systemd-userdbd.service, systemd-userdbd — JSON User/Group Record Query Multiplexer/NSS Compatibility
systemd-userdbd is a system service that multiplexes user/group lookups to all local services that provide JSON user/group record definitions to the system. In addition it synthesizes JSON user/group records from classic UNIX/glibc NSS user/group records in order to provide full backwards compatibility.
Most of systemd-userdbd's functionality is accessible through the userdbctl(1) command.
The user and group records this service provides access to follow the JSON User Record and JSON Group Record definitions. This service implements the User/Group Record Lookup API via Varlink, and multiplexes access other services implementing this API, too. It is thus both server and client of this API.
This service provides two distinct Varlink services:
io.systemd.Multiplexer provides a single, unified API for querying JSON user and
group records. Internally it talks to all other user/group record services running on the system in
parallel and forwards any information discovered. This simplifies clients substantially since they need
to talk to a single service only instead of all of them in
io.systemd.NameSeviceSwitch provides compatibility with classic UNIX/glibc
NSS user records, i.e. converts struct passwd and struct group records as
acquired with APIs such as getpwnam(1) to JSON
user/group records, thus hiding the differences between the services as much as possible.