emqx-coap

emqx-coap is a CoAP Gateway for EMQ X Broker. It translates CoAP messages into MQTT messages and make it possible to communiate between CoAP clients and MQTT clients.

Client Usage Example

libcoap is an excellent coap library which has a simple client tool. It is recommended to use libcoap as a coap client.

To compile libcoap, do following steps:

git clone http://github.com/obgm/libcoap
cd libcoap
./autogen.sh
./configure --enable-documentation=no --enable-tests=no
make

Publish example:

libcoap/examples/coap-client -m put -e 1234  "coap://127.0.0.1/mqtt/topic1?c=client1&u=tom&p=secret"

• topic name is "topic1", NOT "/topic1"
• client id is client1
• username is tom
• password is secret
• payload is a text string "1234"

A mqtt message with topic="topic1", payload="1234" has been published. Any mqtt client or coap client, who has subscribed this topic could receive this message immediately.

Subscribe example:

libcoap/examples/coap-client -m get -s 10 "coap://127.0.0.1/mqtt/topic1?c=client1&u=tom&p=secret"

• topic name is "topic1", NOT "/topic1"
• client id is client1
• username is tom
• password is secret
• subscribe time is 10 seconds

And you will get following result if any mqtt client or coap client sent message with text "1234567" to "topic1":

v:1 t:CON c:GET i:31ae {} [ ]
1234567v:1 t:CON c:GET i:31af {} [ Observe:1, Uri-Path:mqtt, Uri-Path:topic1, Uri-Query:c=client1, Uri-Query:u=tom, Uri-Query:p=secret ]

The output message is not well formatted which hide "1234567" at the head of the 2nd line.

Configure emqx-coap

File: etc/emqx_coap.conf

## The UDP port that CoAP is listening on.
##
## Value: Port
coap.port = 5683

## Interval for keepalive, specified in seconds.
##
## Value: Duration
##  -s: seconds
##  -m: minutes
##  -h: hours
coap.keepalive = 120s

## Whether to enable statistics for CoAP clients.
##
## Value: on | off
coap.enable_stats = off

## Private key file for DTLS
##
## Value: File
coap.keyfile = {{ platform_etc_dir }}/certs/key.pem

## Server certificate for DTLS.
##
## Value: File
coap.certfile = {{ platform_etc_dir }}/certs/cert.pem

• coap.port
  • UDP port for CoAP.
• coap.keepalive
  • Interval for keepalive, in seconds.
• coap.enable_stats
  • To control whether write statistics data into ETS table for dashbord to read.
• coap.certfile
  • server certificate for DTLS
• coap.keyfile
  • private key for DTLS

Load emq-coap

./bin/emqx_ctl plugins load emqx_coap

CoAP Client Observe Operation (subscribe topic)

To subscribe any topic, issue following command:

  GET  coap://localhost/mqtt/{topicname}?c={clientid}&u={username}&p={password}    with OBSERVE=0

• "mqtt" in the path is mandatory.
• replace {topicname}, {clientid}, {username} and {password} with your true values.
• {topicname} and {clientid} is mandatory.
• if clientid is absent, a "bad_request" will be returned.
• {topicname} in URI should be percent-encoded to prevent special characters, such as + and #.
• {username} and {password} are optional.
• if {username} and {password} are not correct, an uauthorized error will be returned.
• topic is subscribed with qos1.

CoAP Client Unobserve Operation (unsubscribe topic)

To cancel observation, issue following command:

  GET  coap://localhost/mqtt/{topicname}?c={clientid}&u={username}&p={password}    with OBSERVE=1

• "mqtt" in the path is mandatory.
• replace {topicname}, {clientid}, {username} and {password} with your true values.
• {topicname} and {clientid} is mandatory.
• if clientid is absent, a "bad_request" will be returned.
• {topicname} in URI should be percent-encoded to prevent special characters, such as + and #.
• {username} and {password} are optional.
• if {username} and {password} are not correct, an uauthorized error will be returned.

CoAP Client Notification Operation (subscribed Message)

Server will issue an observe-notification as a subscribed message.

• Its payload is exactly the mqtt payload.
• payload data type is "application/octet-stream".

CoAP Client Publish Operation

Issue a coap put command to do publishment. For example:

  PUT  coap://localhost/mqtt/{topicname}?c={clientid}&u={username}&p={password}

• "mqtt" in the path is mandatory.
• replace {topicname}, {clientid}, {username} and {password} with your true values.
• {topicname} and {clientid} is mandatory.
• if clientid is absent, a "bad_request" will be returned.
• {topicname} in URI should be percent-encoded to prevent special characters, such as + and #.
• {username} and {password} are optional.
• if {username} and {password} are not correct, an uauthorized error will be returned.
• payload could be any binary data.
• payload data type is "application/octet-stream".
• publish message will be sent with qos0.

CoAP Client Keep Alive

Device should issue a get command periodically, serve as a ping to keep mqtt session online.

  GET  coap://localhost/mqtt/{any_topicname}?c={clientid}&u={username}&p={password}

• "mqtt" in the path is mandatory.
• replace {any_topicname}, {clientid}, {username} and {password} with your true values.
• {any_topicname} is optional, and should be percent-encoded to prevent special characters.
• {clientid} is mandatory. If clientid is absent, a "bad_request" will be returned.
• {username} and {password} are optional.
• if {username} and {password} are not correct, an uauthorized error will be returned.
• coap client should do keepalive work periodically to keep mqtt session online, especially those devices in a NAT network.

CoAP Client NOTES

emq-coap gateway does not accept POST and DELETE requests.

Topics in URI should be percent-encoded, but corresponding uri_path option has percent-encoding converted. Please refer to RFC 7252 section 6.4, "Decomposing URIs into Options":

Note that these rules completely resolve any percent-encoding.

That implies coap client is responsible to convert any percert-encoding into true character while assembling coap packet.

DTLS

emq-coap support DTLS to secure UDP data.

Please config coap.certfile and coap.keyfile in emqx_coap.conf. If certfile or keyfile are invalid, DTLS will be turned off and you could read a error message in system log.

ClientId, Username, Password and Topic

ClientId/username/password/topic in the coap URI are the concepts in mqtt. That is to say, emq-coap is trying to fit coap message into mqtt system, by borrowing the client/username/password/topic from mqtt.

The Auth/ACL/Hook features in mqtt also applies on coap stuff. For example:

• If username/password is not authorized, coap client will get an uauthorized error.
• If username or clientid is not allowed to published specific topic, coap message will be dropped in fact, although coap client will get an acknoledgement from emq-coap.
• If a coap message is published, a 'message.publish' hook is able to capture this message as well.

well-known locations

Discovery always return ","

For example

libcoap/examples/coap-client -m get "coap://127.0.0.1/.well-known/core"

well-known locations

Discovery always return ","

For example

libcoap/examples/coap-client -m get "coap://127.0.0.1/.well-known/core"

License

Apache License Version 2.0

Author

EMQ X Team.