NSS-SYSTEMD(8)

💖 nss-systemd — the manual, but make it make sense 💖

okay bestie, sit down, hydrate, because we're about to talk about nss-systemd (that's the page you're reading, section 8 of the manual, aka nss-systemd(8), aka the little library libnss_systemd.so.2). no cap, this one is actually so good.

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✨ NAME ✨

nss-systemd, libnss_systemd.so.2 — UNIX user and group name resolution for user/group lookup via Varlink. 💅

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🌸 SYNOPSIS 🌸

libnss_systemd.so.2

that's it. that's the whole synopsis. iconic, honestly. no flags, no arguments, just vibes.

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🦋 DESCRIPTION 🦋

So. nss-systemd is a plug-in module for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc). Its whole job? Providing UNIX user and group name resolution for services that implement the User/Group Record Lookup API via Varlink[1]. 👑

Who are those services, you ask? Bestie, the lineup is stacked:

• the system and service manager systemd(1) — for its DynamicUser= feature, see systemd.exec(5) for the deets
• systemd-homed.service(8)
• systemd-machined.service(8)

This module also makes sure that the root and nobody users and groups (i.e. the users/groups with the UIDs/GIDs 0 and 65534) stay resolvable at all times, even if they're not listed in /etc/passwd or /etc/group, or even if those files are straight-up missing. like, they will not ghost you. 🫶

This module preferably utilizes systemd-userdbd.service(8) for resolving users and groups, but also works without the service running. she's flexible, she's adaptable, she's the moment. ✨

To activate the NSS module, add "systemd" to the lines starting with "passwd:", "group:", "shadow:" and "gshadow:" in /etc/nsswitch.conf. easy.

💡 It is recommended to place "systemd" after the "files" entry of the /etc/nsswitch.conf lines so that /etc/passwd, /etc/group, /etc/shadow and /etc/gshadow based mappings take precedence. order matters, bestie, it always does.

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🎀 STATIC DROP-IN JSON USER/GROUP RECORDS 🎀

Besides user/group records acquired via the aforementioned Varlink IPC interfaces and the synthesized root and nobody accounts, this module also makes user and group accounts available to the system that are defined in static drop-in files in the /etc/userdb/, /run/userdb/, /run/host/userdb/ and /usr/lib/userdb/ directories. yes, four directories. no, she's not doing too much, she's doing exactly enough. 💅

This is a simple mechanism to provide static user and group records via JSON drop-in files. Such user records should be defined in the format described by the JSON User Records[2] specification and be placed in one of the aforementioned directories under a file name composed of the user name suffixed with .user, with a world-readable access mode.

A symlink named after the user record's UID formatted in decimal and suffixed with .user pointing to the primary record file should be created as well, in order to allow both lookups by username and by UID. both. she said both. 🔥

Privileged user record data (e.g. hashed UNIX passwords) may optionally be provided as well, in a pair of separate companion files with the .user-privileged suffix. The data should be stored in a regular file named after the user name, suffixed with .user-privileged, and a symlink pointing to it, named after the used numeric UID formatted in decimal with the same suffix.

🔒 These companion files should not be readable to anyone but root. keep them private, bestie.

Example:

-rw-r--r--. 1 root root  723 May 10 foobar.user
-rw-------. 1 root root  123 May 10 foobar.user-privileged
lrwxrwxrwx. 1 root root   19 May 10 4711.user -> foobar.user
lrwxrwxrwx. 1 root root   19 May 10 4711.user-privileged -> foobar.user-privileged

Similarly, group records following the format described in JSON Group Record[3] may be defined, using the file suffixes .group and .group-privileged. 🧚

The primary user/group record files (i.e. those with the .user and .group suffixes) should not contain the "privileged" section as described in the specifications. The privileged user/group record files (i.e. those with the .user-privileged and .group-privileged suffixes) should contain this section, exclusively. no overlap, no crossover, we keep it clean. 🧼

In addition to the two types of user record files and the two types of group record files there's a fifth type of file that may be placed in the searched directories: files that indicate membership of users in groups. Specifically, for every pair of user/group where the user shall be a member of a group a file named "username:groupname.membership" must be created — i.e. the textual UNIX user name, followed by a colon, followed by the textual UNIX group name, suffixed by .membership.

The contents of these files are currently not read, however it is recommended to create them containing an empty JSON object (i.e. "{}"). The mere existence of these files is enough to affect a user/group membership. If a program provides user and/or group record files in the searched directories, it must always also create such files, both for primary and auxiliary group memberships. like, always. non-negotiable. 🙈

⚠️ Note that static user/group records generally do not override conflicting records in /etc/passwd or /etc/group or other account databases. In fact, before dropping in these files a reasonable level of care should be taken to avoid user/group name and UID/GID conflicts. do your homework first, bestie, please.

The systemd-userdb-load-credentials.service service automatically runs at boot and installs these files from user records passed in via system credentials. See userdbctl(1) and systemd.system-credentials(7) for details. ✨

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🌈 CONFIGURATION IN /ETC/NSSWITCH.CONF 🌈

Here is an example /etc/nsswitch.conf file that enables nss-systemd correctly:

passwd:         files systemd
group:          files [SUCCESS=merge] systemd
shadow:         files systemd
gshadow:        files systemd

hosts:          mymachines resolve [!UNAVAIL=return] files myhostname dns
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis

copy it, paste it, slay it. 👑

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🔥 EXAMPLE: MAPPINGS PROVIDED BY SYSTEMD-MACHINED.SERVICE 🔥

The container "rawhide" is spawned using systemd-nspawn(1):

# systemd-nspawn -M rawhide --boot --network-veth --private-users=pick
Spawning container rawhide on /var/lib/machines/rawhide.
Selected user namespace base 20119552 and range 65536.
...

$ machinectl --max-addresses=3
MACHINE CLASS     SERVICE        OS     VERSION ADDRESSES
rawhide container systemd-nspawn fedora 30      169.254.40.164 fe80::94aa:3aff:fe7b:d4b9

$ getent passwd vu-rawhide-0 vu-rawhide-81
vu-rawhide-0:*:20119552:65534:vu-rawhide-0:/:/usr/sbin/nologin
vu-rawhide-81:*:20119633:65534:vu-rawhide-81:/:/usr/sbin/nologin

$ getent group vg-rawhide-0 vg-rawhide-81
vg-rawhide-0:*:20119552:
vg-rawhide-81:*:20119633:

$ ps -o user:15,pid,tty,command -e|grep '^vu-rawhide'
vu-rawhide-0      692 ?        /usr/lib/systemd/systemd
vu-rawhide-0      731 ?        /usr/lib/systemd/systemd-journald
vu-rawhide-192    734 ?        /usr/lib/systemd/systemd-networkd
vu-rawhide-193    738 ?        /usr/lib/systemd/systemd-resolved
vu-rawhide-0      742 ?        /usr/lib/systemd/systemd-logind
vu-rawhide-81     744 ?        /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
vu-rawhide-0      746 ?        /usr/sbin/sshd -D ...
vu-rawhide-0      752 ?        /usr/lib/systemd/systemd --user
vu-rawhide-0      753 ?        (sd-pam)
vu-rawhide-0     1628 ?        login -- zbyszek
vu-rawhide-1000  1630 ?        /usr/lib/systemd/systemd --user
vu-rawhide-1000  1631 ?        (sd-pam)
vu-rawhide-1000  1637 pts/8    -zsh

literally look at those UIDs pop into existence. that's the module doing her thing. 💅✨

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🫶 SEE ALSO 🫶

systemd(1), systemd.exec(5), nss-resolve(8), nss-myhostname(8), nss-mymachines(8), systemd-userdbd.service(8), systemd-homed.service(8), systemd-machined.service(8), userdbctl(1), systemd.system-credentials(7), nsswitch.conf(5), getent(1)

go read them, they're all friends of hers. 🦋

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📝 NOTES 📝

1. User/Group Record Lookup API via Varlink https://systemd.io/USER_GROUP_API
2. JSON User Records https://systemd.io/USER_RECORD
3. JSON Group Record https://systemd.io/GROUP_RECORD

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okay that's the whole thing, bestie, you ate that. this page is from systemd version 262.1, and it's filed under NSS-SYSTEMD(8) in the manual. go forth and resolve some users. 💖🌸🎀🦋👑