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>Users and Groups in the Debian System</TITLE
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><DIV
CLASS="BOOK"
><A
NAME="USERS-AND-GROUPS"
></A
><DIV
CLASS="TITLEPAGE"
><H1
CLASS="TITLE"
><A
NAME="AEN2"
>Users and Groups in the Debian System</A
></H1
><H3
CLASS="AUTHOR"
><A
NAME="AEN6"
></A
>Joey Hess</H3
><H3
CLASS="AUTHOR"
><A
NAME="AEN10"
></A
>Colin Watson</H3
><H3
CLASS="AUTHOR"
><A
NAME="AEN14"
></A
>David Mandelberg</H3
><P
CLASS="COPYRIGHT"
>Copyright © 2001, 2002 Joey Hess</P
><P
CLASS="COPYRIGHT"
>Copyright © 2005 David Mandelberg</P
><P
CLASS="COPYRIGHT"
>Copyright © 2001-2022 Colin Watson</P
><DIV
CLASS="LEGALNOTICE"
><P
></P
><A
NAME="AEN28"
></A
><P
> This document is free; you can redistribute it and/or modify it
under the terms of version 2 of the GNU General Public License as
published by the Free Software Foundation.
</P
><P
> This document is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
</P
><P
> You should have received a copy of the GNU General Public License
along with this document; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
02110-1301, USA.
</P
><P
></P
></DIV
><HR></DIV
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
>1. <A
HREF="#INTRODUCTION"
>Introduction</A
></DT
><DT
>2. <A
HREF="#ENTRIES"
>Users and Groups</A
></DT
></DL
></DIV
><DIV
CLASS="CHAPTER"
><HR><H1
><A
NAME="INTRODUCTION"
></A
>Chapter 1. Introduction</H1
><P
> The Debian base-passwd package contains the master versions of
<TT
CLASS="FILENAME"
>/etc/passwd</TT
> and <TT
CLASS="FILENAME"
>/etc/group</TT
>.
The <B
CLASS="COMMAND"
>update-passwd</B
> tool keeps the entries in these
master files in sync on all Debian systems. They comprise only "global
static" ids: that is, those which are reserved globally for the
benefit of packages which need to include files owned by those users
or groups, or need the ids compiled into binaries. Since this
reservation is a serious restriction, these ids must be allocated by
the base-passwd maintainer on request. In general, packages should
avoid requesting such ids where possible and instead allocate system
users or groups dynamically. See Debian Policy for further details.
</P
><P
> The Debian Policy Manual reserves ranges for these global static users
and groups, but it makes no attempt to allocate individual numbers or
define their purposes. This document fills that gap by describing the
purposes of the individual entries in these master files.
</P
><P
> This is a work in progress. Items in need of feedback are marked with
the "HELP" tag. Please send mail to
<CODE
CLASS="EMAIL"
><<A
HREF="mailto:base-passwd@packages.debian.org"
>base-passwd@packages.debian.org</A
>></CODE
> or file a bug with the
Debian bug tracking system if you have more information.
</P
></DIV
><DIV
CLASS="CHAPTER"
><HR><H1
><A
NAME="ENTRIES"
></A
>Chapter 2. Users and Groups</H1
><P
> Many users have a corresponding group, and these pairs will be treated
together.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>root</DT
><DD
><P
> Root is (typically) the superuser.
</P
></DD
><DT
>daemon</DT
><DD
><P
> Some unprivileged daemons that need to be able to write to some
files on disk run as daemon.daemon (<B
CLASS="COMMAND"
>portmap</B
>,
<B
CLASS="COMMAND"
>atd</B
>, <B
CLASS="COMMAND"
>lambdamoo</B
>,
<B
CLASS="COMMAND"
>mon</B
>, 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.
</P
><P
> 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."
</P
></DD
><DT
>bin</DT
><DD
><P
> 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 <TT
CLASS="FILENAME"
>/bin</TT
>? It is not mentioned in
the FHS, Debian Policy, or the changelogs of base-passwd or
base-files.
</P
><P
> 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."
</P
></DD
><DT
>sys</DT
><DD
><P
> Historically, the sys user and group
<A
HREF="http://article.olduse.net/109@Autzoo.UUCP"
TARGET="_top"
>owned the
kernel sources and some related items like the include
files</A
>, but this was obsoleted long ago in favour of bin
(now itself legacy; see above).
</P
></DD
><DT
>sync</DT
><DD
><P
> The shell of user sync is <TT
CLASS="FILENAME"
>/bin/sync</TT
>. 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.
</P
></DD
><DT
>games</DT
><DD
><P
> Many games are sgid to games so they can write their high score
files. This is explained in Debian Policy.
</P
></DD
><DT
>man</DT
><DD
><P
> The <B
CLASS="COMMAND"
>man</B
> program (sometimes) runs as user man,
so it can write cat pages to <TT
CLASS="FILENAME"
>/var/cache/man</TT
>
and update its databases there.
</P
></DD
><DT
>lp</DT
><DD
><P
> The <TT
CLASS="FILENAME"
>lp*</TT
> 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.
</P
><P
> The <B
CLASS="COMMAND"
>lpr</B
> system keeps its spool directories
owned by lp/lp. Its daemon and frontend tools (through setuid)
run as user root.
</P
><P
> HELP: what do other print systems (<B
CLASS="COMMAND"
>rlpr</B
>,
<B
CLASS="COMMAND"
>lprng</B
>, ...) do?
</P
></DD
><DT
>mail</DT
><DD
><P
> Mailboxes in <TT
CLASS="FILENAME"
>/var/mail</TT
> 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.
</P
></DD
><DT
>news</DT
><DD
><P
> Various news servers and other associated programs (such as
<B
CLASS="COMMAND"
>suck</B
>) use user and group news in various
ways. Files in the news spool are often owned by user and group
news. Programs such as <B
CLASS="COMMAND"
>inews</B
> that can be used
to post news are typically sgid news.
</P
></DD
><DT
>uucp</DT
><DD
><P
> The uucp user and group is used by the UUCP subsystem. It owns
spool and configuration files. Users in the uucp group may run
<B
CLASS="COMMAND"
>uucico</B
>.
</P
></DD
><DT
>proxy</DT
><DD
><P
> 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
<B
CLASS="COMMAND"
>pdnsd</B
>, and <B
CLASS="COMMAND"
>squid</B
> runs as
user proxy.
</P
></DD
><DT
>majordom</DT
><DD
><P
> Majordomo has a statically allocated uid on Debian systems for
historical reasons. It is not installed on new systems.
</P
></DD
><DT
>postgres</DT
><DD
><P
> Postgresql databases are owned by this user and group.
</P
></DD
><DT
>www-data</DT
><DD
><P
> Some web servers run as www-data. Web content should
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> 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.
</P
></DD
><DT
>backup</DT
><DD
><P
> Presumably so backup/restore responsibilities can be locally
delegated to someone without full root permissions?
</P
><P
> HELP: Is that right? Amanda reportedly uses this, details?
</P
></DD
><DT
>operator</DT
><DD
><P
> 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 <B
CLASS="COMMAND"
>sudo</B
> 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.
</P
><P
> The operator group is used by <B
CLASS="COMMAND"
>dump -n</B
> to
notify logged-in operators via <B
CLASS="COMMAND"
>wall</B
> 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.)
</P
></DD
><DT
>list</DT
><DD
><P
> Mailing list archives and data are owned by this user and group.
Some mailing list programs may run as this user as well.
</P
></DD
><DT
>irc</DT
><DD
><P
> Used by IRC daemons. A statically allocated user is needed only
because of a bug in <B
CLASS="COMMAND"
>ircd</B
>: it
<CODE
CLASS="FUNCTION"
>setuid()</CODE
>s itself to a compiled-in user id
on startup.
</P
></DD
><DT
>gnats</DT
><DD
><P
> Used by <B
CLASS="COMMAND"
>gnats</B
>. This had a statically
allocated user and group for purely historical reasons (it could
equally well use a dynamic system user and group). Since
<B
CLASS="COMMAND"
>gnats</B
> has been removed from Debian, this user
and group are no longer created by default; they had uid/gid 41.
</P
></DD
><DT
>nobody, nogroup</DT
><DD
><P
> 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.
</P
><P
> (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.)
</P
><P
> If root-squashing is in use over NFS, root access from the
client is performed as user nobody on the server.
</P
></DD
><DT
>messagebus</DT
><DD
><P
> The dbus daemon (<B
CLASS="COMMAND"
>dbus-daemon-1</B
>) runs as this
user and group.
</P
></DD
><DT
>postfix</DT
><DD
><P
> Used by the <B
CLASS="COMMAND"
>postfix</B
> MTA.
</P
></DD
><DT
>gdm</DT
><DD
><P
> GDM (GNOME Display Manager) runs as this user/group.
</P
></DD
><DT
>saned</DT
><DD
><P
> Added by sane-utils, but appear to be unused.
</P
></DD
><DT
>klog</DT
><DD
><P
> Used by <B
CLASS="COMMAND"
>klogd</B
>, the kernel logger.
</P
><P
> </P
></DD
><DT
>syslog</DT
><DD
><P
> Used by syslog, the general purpose logger.
</P
></DD
></DL
></DIV
><P
> Other groups have no associated user.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>adm</DT
><DD
><P
> Group adm is used for system monitoring tasks. Members of this
group can read many log files in <TT
CLASS="FILENAME"
>/var/log</TT
>,
and can use <B
CLASS="COMMAND"
>xconsole</B
>.
</P
><P
> Historically, <TT
CLASS="FILENAME"
>/var/log</TT
> was
<TT
CLASS="FILENAME"
>/usr/adm</TT
> (and later
<TT
CLASS="FILENAME"
>/var/adm</TT
>), thus the name of the group.
</P
><P
> 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 ...
</P
></DD
><DT
>tty</DT
><DD
><P
> Tty devices and <TT
CLASS="FILENAME"
>/dev/vcs*</TT
> are owned by this
group. This is used by <B
CLASS="COMMAND"
>write</B
> and
<B
CLASS="COMMAND"
>wall</B
> to enable them to write to other
people's ttys.
</P
></DD
><DT
>disk</DT
><DD
><P
> Raw access to disks. Mostly equivalent to root access.
</P
><P
> HELP: Well, I have some disk devices in
<TT
CLASS="FILENAME"
>/dev</TT
> owned by the group, but I can't see
the point. On another system, I noticed that some of the files
<B
CLASS="COMMAND"
>lilo</B
> puts in <TT
CLASS="FILENAME"
>/boot</TT
> 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.
</P
></DD
><DT
>kmem</DT
><DD
><P
> <TT
CLASS="FILENAME"
>/dev/kmem</TT
> 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.
</P
></DD
><DT
>dialout</DT
><DD
><P
> Full and direct access to serial ports. Members of this group
can reconfigure the modem, dial anywhere, etc.
</P
></DD
><DT
>dip</DT
><DD
><P
> The group's name stands for "Dialup IP". Being in group dip
allows you to use tools such as <B
CLASS="COMMAND"
>pppd</B
>,
<B
CLASS="COMMAND"
>pon</B
>, and <B
CLASS="COMMAND"
>poff</B
> to make
dialup connections to other systems using predefined
configuration file(s) in the <TT
CLASS="FILENAME"
>/etc/ppp/peers</TT
>
directory.
</P
></DD
><DT
>fax</DT
><DD
><P
> Allows members to use fax software to send or receive faxes.
</P
></DD
><DT
>voice</DT
><DD
><P
> Voicemail, useful for systems that use modems as answering
machines.
</P
></DD
><DT
>cdrom</DT
><DD
><P
> This group can be used locally to give a set of users access to
a CD-ROM drive.
</P
></DD
><DT
>floppy</DT
><DD
><P
> This group can be used locally to give a set of users access to
a floppy drive.
</P
></DD
><DT
>tape</DT
><DD
><P
> This group can be used locally to give a set of users access to
a tape drive.
</P
></DD
><DT
>sudo</DT
><DD
><P
> Members of this group may run any command as any user when using
<B
CLASS="COMMAND"
>sudo</B
> or <B
CLASS="COMMAND"
>pkexec</B
> (from the
policykit-1 package, independently of whether the sudo package
is installed).
</P
></DD
><DT
>audio</DT
><DD
><P
> This group can be used locally to give a set of users access to
an audio device.
</P
></DD
><DT
>src</DT
><DD
><P
> This group owns source code, including files in
<TT
CLASS="FILENAME"
>/usr/src</TT
>. It can be used locally to give a
user the ability to manage system source code.
</P
><P
> HELP: <TT
CLASS="FILENAME"
>/usr/src</TT
> 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
<TT
CLASS="FILENAME"
>/usr/src</TT
> owned and writable by the group
(and files in tarballs dropped there likewise)?
</P
></DD
><DT
>shadow</DT
><DD
><P
> <TT
CLASS="FILENAME"
>/etc/shadow</TT
> is readable by this group. Some
programs that need to be able to access the file are setgid
shadow.
</P
></DD
><DT
>utmp</DT
><DD
><P
> This group can write to <TT
CLASS="FILENAME"
>/run/utmp</TT
>,
<TT
CLASS="FILENAME"
>/var/log/wtmp</TT
>,
<TT
CLASS="FILENAME"
>/var/log/lastlog</TT
>, and similar files.
Programs that need to be able to write to them (such as X
terminal emulators) are setgid utmp.
</P
></DD
><DT
>video</DT
><DD
><P
> This group can be used locally to give a set of users access to
a video device.
</P
></DD
><DT
>plugdev</DT
><DD
><P
> Members of this group can access removable devices in limited
ways without explicit configuration in
<TT
CLASS="FILENAME"
>/etc/fstab</TT
>. This is useful for local users
who expect to be able to insert and use CDs, USB drives, and so
on.
</P
><P
> Since <B
CLASS="COMMAND"
>pmount</B
> (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.
</P
></DD
><DT
>staff</DT
><DD
><P
> Allows users to add local modifications to the system
(<TT
CLASS="FILENAME"
>/usr/local</TT
>, <TT
CLASS="FILENAME"
>/home</TT
>)
without needing root privileges. Compare with group 'adm', which
is more related to monitoring/security.
</P
><P
> Note that the ability to modify <TT
CLASS="FILENAME"
>/usr/local</TT
>
is effectively equivalent to root access (since
<TT
CLASS="FILENAME"
>/usr/local</TT
> is intentionally on search paths
ahead of <TT
CLASS="FILENAME"
>/usr</TT
>), 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.
</P
></DD
><DT
>users</DT
><DD
><P
> 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.
</P
></DD
><DT
>lpadmin</DT
><DD
><P
> Allows a user to add, modify, and remove printers from foomatic,
cups, and possibly other printer databases.
</P
></DD
><DT
>sasl</DT
><DD
><P
> Users in this group have read/write access to
<TT
CLASS="FILENAME"
>/etc/sasldb</TT
> and/or
<TT
CLASS="FILENAME"
>/etc/sasldb2</TT
>, wich are used to
authentication with sasl. This is commonly used by IMAP, POP,
and SMTP servers for authentication.
</P
></DD
><DT
>scanner</DT
><DD
><P
> Users in this group can use scanner(s).
</P
></DD
><DT
>_ssh</DT
><DD
><P
> <B
CLASS="COMMAND"
>ssh-agent</B
> is setgid to _ssh in order to
prevent ptrace attacks.
</P
></DD
><DT
>libvirt</DT
><DD
><P
> 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.
</P
></DD
></DL
></DIV
><P
> Some users have no corresponding group.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>sshd</DT
><DD
><P
> Unprivileged user used by the privilege-separated
<B
CLASS="COMMAND"
>sshd</B
> when communicating with the network
before successful authentication.
</P
></DD
><DT
>fetchmail</DT
><DD
><P
> Used by the <B
CLASS="COMMAND"
>fetchmail</B
> program.
</P
></DD
><DT
>cupsys</DT
><DD
><P
> <ACRONYM
CLASS="ACRONYM"
>CUPS</ACRONYM
> (Common Un*x Printing System) runs as
this user. It is in group lp, so it can access printer devices.
</P
></DD
></DL
></DIV
></DIV
></DIV
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