The juju info interface is a special and implicit relationship that works with any charm. It is mainly useful for subordinate charms that can add functionality to any exisiting machine without the host charm being aware of it.
{{endpoint_name}}.connected
Note: This flag keys off of what the charm author names the relationship endpoint, which should not be the name of the interface:
An example of a properly implemented relationship would resemble the following:
requires:
host-system:
interface: juju-info
This might then be used in your charm would like:
@when_any('host-system.connected')
def handle_host():
host = endpoint_from_flag('host-system.connected')
for address in host.addresses:
hookenv.log('Connected to: {}'.format(address))