Root is (typically) the superuser.

Some unprivileged daemons that need to be able to write to some files on disk run as daemon.daemon (portmap, atd, lambdamoo, mon, and others). Daemons that don't need to own any files sometimes run as nobody.nogroup instead; it is generally better practice to use a dedicated user, and more complex or security-conscious daemons certainly do this. The daemon user is also handy for locally installed daemons, probably.

LSB 1.3 lists daemon as legacy, and says: "The 'daemon' UID/GID was used as an unprivileged UID/GID for daemons to execute under in order to limit their access to the system. Generally daemons should now run under individual UID/GIDs in order to further partition daemons from one another."

HELP: No files on my system are owned by user or group bin. What good are they? Historically they were probably the owners of binaries in /bin? It is not mentioned in the FHS, Debian Policy, or the changelogs of base-passwd or base-files.

LSB 1.3 lists bin as legacy, and says: "The 'bin' UID/GID is included for compatibility with legacy applications. New applications should no longer use the 'bin' UID/GID."

Historically, the sys user and group owned the kernel sources and some related items like the include files, but this was obsoleted long ago in favour of bin (now itself legacy; see above).

The shell of user sync is /bin/sync. Thus, if its password is set to something easy to guess (such as ""), anyone can sync the system at the console even if they have no account on the system.

Many games are sgid to games so they can write their high score files. This is explained in Debian Policy.

The man program (sometimes) runs as user man, so it can write cat pages to /var/cache/man and update its databases there.

The lp* devices are writable by this group so that users in it can access the parallel ports directly. Traditionally this job is taken by a printer daemon instead which will only need to run in this group.

The lpr system keeps its spool directories owned by lp/lp. Its daemon and frontend tools (through setuid) run as user root.

HELP: what do other print systems (rlpr, lprng, ...) do?

Mailboxes in /var/mail are owned and writable by group mail, as is explained in Debian Policy. The user and group is used for other purposes as well by various MTAs and MUAs.

Various news servers and other associated programs (such as suck) use user and group news in various ways. Files in the news spool are often owned by user and group news. Programs such as inews that can be used to post news are typically sgid news.

The uucp user and group is used by the UUCP subsystem. It owns spool and configuration files. Users in the uucp group may run uucico.

Like daemon, this user and group is used by some daemons (specifically, proxy daemons) that don't have dedicated user ids and that need to own files. For example, group proxy is used by pdnsd, and squid runs as user proxy.

Majordomo has a statically allocated uid on Debian systems for historical reasons. It is not installed on new systems.

Postgresql databases are owned by this user and group.

Some web servers run as www-data. Web content should not be owned by this user, or a compromised web server would be able to rewrite a web site. Data written out by web servers will be owned by www-data.

Presumably so backup/restore responsibilities can be locally delegated to someone without full root permissions?

HELP: Is that right? Amanda reportedly uses this, details?

Historically, the operator user account was used by system operators with low privilege to dump filesystem backups to tape, and was in the root group so that it could do this. In Debian, the use of a utility such as sudo to gain privilege is preferred over such highly-special-purpose accounts, and the operator user is no longer created by default. It had uid 37.

The operator group is used by dump -n to notify logged-in operators via wall when it requires operator attention. This is a historical use, and new applications should not behave this way. (If nothing else, the group should be configurable.)

Mailing list archives and data are owned by this user and group. Some mailing list programs may run as this user as well.

Used by IRC daemons. A statically allocated user is needed only because of a bug in ircd: it setuid()s itself to a compiled-in user id on startup.

Used by gnats. This had a statically allocated user and group for purely historical reasons (it could equally well use a dynamic system user and group). Since gnats has been removed from Debian, this user and group are no longer created by default; they had uid/gid 41.

Daemons that need not own any files sometimes run as user nobody and group nogroup, although using a dedicated user is far preferable. Thus, no files on a system should be owned by this user or group.

(Technically speaking, it does no harm for a file to be owned by group nogroup as long as the ownership confers no additional privileges, that is if the group and other permission bits are equal. However, this is sloppy practice and should be avoided.)

If root-squashing is in use over NFS, root access from the client is performed as user nobody on the server.

The dbus daemon (dbus-daemon-1) runs as this user and group.

Used by the postfix MTA.

GDM (GNOME Display Manager) runs as this user/group.

Added by sane-utils, but appear to be unused.

Used by klogd, the kernel logger.

Used by syslog, the general purpose logger.

Group adm is used for system monitoring tasks. Members of this group can read many log files in /var/log, and can use xconsole.

Historically, /var/log was /usr/adm (and later /var/adm), thus the name of the group.

HELP: Perhaps policy should state the purpose of this group so users may be safely added to it, in certainty that all they'll be able to do is read logs. Wouldn't hurt to rename it 'log' either ...

Tty devices and /dev/vcs* are owned by this group. This is used by write and wall to enable them to write to other people's ttys.

Raw access to disks. Mostly equivalent to root access.

HELP: Well, I have some disk devices in /dev owned by the group, but I can't see the point. On another system, I noticed that some of the files lilo puts in /boot are also owned by disk. I can imagine local uses for such a group, like if you want to give some users in the group direct access to some hard disk. But these uses I've found on my systems seem to preclude doing that easily; if I put a user in group disk here, they'd have write access to the root filesystem.

/dev/kmem and similar files are readable by this group. This is mostly a BSD relic, but any programs that need direct read access to the system's memory can thus be made setgid kmem.

Full and direct access to serial ports. Members of this group can reconfigure the modem, dial anywhere, etc.

The group's name stands for "Dialup IP". Being in group dip allows you to use tools such as pppd, pon, and poff to make dialup connections to other systems using predefined configuration file(s) in the /etc/ppp/peers directory.

Allows members to use fax software to send or receive faxes.

Voicemail, useful for systems that use modems as answering machines.

This group can be used locally to give a set of users access to a CD-ROM drive.

This group can be used locally to give a set of users access to a floppy drive.

This group can be used locally to give a set of users access to a tape drive.

Members of this group may run any command as any user when using sudo or pkexec (from the policykit-1 package, independently of whether the sudo package is installed).

This group can be used locally to give a set of users access to an audio device.

This group owns source code, including files in /usr/src. It can be used locally to give a user the ability to manage system source code.

HELP: /usr/src is owned by group src and is setgid. This doesn't make files put there by foo-src packages necessarily be owned by group src though. If the intent is to make group src be able to manage source code, perhaps policy should say that foo-src packages make files in /usr/src owned and writable by the group (and files in tarballs dropped there likewise)?

/etc/shadow is readable by this group. Some programs that need to be able to access the file are setgid shadow.

This group can write to /run/utmp, /var/log/wtmp, /var/log/lastlog, and similar files. Programs that need to be able to write to them (such as X terminal emulators) are setgid utmp.

This group can be used locally to give a set of users access to a video device.

Members of this group can access removable devices in limited ways without explicit configuration in /etc/fstab. This is useful for local users who expect to be able to insert and use CDs, USB drives, and so on.

Since pmount (the original implementor of group plugdev) always mounts with the nodev and nosuid options and applies other checks, this group is not intended to be root-equivalent in the ways that the ability to mount filesystems might ordinarily allow. Implementors of semantics involving this group should be careful not to allow root-equivalence.

Allows users to add local modifications to the system (/usr/local, /home) without needing root privileges. Compare with group 'adm', which is more related to monitoring/security.

Note that the ability to modify /usr/local is effectively equivalent to root access (since /usr/local is intentionally on search paths ahead of /usr), and so you should only add trusted users to this group. Be careful in environments using NFS since acquiring another non-root user's privileges is often easier in such environments.

While Debian systems use the user-group system by default (each user has their own group), some prefer to use a more traditional group system. In that system, each user is a member of the 'users' group.

Allows a user to add, modify, and remove printers from foomatic, cups, and possibly other printer databases.

Users in this group have read/write access to /etc/sasldb and/or /etc/sasldb2, wich are used to authentication with sasl. This is commonly used by IMAP, POP, and SMTP servers for authentication.

Users in this group can use scanner(s).

ssh-agent is setgid to _ssh in order to prevent ptrace attacks.

Access to the system libvirt daemon is controlled by this group. Membership in this group gives full daemon access including (but not restricted to) managing virtual machines.

Unprivileged user used by the privilege-separated sshd when communicating with the network before successful authentication.

Used by the fetchmail program.

CUPS (Common Un*x Printing System) runs as this user. It is in group lp, so it can access printer devices.