Comment by jchw

Yes but the systemd developers don't want to implement their own protocols with e.g. ACL checking, and given some of their track record I kind of think you don't want them to, either. I'm pretty sure the error conditions would be even more bespoke if they "just" used UNIX domain sockets directly. Don't get me wrong, there's nothing particularly wrong with UNIX domain sockets, but there's no "standard" protocols for communicating over UDS.