rpcd: OpenWrt ubus RPC daemon for backend server

In OpenWrt we commonly use ubus for all kinds of communication. It can provide info from various software as well as request various actions. Nevertheless, not every part of OpenWrt has a daemon that can register itself using ubus. For an example uci and opkg are command-line tools without any background process running all the time.

It would be not efficient to write a daemon for every software like this and run them independently. This is why rpcd was developed. It’s a tiny daemon with support for plugins using trivial API. It loads library .so files and calls init function of each of them.

The code is published under ISC license and can be found via git at https://git.openwrt.org/project/rpcd.git.

There are few small plugins distributed with the rpcd sources. Two of them (session and uci) are built-in, others are optional and have to be build as separated .so libraries. Apart from that there are other projects providing their own plugins.

It is possible to expose shell script functionality over ubus by using rpcd plugin executables functionality. Executables stored in /usr/libexec/rpcd/ directory will be run by rpcd. Lets look at the following example:

mkdir -p /usr/libexec/rpcd
cat << "EOF" > /usr/libexec/rpcd/foo
case "$1" in
		echo '{ "bar": { "arg1": true, "arg2": 32, "arg3": "str" }, "toto": { }, "failme": {} }'
		case "$2" in
				# read the arguments
				read input;
				# optionally log the call
				logger -t "foo" "call" "$2" "$input"
				# return json object or an array
				echo '{ "hello": "world" }'
				# return json object
				echo '{ "something": "somevalue" }'
                                # return invalid
                                echo '{asdf/3454'
chmod +x /usr/libexec/rpcd/foo
service rpcd restart

This will create new ubus functions which then can be used (after restarting rpcd):

$ ubus -v list foo
'foo' @a9482c5b
$ ubus -S call foo bar '{"arg1": true }'
$ ubus -S call foo toto
$ ubus -S call foo failme
$ echo $?

On startup rpcd will call all executables in /usr/libexec/rpcd/ with argv[1] set to “list”. For a plugin, which responds with a valid list of methods and signatures, ubus method with appropriate arguments will be created. When a method provided by the plugin is about to be invoked, rpcd calls the binary with argv[1] set to call and argv[2] set to the invoked method name.

The actual data is then sent by the ubus client via stdin. I.e. if you're testing the script itself, you need to use

echo '{"arg1": 42}' | yourscript call yourmethod

You cannot simply use yourscript call yourmethod '{“arg1”: 42}' as you might have expected.

The method signature is a simple object containing key:value pairs. The argument type is inferred from the value. If the value is a string (regardless of the contents) it is registered as string, if the value is a bool true or false, its registered as bool, if the value is an integer, it is registered as either int8, int16, int32 or int64 depending on the value i.e. “foo”: 16 will be INT16, “foo”: 64 will be INT64, “foo”: 8 will be INT8 and everything else will be INT32.

It is enough to issue service rpcd reload to make it pick up new plugin executables, that way one does not lose active sessions.

NOTE: At least on CC builds, reload is not enough, and you must restart to pickup new plugins and changes to existing plugins.

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  • Last modified: 2023/10/14 06:08
  • by vgaetera