yggdrasil is a system daemon that subscribes to topics on an MQTT broker and routes any data received on the topics to an appropriate child "worker" process, exchanging data with its worker processes through a D-Bus message broker.

The easiest way to compile and install yggdrasil is using meson. Because yggdrasil runs as a privileged system daemon, systemd unit files and D-Bus policy files must be installed in specific directories in order for the service to start.

Generally, it is recommended to follow your distribution's packaging guidelines for compiling Go programs and installing projects using meson. What follows is a generally acceptable set of steps to setup, compile and install yggdrasil using meson.

# Set up the project according to distribution-specific directory locations
meson setup --prefix /usr/local --sysconfdir /etc --localstatedir /var builddir
# Compile
meson compile -C builddir
# Install
meson install -C builddir

meson includes an optional --destdir to its install subcommand to aid in packaging.

Configuration of yggd can be done by specifying values in a configuration file or via command line arguments. Command-line arguments take precedence over configuration file values. The configuration file is TOML.

The system-wide configuration file is located at /etc/yggdrasil/config.toml (assuming SYSCONFDIR=/etc, as the example above). The location of the file may be overridden by passing the --config command-line argument to yggd.

In order to run yggd under certain conditions (such as connecting to a broker that requires mTLS authentication), a valid certificate must first be created and written to the filesystem.

One way of generating a valid certificate is to first register the system with an RHSM provider. The simplest way to do this is to create a free Red Hat Developer account. On a Red Hat Enterprise Linux system, run subscription-manager register, using the developer account username and password.

sudo subscription-manager register --username [email protected] --password sw0rdf1sh

Once the system is successfully registered with RHSM, yggd can be launched, using the certificate key pair:

sudo /usr/sbin/yggd --cert-file /etc/pki/consumer/cert.pem --key-file /etc/pki/consumer/key.pem

A set of tags may be defined to associate additional key/value data with a host when connecting to the broker. To do this, create the file /etc/yggdrasil/tags.toml (assuming SYSCONFDIR=/etc, as the example above). The contents of the file may be any number of TOML key/value pairs. However, a limited number of TOML values are accepted as tag values (strings, integers, booleans, floats, Local Date, Local Time, Offset Date-Time and Local Date-Time).

yggdrasil uses D-Bus as an IPC framework to enable communication between workers and the dispatcher, as well as exporting an API enabling other services to interact with it. yggd follows the normal logic for determining which bus to connect to. It will attempt to connect to the session bus if DBUS_SESSION_BUS_ADDRESS is defined. Otherwise, it will attempt to connect to the system bus. On the system bus, if the process is not running as root, the installed D-Bus security policy will deny the process from claiming the com.redhat.Yggdrasil1 and com.redhat.Yggdrasil1.Dispatcher1 names, and the process will exit.

The systemd unit yggrasil.service starts yggd, using the logic described above.

systemctl enable --now yggdrasil

Multiple instances of yggd can be run by using the templated systemd units. Each instance requires a private D-Bus session. The socket and broker are started with [email protected] and [email protected], respectively. A templated yggdrasil unit is available to automatically connect yggd to the D-Bus broker started by [email protected].

systemctl enable --now [email protected]
systemctl enable --now [email protected]
systemctl enable --now [email protected]

This will define and create an abstract UNIX domain socket named yggd_bunnies. It will start a dbus-broker process running in the "user" scope, connecting to the yggd_bunnies socket defined previously. Finally, yggd is launched with a specific configuration file as the value of the --config argument: /etc/yggdrasil/yggdrasil-bunnies.toml.

A functional worker program must connect to the message bus as determined by the DBUS_SESSION_BUS_ADDRESS environment variable, connecting to a session bus if the value is defined, otherwise connecting to the system bus. Once connected to the bus:

• The program must export an object on the bus that implements the com.redhat.Yggdrasil1.Worker1 interface.
• The object must be exported at a path under /com/redhat/Yggdrasil1/Worker1 that includes the directive name (i.e. /com/redhat/Yggdrasil1/Worker1/echo).
• The worker must claim a well-known name that begins with com.redhat.Yggdrasil1.Worker1 and includes its directive as the final segment in reverse-domain-name notation (i.e. com.redhat.Yggdrasil1.Worker1.echo).

A worker can transmit data back to a destination by calling the com.redhat.Yggdrasil1.Dispatcher1.Transmit method.

Package worker implements the above requirements implicitly, enabling workers to be written without needing to worry about much of the D-Bus requirements outlined above.

See worker/echo for a reference implementation of a worker program.