To activate the NSS module, add "resolve" to the line starting with "hosts:" in /etc/nsswitch.conf. Specifically, it is recommended to place "resolve" early in /etc/nsswitch.conf's "hosts:" line (but after the "files" or "mymachines" entries), right before the "dns" entry if it exists, followed by "[!UNAVAIL=return]", to ensure DNS queries are always routed via systemd-resolved(8) if it is running, but are routed to nss-dns if this service is not available.
Note that systemd-resolved will synthesize DNS resource records in a few cases, for example for "localhost" and the current hostname, see systemd-resolved(8) for the full list. This duplicates the functionality of nss-myhostname(8), but it is still recommended (see examples below) to keep nss-myhostname configured in /etc/nsswitch.conf, to keep those names resolveable if systemd-resolved is not running.
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