernetes secret.

Set up the root certificate as a Kubernetes secret

If you have an existing CA certificate, create a Kubernetes secret with the CA certificate and private key PEM files. Run the following command and you have set up the root certificate as a Kubernetes secret and can skip the next section.

kubectl create secret tls test-ca --cert tls.crt --key tls.key -n azure-iot-operations

If you don't have a CA certificate, cert-manager can generate a root CA certificate for you. Using cert-manager to generate a root CA certificate is known as bootstrapping a CA issuer with a self-signed certificate.

  1. Start by creating ca.yaml:

    apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: selfsigned-ca-issuer
  namespace: azure-iot-operations
spec:
  selfSigned: {}
---
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: selfsigned-ca-cert
  namespace: azure-iot-operations
spec:
  isCA: true
  commonName: test-ca
  secretName: test-ca
  issuerRef:
    # Must match Issuer name above
    name: selfsigned-ca-issuer
    # Must match Issuer kind above
    kind: Issuer
    group: cert-manager.io
  # Override default private key config to use an EC key
  privateKey:
    rotationPolicy: Always
    algorithm: ECDSA
    size: 256

  2. Create the self-signed CA certificate with the following command:

    kubectl apply -f ca.yaml

Cert-manager creates a CA certificate using its defaults. The properties of this certificate can be changed by modifying the Certificate specification. See cert-manager documentation for a list of valid options.

Distribute the root certificate

The prior example stores the CA certificate in a Kubernetes secret called test-ca. The certificate in PEM format can be retrieved from the secret and stored in a file ca.crt with the following command:

kubectl get secret test-ca -n azure-iot-operations -o json | jq -r '.data["tls.crt"]' | base64 -d > ca.crt

This certificate must be distributed and trusted by all clients. For example, use --cafile flag for a mosquitto client.

Create issuer based on CA certificate

Cert-manager needs an issuer based on the CA certificate generated or imported in the earlier step. Create the following file as issuer-ca.yaml:

apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: my-issuer
  namespace: azure-iot-operations
spec:
  ca:
    # Must match secretName of generated or imported CA cert
    secretName: test-ca

Create the issuer with the following command:

kubectl apply -f issuer-ca.yaml

The prior command creates an issuer for issuing the TLS server certificates. Note the name and kind of the issuer. In the example, name my-issuer and kind Issuer. These values are set in the BrokerListener resource later.

Enable TLS automatic certificate management for a port

The following is an example of a BrokerListener resource that enables TLS on port 8884 with automatic certificate management.

  1. In the Azure portal, go to your IoT Operations instance.

  2. Under Azure IoT Operations resources, select MQTT Broker.

  3. Select or create a listener. You can only create one listener per service type. If you already have a listener of the same service type, you can add more ports to the existing listener.

  4. You can add TLS settings to the listener by selecting the TLS on an existing port or by adding a new port.

    Screenshot using Azure portal to create MQTT broker for load balancer listener with automatic TLS certificates.

    Enter the following settings:

    Setting Description
    Port Port number on which the BrokerListener listens for MQTT connections. Required.
    Authentication The authentication resource reference.
    Authorization The authorization resource reference.
    TLS Select the Add button.
    Issuer name Name of the cert-manager issuer. Required.
    Issuer kind Kind of the cert-manager issuer. Required.
    Issuer group Group of the cert-manager issuer. Required.
    Private key algorithm Algorithm for the private key.
    Private key rotation policy Policy for rotating the private key.
    DNS names DNS subject alternate names for the certificate.
    IP addresses IP addresses of the subject alternate names for the certificate.
    Secret name Kubernetes secret containing an X.509 client certificate.
    Duration Total lifetime of the TLS server certificate Defaults to 90 days.
    Renew before When to begin renewing the certificate.

  5. Select Save to save the TLS settings.

Connect to the broker with TLS

Once the server certificate is configured, TLS is enabled. To test with mosquitto:

mosquitto_pub -h $HOST -p 8884 -V mqttv5 -i "test" -t "test" -m "test" --cafile ca.crt

The --cafile argument enables TLS on the mosquitto client and specifies that the client should trust all server certificates issued by the given file. You must specify a file that contains the issuer of the configured TLS server certificate.

Replace $HOST with the appropriate host:

  • If connecting from within the same cluster, replace with the service name given (my-new-tls-listener in example) or the service CLUSTER-IP.
  • If connecting from outside the cluster, the service EXTERNAL-IP.

Remember to specify authentication methods if needed.

Default root CA and issuer

To help you get started, Azure IoT Operations is deployed with a default "quickstart" root CA and issuer for TLS server certificates. You can use this issuer for development and testing. For more information, see Default root CA and issuer for TLS server certificates.

For production, you must configure a CA issuer with a certificate from a trusted CA, as described in the previous sections.

Configure TLS with manual certificate management

To manually configure MQTT broker to use a specific TLS certificate, specify it in a BrokerListener resource with a reference to a Kubernetes secret. Then deploy it using kubectl. This article shows an example to configure TLS with self-signed certificates for testing.

Create certificate authority with Step CLI

Step is a certificate manager that can quickly get you up and running when creating and managing your own private CA.

  1. Install Step CLI and create a root certificate authority (CA) certificate and key.

    step certificate create --profile root-ca "Example Root CA" root_ca.crt root_ca.key

  2. Create an intermediate CA certificate and key signed by the root CA.

    step certificate create --profile intermediate-ca "Example Intermediate CA" intermediate_ca.crt intermediate_ca.key \
--ca root_ca.crt --ca-key root_ca.key

Create server certificate

Use Step CLI to create a server certificate from the signed by the intermediate CA.

step certificate create mqtts-endpoint mqtts-endpoint.crt mqtts-endpoint.key \
--profile leaf \
--not-after 8760h \
--san mqtts-endpoint \
--san localhost \
--ca intermediate_ca.crt --ca-key intermediate_ca.key \
--no-password --insecure

Here, mqtts-endpoint and localhost are the Subject Alternative Names (SANs) for MQTT broker's frontend in Kubernetes and local clients, respectively. To connect over the internet, add a --san with an external IP. The --no-password --insecure flags are used for testing to skip password prompts and disable password protection for the private key because it's stored in a Kubernetes secret. For production, use a password and store the private key in a secure location like Azure Key Vault.

Certificate key algorithm requirements

Both EC and RSA keys are supported, but all certificates in the chain must use the same key algorithm. If you import your own CA certificates, ensure that the server certificate uses the same key algorithm as the CAs.

Import server certificate chain as a Kubernetes secret

  1. Create a full server certificate chain, where the order of the certificates matters: the server certificate is the first one in the file, the intermediate is the second.

    cat  mqtts-endpoint.crt intermediate_ca.crt  > server_chain.crt

  2. Create a Kubernetes secret with the server certificate chain and server key using kubectl.

    kubectl create secret tls server-cert-secret -n azure-iot-operations \
--cert server_chain.crt \
--key mqtts-endpoint.key

Enable TLS manual certificate management for a port

The following is an example of a BrokerListener resource that enables TLS on port 8884 with manual certificate management.

  1. In the Azure portal, navigate to your IoT Operations instance.

  2. Under Azure IoT Operations resources, select MQTT Broker.

  3. Select or create a listener. You can only create one listener per service type. If you already have a listener of the same service type, you can add more ports to the existing listener.

  4. You can add TLS settings to the listener by selecting the TLS on an existing port or by adding a new port.

    Screenshot using Azure portal to create MQTT broker for load balancer listener with manual TLS certificates.

    Enter the following settings:

    Setting Description
    Port Port number on which the BrokerListener listens for MQTT connections. Required.
    Authentication The authentication resource reference.
    Authorization The authorization resource reference.
    TLS Select the Add button.
    Secret name Kubernetes secret containing an X.509 client certificate.

  5. Select Save to save the TLS settings.

Connect to the broker with TLS

To test the TLS connection with mosquitto client, publish a message and pass the root CA certificate in the parameter --cafile.

mosquitto_pub -d -h localhost -p 8885 -i "my-client" -t "test-topic" -m "Hello" --cafile root_ca.crt

Client my-client sending CONNECT
Client my-client received CONNACK (0)
Client my-client sending PUBLISH (d0, q0, r0, m1, 'test-topic', ... (5 bytes))
Client my-client sending DISCONNECT

Tip

To use localhost, the port must be available on the host machine. For example, kubectl port-forward svc/mqtts-endpoint 8885:8885 -n azure-iot-operations. With some Kubernetes distributions like K3d, you can add a forwarded port with k3d cluster edit $CLUSTER_NAME --port-add 8885:8885@loadbalancer.

Note

To connect to the broker you need to distribute root of trust to the clients, also known as trust bundle. In this case the root of trust is the self-signed root CA created Step CLI. Distribution of root of trust is required for the client to verify the server certificate chain. If your MQTT clients are workloads on the Kubernetes cluster you also need to create a ConfigMap with the root CA and mount it in your Pod.

Remember to specify username, password, etc. if MQTT broker authentication is enabled.

Use external IP for the server certificate

To connect with TLS over the internet, MQTT broker's server certificate must have its external hostname as a SAN. In production, this is usually a DNS name or a well-known IP address. However, during dev/test, you might not know what hostname or external IP is assigned before deployment. To solve this, deploy the listener without the server certificate first, then create the server certificate and secret with the external IP, and finally import the secret to the listener.

If you try to deploy the example TLS listener manual-tls-listener but the referenced Kubernetes secret server-cert-secret doesn't exist, the associated service gets created, but the pods don't start. The service is created because the operator needs to reserve the external IP for the listener.

kubectl get svc mqtts-endpoint -n azure-iot-operations

NAME                   TYPE           CLUSTER-IP      EXTERNAL-IP   PORT(S)             AGE
mqtts-endpoint         LoadBalancer   10.X.X.X        172.X.X.X     8885:30674/TCP      1m15s

However, this behavior is expected and it's okay to leave it like this while we import the server certificate. The health manager logs mention MQTT broker is waiting for the server certificate.

kubectl logs -l app=health-manager -n azure-iot-operations

...
<6>2023-11-06T21:36:13.634Z [INFO] [1] - Server certificate server-cert-secret not found. Awaiting creation of secret.

Note

Generally, in a distributed system, pods logs aren't deterministic and should be used with caution. The right way for information like this to surface is through Kubernetes events and custom resource status, which is in the backlog. Consider the previous step as a temporary workaround.

Even though the frontend pods aren't up, the external IP is already available.

kubectl get svc mqtts-endpoint -n azure-iot-operations

NAME                   TYPE           CLUSTER-IP      EXTERNAL-IP   PORT(S)             AGE
mqtts-endpoint         LoadBalancer   10.X.X.X        172.X.X.X     8885:30674/TCP      1m15s

From here, follow the same steps as previously to create a server certificate with this external IP in --san and create the Kubernetes secret in the same way. Once the secret is created, it's automatically imported to the listener.

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