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>Users and Groups in the Debian System</TITLE
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CLASS="BOOK"
><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
>Copyright &copy; 2001, 2002, 2003, 2004, 2005, 2007 Joey Hess, Colin Watson, David Mandelberg<DIV
CLASS="LEGALNOTICE"
><A
NAME="AEN28"
></A
>	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.
 	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.
 	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.
 </DIV
><HR></DIV
><DIV
CLASS="TOC"
><DL
><DT
>Table of Contents</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
> The Debian base-passwd package contains the master versions of
 <TT
CLASS="FILENAME"
>/etc/passwd</TT
> and <TT
CLASS="FILENAME"
>/etc/group</TT
>.
 The update-passwd 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.
 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.
 This is a work in progress. Items in need of feedback are marked with
 the "HELP" tag. Please send mail to
 <CODE
CLASS="EMAIL"
>&#60;<A
HREF="mailto:base-passwd@packages.debian.org"
>base-passwd@packages.debian.org</A
>&#62;</CODE
> or file a bug with the
 Debian bug tracking system if you have more information.
 </DIV
><DIV
CLASS="CHAPTER"
><HR><H1
><A
NAME="ENTRIES"
></A
>Chapter 2. Users and Groups</H1
> Many users have a corresponding group, and these pairs will be treated
 together.
 <DIV
CLASS="VARIABLELIST"
><DL
><DT
>root</DT
><DD
>	 Root is (typically) the superuser.
	 </DD
><DT
>daemon</DT
><DD
>	 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."
	 </DD
><DT
>bin</DT
><DD
>	 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.
	 	 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."
	 </DD
><DT
>sys</DT
><DD
>	 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).
	 </DD
><DT
>sync</DT
><DD
>	 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.
	 </DD
><DT
>games</DT
><DD
>	 Many games are sgid to games so they can write their high score
	 files. This is explained in Debian Policy.
	 </DD
><DT
>man</DT
><DD
>	 The man 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.
	 </DD
><DT
>lp</DT
><DD
>	 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.
	 	 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?
	 </DD
><DT
>mail</DT
><DD
>	 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.
	 </DD
><DT
>news</DT
><DD
>	 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.
	 </DD
><DT
>uucp</DT
><DD
>	 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.
	 </DD
><DT
>proxy</DT
><DD
>	 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.
	 </DD
><DT
>majordom</DT
><DD
>	 Majordomo has a statically allocated uid on Debian systems for
	 historical reasons. It is not installed on new systems.
	 </DD
><DT
>postgres</DT
><DD
>	 Postgresql databases are owned by this user and group.
	 </DD
><DT
>www-data</DT
><DD
>	 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.
	 </DD
><DT
>backup</DT
><DD
>	 Presumably so backup/restore responsibilities can be locally
	 delegated to someone without full root permissions?
	 	 HELP: Is that right? Amanda reportedly uses this, details?
	 </DD
><DT
>operator</DT
><DD
>	 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.)
	 </DD
><DT
>list</DT
><DD
>	 Mailing list archives and data are owned by this user and group.
	 Some mailing list programs may run as this user as well.
	 </DD
><DT
>irc</DT
><DD
>	 Used by IRC daemons. A statically allocated user is needed only
	 because of a bug in ircd: it
	 <CODE
CLASS="FUNCTION"
>setuid()</CODE
>s itself to a compiled-in user id
	 on startup.
	 </DD
><DT
>gnats</DT
><DD
>	 Used by gnats. This has a statically
	 allocated user and group for purely historical reasons (it could
	 equally well use a dynamic system user and group), but it's
	 cumbersome to change now.
	 </DD
><DT
>nobody, nogroup</DT
><DD
>	 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.
	 </DD
><DT
>messagebus</DT
><DD
>	 The dbus daemon (dbus-daemon-1) runs as this
	 user and group.
	 </DD
><DT
>postfix</DT
><DD
>	 Used by the postfix MTA.
	 </DD
><DT
>gdm</DT
><DD
>	 GDM (GNOME Display Manager) runs as this user/group.
	 </DD
><DT
>saned</DT
><DD
>	 Added by sane-utils, but appear to be unused.
	 </DD
><DT
>klog</DT
><DD
>	 Used by klogd, the kernel logger.
	 	</DD
><DT
>syslog</DT
><DD
>	 Used by syslog, the general purpose logger.
	 </DD
></DL
></DIV
> Other groups have no associated user.
 <DIV
CLASS="VARIABLELIST"
><DL
><DT
>adm</DT
><DD
>	 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 xconsole.
	 	 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.
	 	 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 ...
	 </DD
><DT
>tty</DT
><DD
>	 Tty devices and <TT
CLASS="FILENAME"
>/dev/vcs*</TT
> are owned by this
	 group. This is used by write and
	 wall to enable them to write to other
	 people's ttys.
	 </DD
><DT
>disk</DT
><DD
>	 Raw access to disks. Mostly equivalent to root access.
	 	 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
	 lilo 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.
	 </DD
><DT
>kmem</DT
><DD
>	 <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.
	 </DD
><DT
>dialout</DT
><DD
>	 Full and direct access to serial ports. Members of this group
	 can reconfigure the modem, dial anywhere, etc.
	 </DD
><DT
>dip</DT
><DD
>	 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 <TT
CLASS="FILENAME"
>/etc/ppp/peers</TT
>
	 directory.
	 </DD
><DT
>fax</DT
><DD
>	 Allows members to use fax software to send or receive faxes.
	 </DD
><DT
>voice</DT
><DD
>	 Voicemail, useful for systems that use modems as answering
	 machines.
	 </DD
><DT
>cdrom</DT
><DD
>	 This group can be used locally to give a set of users access to
	 a CD-ROM drive.
	 </DD
><DT
>floppy</DT
><DD
>	 This group can be used locally to give a set of users access to
	 a floppy drive.
	 </DD
><DT
>tape</DT
><DD
>	 This group can be used locally to give a set of users access to
	 a tape drive.
	 </DD
><DT
>sudo</DT
><DD
>	 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).
	 </DD
><DT
>audio</DT
><DD
>	 This group can be used locally to give a set of users access to
	 an audio device.
	 </DD
><DT
>src</DT
><DD
>	 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.
	 	 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)?
	 </DD
><DT
>shadow</DT
><DD
>	 <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.
	 </DD
><DT
>utmp</DT
><DD
>	 This group can write to <TT
CLASS="FILENAME"
>/var/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.
	 </DD
><DT
>video</DT
><DD
>	 This group can be used locally to give a set of users access to
	 a video device.
	 </DD
><DT
>plugdev</DT
><DD
>	 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.
	 	 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.
	 </DD
><DT
>staff</DT
><DD
>	 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.
	 	 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.
	 </DD
><DT
>users</DT
><DD
>	 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.
	 </DD
><DT
>lpadmin</DT
><DD
>	 Allows a user to add, modify, and remove printers from foomatic,
	 cups, and possibly other printer databases.
	 </DD
><DT
>sasl</DT
><DD
>	 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.
	 </DD
><DT
>scanner</DT
><DD
>	 Users in this group can use scanner(s).
	 </DD
><DT
>ssh</DT
><DD
>	 ssh-agent is setgid to ssh in order to
	 prevent ptrace attacks.
	 </DD
></DL
></DIV
> Some users have no corresponding group.
 <DIV
CLASS="VARIABLELIST"
><DL
><DT
>sshd</DT
><DD
>	 Unprivileged user used by the privilege-separated
	 sshd when communicating with the network
	 before successful authentication.
	 </DD
><DT
>fetchmail</DT
><DD
>	 Used by the fetchmail program.
	 </DD
><DT
>cupsys</DT
><DD
>	 <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. 
	 </DD
></DL
></DIV
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