nss-systemd is a plug-in module for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc), providing UNIX user and group name resolution for dynamic users and groups allocated through the DynamicUser= option in systemd unit files. See systemd.exec(5) for details on this option.
This module also ensures that the root and nobody users and groups (i.e. the users/groups with the UIDs/GIDs 0 and 65534) remain resolvable at all times, even if they aren't listed in /etc/passwd or /etc/group, or if these files are missing.
To activate the NSS module, add "systemd" to the lines starting with "passwd:" and "group:" in /etc/nsswitch.conf.
Here is an example /etc/nsswitch.conf file that enables nss-systemd correctly:
passwd: compat mymachines systemd group: compat mymachines systemd shadow: compat hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname networks: files protocols: db files services: db files ethers: db files rpc: db files netgroup: nis