UID/GID allocation

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The departmental Unix LDAP servers (ldap-serv1.cl.cam.ac.uk, etc.) export lists of users and groups, equivalent to the local Unix files /etc/passwd and /etc/group. This page documents a policy agreed on 2015-07-09 for how in future the numerical user and group IDs exported by the departmental Unix LDAP servers should be allocated.

One important goal of this policy is to ensure that self-managed machines can import the user and group tables offered by the departmental LDAP servers without risking collisions with entries defined locally on the client machine by the operating system or the user.

Rules and considerations

  1. The LDAP servers should not export any numeric UID or GID with a value below 1100.
    • This mainly helps to avoid numeric collisions with UID and GID allocations by operating systems, which can cover the whole range 0-999.
    • This helps to avoid collisions with locally created users (e.g., family members on a private laptop), which some operating systems allocate starting at 1000 upwards.
  2. The LDAP servers should also not export any numeric UID or GID of -2, -1, 32767, 65534, or 65535.
    • This helps to avoid collisions with the POSIX return value -1 of some related system calls, as well as with the entries "nobody" and "nogroup" used on some systems.
  3. Departmental numeric UID and GID allocations should preferably only use 4-digit decimal numbers, at least until the available space there is exhausted.
    • this leaves numbers 10000- available for temporary local use, such as remapping of UIDs/GIDs by Linux containers by adding an integer multiple of 10000 for each container namespace
    • Unix tools typically display UID/GID values as decimal numbers and therefore the structure of the number space should be easy to recognize in decimal
    • 4-digit just looks neater in "id", etc. than a mix of 4- and 5-digit numbers
    • 4-digit UIDs are going to last at least until the year 2050 at current growth rates (~150 new entries every year, e.g. October 2009: max_uid=2659, October 2014: max_uid=3365).
  4. UID and GID allocations should be grouped together by type (regular users, pseudo-users, etc.)
    • This is mostly for the benefit of users who see lists of UIDs and GIDs sorted by numeric value, e.g. with the Unix "id" command or in administrative tools and tables.
  5. For each user name and numeric UID, the corresponding identical group name and numeric GID is reserved for the respective personal group.
    • Personal groups have the eponymous user as their sole member.
  6. Names of users and groups should be chosen to avoid likely collisions with operating-system allocations
  7. Files stored on the departmental file space should avoid using any numeric UID or GID value below 1100 or above 9999
    • the meaning of allocations in this range can differ between NFS client machines.

UID range allocation

  • 0000-0999: not used in LDAP, reserved for OS-specific system entries or client-specific user entries
  • 1000-1099: not used in LDAP, reserved for client-specific user entries
  • 1100-8999: departmental users (real people as CRSId)
  • 9000-9499: departmental pseudo users (role accounts, daemon accounts, group accounts, etc.)
  • 9500-9999: not used, to avoid collisions of corresponding personal groups with departmental groups
  • 10000-: not used in LDAP, reserved for client-specific temporary allocations (e.g., Linux container namespace remapping)

GID range allocation

  • 0000-0999: not used by LDAP, reserved for OS-specific entries or personal groups of client-specific user entries
  • 1000-1099: not used in LDAP, reserved for personal groups of client-specific user entries
  • 1100-9499: departmental personal groups associated with the corresponding UID name and value
  • 9500-9999: departmental groups
  • 10000-: not used in LDAP, reserved for client-specific temporary allocations (e.g., Linux container namespace remapping)

Migration of legacy entries

As of 2014-09-10, there are still 53 UIDs and 103 GIDs allocated in the LDAP tables in the range 0-1099. They should all be moved to higher 4-digit positions eventually, according to the new scheme. There is also one regular user at 15384. Some cause more problems than others, or are easier to move than others, and should therefore be prioritized.

  • existing departmental groups in the range 500-999 can be moved to 9500-9999 by adding 9000 to their GID
  • existing departmental pseudo-users (and associated personal groups) in the range 500-999 can be moved to 9000-9499 by adding 8500 to their UID/GID:
weather:501:dtg weather data gathering user           -> 9001
dtg-backup:502:dtg data backup user                   -> 9002
qosmos:503:qosmos box file transfer user              -> 9003
wwwsvn:505:WWW SVN user id                            -> 9005
nprobe:506:nprobe pseudo-user                         -> 9006
dtg-time:507:DTG TIME project                         -> 9007
apache-fp:509:apache fp                               -> 9009
wwwupdate:510:WWW updater                             -> 9010
ivc:511:ivc pseudo user                               -> 9011
ucard:512:University Card Office                      -> 9012
sec-repos:519:Security group svn repository           -> 9019
ugprac:561:Undergraduate Practicals                   -> 9061
energy:591:Energy logs from monitors                  -> 9091
keytab:596:keytab                                     -> 9096
  • particular care is needed where existing departmental groups collide numerically with existing departmental pseudo-users and their personal groups:
507: dtg-time = lt-video
  • existing departmental regular users outside this allocation scheme should be moved to new 4-digit locations that would otherwise be allocated to the next new users, with particular priority to be be given to those in the range 101-128 (which is densely populated by Linux system users):
maj1:101:Martyn Johnson
pb22:104:Piete Brooks
acn1:107:Arthur Norman
mjcg:110:Mike Gordon
pr10:111:Peter Robinson
jf15:128:Jon Fairbairn
lp15:138:Larry Paulson
ceb4:145:Caroline Blackmun
gt19:178:Graham Titmus
iml1:243:Ian Leslie
gw104:244:Glynn Winskel
ah12:260:Andy Hopper
am21:300:Alan Mycroft
fhk1:301:Frank King
mr10:302:Martin Richards
drm10:336:Derek McAuley
jmb25:341:Jean Bacon
rf10:344:Robin Fairbairns
ejb1:412:Ted Briscoe
aac10:432:Ann Copestake
djg11:1004:David Greaves
km10:1077:Ken Moody
sqlcache:10001:Linux cache of SQL server data
visitor1:11105:Visitor account 1
visitor2:11106:Visitor account 2
visitor3:11107:Visitor account 3
visitor4:11108:Visitor account 4
visitor5:11109:Visitor account 5
tlh20:15384:Tim Harris
Moving Linux desktop users with UID < 500 will immediately benefit them by resolving an annoying XDM malfunction on Ubuntu 14.04: they will finally be classified as human users and will therefore be able to login comfortably, as they will no longer be excluded from those offered for mouse selection in the login GUI.
See also Moving the UID/GID of a user.
  • Some active pseudo-users are currently located among the regular users:
visitor6:3225:Visitor account 6
visitor7:3226:Visitor account 7
visitor8:3227:Visitor account 8
visitor9:3228:Visitor account 9
visitor0:3229:Visitor account 0
  • Some departmental groups are currently located among the personal groups of regular users, which will soon cause problems when adding personal groups for future new users:
WARNING: The current (2015-04-29) highest regular user is has a UID of 3487. Once that number reaches 3600, the personal group of new users will collide with the dtg-* groups!

Background information

Linux conventions

The Linux Standard Base Core Specification specifies that UID values in the range 0 to 99 should be statically allocated by the system, and shall not be created by applications, while UIDs from 100 to 499 should be reserved for dynamic allocation by system administrators and post install scripts. See also LSB 3.0, Section 9.3: UID Ranges.

In practice, Linux distributions start allocating local regular user IDs from either 500 (Red Hat) or 1000 (SUSE, Debian). On many Linux systems, these ranges are specified in /etc/login.defs, for useradd and similar tools.

Some tools distinguish between real users and system pseudo-users based on the UID range. On Ubuntu, the LightDM display manager configuration file /etc/lightdm/users.conf contains per default the line “minimum-uid=500” and users with a lower UID are not shown in the login-screen greeter. However, if AccountsService is installed, lightdm [ allegedly] uses its definition instead, which appears to be configured at compile time.

Mac OS X conventions

Mac OS X allocates locally created users from 500 upwards and uses the UIDs and GIDs 0-499 for the operating system.

FreeBSD conventions

For both UIDs and GIDs, the range 0-49 is reserved for core OS allocations, and the range 50-999 can be allocated by package porters for use by specific software packages in the files ports/UIDs and ports/GIDs. These ranges are already quite densely populated. See also FreeBSD Porter's Handbook, Section 6.26: Adding Users and Groups.

Personal groups

For each user, a corresponding group can be created that has the same name and numeric identifier, known as the personal group. Such personal groups have no other members and make collaboration with other users in shared directories easier, by allowing users to habitually work with umask 0002. This way, newly created files can have by default write permissions enabled for group members, because this will normally only enable write access for members of the personal group, that is only for the file's owner. However, if a file is created in a shared directory that belongs to another group and has the setgid bit set, then the created file will automatically become writeable to members of that directory's group as well.

On many Linux systems, the USERGROUPS_ENAB variable in /etc/login.defs controls whether commands like useradd or userdel automatically add or delete an associated personal group.

Reviewing tools

List and test UIDs from LDAP server:

$ /homes/mgk25/proj/filer/ldap_uids.pl

List and test GIDs from LDAP server:

$ /homes/mgk25/proj/filer/ldap_gids.pl

(Requires Ubuntu package libnet-ldap-perl)

See also