Easy ubus Daemons with rpcd

In this article we will see how to create a simple ubus daemon with RPCd.

Ubus is an amazing piece of software that allow inter-process communication inside and in between nodes.

For inter-process communications it uses a common bus with synchronous and asynchronous calling, publish/subscribe and event paradigms.

For in-between nodes communication it provices a JSON-RPC 2.0 Server (and I hope will do WebSockets soon!).

For more information on how ubus works check out OpenWRT’s Wiki page.

If you want to expose simple functionality to ubus (or you are prototyping an interface before creating an efficient daemon) you can use rpcd for this.

This is a simplified example based on the one in the wiki

$ cat << EOF > /usr/libexec/rpcd/banner
#!/bin/sh

# The RPCd interfaces with commands via two methods: list and call.
case "$1" in
	list)
                # List method must return the list of methods and parameters that the daemon will accept. Only methods listed here will available to call.
		echo '{ "append": { "content": "str"}, "show": { } }'
	;;
	call)
                # The way rpcd calls the methods is by calling your script like this: <script-name> call <method-name> << <json-input>
                # So, in order to retrieve the called method, you need to read $2
		case "$2" in
			append)
				# And in order to retrieve the method parameter, you need to read stdin
				read input;
                                CONTENT=`echo $input | jsonfilter -e '@.content'` 
                                echo $CONTENT >> /etc/banner
				echo '{ "result": "ok" }'
			;;
			show)
				# return json object or an array
				echo -n '{ "content": "`; echo $(cat /etc/banner | text_to_json); echo '"}'
			;;
		esac
	;;
esac

function text_to_json() {
JSON_TOPIC_RAW=`echo $1`
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//\\/\\\\} # \ 
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//\//\\\/} # / 
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//\'/\\\'} # ' (not strictly needed ?)
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//\"/\\\"} # " 
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//   /\\t} # \t (tab)
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//
/\\\n} # \n (newline)
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//^M/\\\r} # \r (carriage return)
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//^L/\\\f} # \f (form feed)
JSON_TOPIC_RAW=${JSON_TOPIC_RAW//^H/\\\b} # \b (backspace)
echo JSON_TOPIC_RAW
}
EOF
$ chmod +x /usr/libexec/rpcd/banner
$ service rpcd restart

And then we can try the new daemon:

$ ubus -v list banner
'banner' @xxxxxxx
    "append":{"content":"String"}
    "show":{}

$ ubus call banner show
LEDE blah blah
...

$ ubus call banner append 'Hello world!'
LEDE blah blah
...
Hello world!

Update:
@Rundfreifunk comment suggested that if you want to use this RPC commands via JSON-RPC, you need to add authentication parameters.
In this case, let’s suppose that we want to grant anonymous read access to the banner.show method.

We could do that like this:

$cat << EOF > /usr/share/rpcd/acl.d/banner.json
{
  "unauthenticated": {
    "description": "Alfred data types via batman-adv meshrouting",
    "read": {
      "ubus": {
        "banner": ["show"]
      }
    }
  }
}

The documentation related to ACLs can be found in this OpenWRT wiki page.