CoAP Adapter Configuration

The CoAP protocol adapter exposes CoAP based endpoints for Eclipse Hono™’s south bound Telemetry, Event and Command & Control APIs.

The adapter is implemented as a Quarkus application using Eclipse Californium™ for implementing the CoAP protocol handling. It can be run either directly from the command line or by means of starting the corresponding Docker image created from it.

Service Configuration

The following table provides an overview of the configuration variables and corresponding system properties for configuring the CoAP adapter.

The variables only need to be set if the default value needs to be changed.

In addition to the options described in the table above, this component supports the following standard configuration options:

• Common Java VM Options
• Common vert.x Options
• Common Protocol Adapter Options
• Monitoring Options

Port Configuration

The CoAP protocol adapter can be configured to listen for connections on

• a secure port only (default) or
• an insecure port only or
• both a secure and an insecure port (dual port configuration)

The CoAP protocol adapter will fail to start if none of the ports is configured properly.

Secure Port Only

The protocol adapter opens a DTLS secured port if any of the following criteria are met

• The HONO_COAP_KEYSTOREPATH and HONO_COAP_KEYSTOREPASSWORD environment variables are set in order to load a key and certificate from a password protected key store or
• the HONO_COAP_KEYPATH and HONO_COAP_CERTPATH environment variables are set in order to load a key and certificate from two separate PEM files in PKCS8 format or
• the HONO_COAP_PORT environment variable is set to a valid port number.

When starting up, the protocol adapter will bind a DTLS secured UDP socket to the configured port. If the port is not set explicitly, the default CoAP secure port 5684 is used.

The HONO_COAP_BINDADDRESS variable can be used to specify the network interface that the port should be exposed on. By default the port is bound to the loopback device only, i.e. the port will only be accessible from the local host. Setting this variable to 0.0.0.0 will let the port being bound to all network interfaces (be careful not to expose the port unintentionally to the outside world).

Insecure Port Only

The secure port will mostly be required for production scenarios. However, it might be desirable to expose a non-DTLS secured port instead, e.g. for testing purposes. In any case, the non-secure port needs to be explicitly enabled by

• explicitly setting HONO_COAP_AUTHENTICATIONREQUIRED to false and either
  • explicitly setting HONO_COAP_INSECUREPORT to a valid port number or
  • implicitly configuring the default insecure CoAP port (5683) by setting HONO_COAP_INSECUREPORTENABLED to true.

The protocol adapter issues a warning on the console if HONO_COAP_INSECUREPORT is set to the default secure CoAP port (5684).

The HONO_COAP_INSECUREPORTBINDADDRESS variable can be used to specify the network interface that the port should be exposed on. By default the port is bound to the loopback device only, i.e. the port will only be accessible from the local host. This variable might be used to e.g. expose the non-DTLS secured port on a local interface only, thus providing easy access from within the local network, while still requiring encrypted communication when accessed from the outside over public network infrastructure.

Setting this variable to 0.0.0.0 will let the port being bound to all network interfaces (be careful not to expose the port unintentionally to the outside world).

Dual Port

The protocol adapter may be configured to open both a secure and a non-secure port at the same time simply by configuring both ports as described above. For this to work, both ports must be configured to use different port numbers, otherwise startup will fail.

Ephemeral Ports

Both the secure as well as the insecure port numbers may be explicitly set to 0. The protocol adapter will then use arbitrary (unused) port numbers determined by the operating system during startup.

Authentication

The CoAP protocol is UDP based and as such uses the DTLS protocol to secure the communication between a client (device) and a server (adapter). The CoAP adapter also uses the DTLS handshake to prove its identity to devices and to authenticate the devices themselves. The DTLS protocol allows for different cipher suites to be used for doing so. These suites mainly differ from each other in the type of secret being used for proving the participants’ identity to each other.

One class of suites is based on a secret that is shared between the client and the server, very much like in a username/password based authentication scheme. This class of suites is called pre-shared key or PSK-based and is very popular for use cases where the devices are very constrained regarding CPU and memory. Another class of cipher suites is based on certificates which use asymmetric encryption for proving possession of the secret (the private key).

The CoAP adapter supports cipher suites from both classes but only supports cipher suites from the latter class which use ECDSA algorithm for authentication. In particular, this means that the client and/or server need to use elliptic curve cryptography (ECC) based keys instead of RSA based ones.

When enabling the secure port without configuring an ECC based key and certificate, the adapter will only use PSK based cipher suites for authentication. When configuring an ECC based key and certificate, the adapter will also offer certificate based cipher suites to the client to use for authentication.

In any case the device’s credentials need to be registered with the device registry. Please refer to the Standard Credential Types and the Device Registry Management API for additional information.