Central management of subordinate user and group ids

Subordinate ids are a Linux Kernel feature to grant a user additional user and group id ranges. Amongst others the feature can be used by container runtime engines to implement rootless containers. Traditionally subordinate id ranges are configured in /etc/subuid and /etc/subgid.

To make rootless containers in a large environment as easy as pie, IPA gains the ability to centrally manage and assign subordinate id ranges. SSSD and shadow-util are extended to read subordinate ids from IPA and provide them to userspace tools.

Overview

Feature requests

Man pages

Articles / blog posts

Design choices

Some design choices are owed to the circumstance that uids and gids are limited datatypes. The Linux Kernel and userland defines uid_t and gid_t as unsigned 32bit integers (uint32_t), which limits possible values for numeric user and group ids to 0 .. 2^32-2. (uid_t)-1 is reserved for error reporting. On the other hand the user nobody typically has uid 65534 / gid 65534. This means we need to assign 65,536 subordinate ids to every user. The theoretical maximum amount of subordinate ranges is less than 65,536 (65536 * 65536 == 2^32). ``logins.def` <https://man7.org/linux/man-pages/man5/login.defs.5.html>`_ also uses 65536 as default setting for SUB_UID_COUNT.

The practical limit is far smaller. Subordinate ids should not overlap with system accounts, local user accounts, IPA user accounts, and mapped accounts from Active Directory. Therefore IPA uses the upper half of the uid_t range (>= 2^31 == 2,147,483,648) for subordinate ids. The high bit is rarely used. IPA limits general numeric ids (uidNumber, gidNumber, ID ranges) to maximum values of signed 32bit integer (2^31-1) for backwards compatibility with XML-RPC. logins.def defaults to SUB_UID_MAX 600,100,000.

A default subordinate id count of 65,536 and a total range of approx. 2.1 billion limits IPA to slightly more than 32,000 possible ranges. It may sound like a lot of users, but there are much bigger installations of IPA. For comparison Fedora Accounts has over 120,000 users stored in IPA.

For that reason we treat subordinate id space as premium real estate and don’t auto-map or auto-assign subordinate ids by default. Instead we give the admin several options to assign them manually, semi-manual, or automatically.

Revision 1 limitation

The first revision of the feature is deliberately limited and restricted. We are aiming for a simple implementation that covers basic use cases. Some restrictions may be lifted in the future.

  • subuid and subgids cannot be set independently. They are always set to the same value.

  • counts are hard-coded to value 65536.

  • once assigned subids cannot be removed.

  • IPA does not support multiple subordinate id ranges, yet. Contrary to /etc/subuid, users are limited to one set of subordinate ids. The limitation is implemented with a unique index on owner reference and can be lifted in the future.

  • subids are auto-assigned from 389-DS DNA plugin (dnaInterval).

  • subids are allocated from hard-coded range [2147483648..4294901767] (2^31 to 2^32-1-65536), which is the upper 2.1 billion uids of uid_t (uint32_t). The range can hold little 32,767 subordinate id ranges.

  • Active Directory support is out of scope and may be provided in the future.

Finally, it is important to note that new installations come with a new idrange for subordinate ids. The new $REALM_subid_range entry uses range type ipa-ad-trust instead of range type ipa-local-subid for backwards compatibility with older SSSD clients, see SSSD #5571.

dn: cn=TESTREALM.TEST_subid_range,cn=ranges,cn=etc,dc=testrealm,dc=test
objectClass: top
objectClass: ipaIDrange
objectClass: ipaTrustedADDomainRange
cn: TESTREALM.TEST_subid_range
ipaBaseID: 2147483648
ipaIDRangeSize: 2147352576
ipaBaseRID: 2147283648
ipaNTTrustedDomainSID: S-1-5-21-738065-838566-1351619967
ipaRangeType: ipa-ad-trust

The additional idrange entry is defined as: cn=$REALM_subid_range,cn=ranges,cn=etc,dc=ipa,dc=test, which is visible with ipa idrange-find.

Subid assignment example

>>> import itertools
>>> def subids():
...     for n in itertools.count(start=0):
...         start = SUBID_RANGE_START + (n * SUBID_COUNT)
...         last = start + SUBID_COUNT - 1
...         yield (start, last)
...
>>> gen = subids()
>>> next(gen)
(2147483648, 2147549183)
>>> next(gen)
(2147549184, 2147614719)
>>> next(gen)
(2147614720, 2147680255)

The first user has 65565 subordinate ids from uid/gid 2147483648 to 2147549183, the next user has 2147549184 to 2147614719, and so on. The range count includes the start value.

An installation with multiple servers, 389-DS’ DNA plug-in takes care of delegating and assigning chunks of subid ranges to servers. The DNA plug-in guarantees uniqueness across servers.

LDAP

LDAP schema extension

The subordinate id feature introduces a new auxiliar object class ipaSubordinateId with five required attributes ipaOwner, ipaSubUidNumber, ipaSubUidCount, ipaSubGidNumber, and ipaSubGidCount. The attributes with number suffix store the start value of the interval. The count attributes contain the size of the interval including the start value. The maximum subid is ipaSubUidNumber + ipaSubUidCount - 1. The ipaOwner attribute is a reference to the owning user.

All count and number attributes are single-value INTEGER type with standard integer matching rules. OIDs 2.16.840.1.113730.3.8.23.8 and 2.16.840.1.113730.3.8.23.11 are reserved for future use.

attributeTypes: (
  2.16.840.1.113730.3.8.23.7
  NAME 'ipaSubUidNumber'
  DESC 'Numerical subordinate user ID (range start value)'
  EQUALITY integerMatch ORDERING integerOrderingMatch
  SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE
  X-ORIGIN 'IPA v4.9'
)
attributeTypes: (
  2.16.840.1.113730.3.8.23.8
  NAME 'ipaSubUidCount'
  DESC 'Subordinate user ID count (range size)'
  EQUALITY integerMatch ORDERING integerOrderingMatch
  SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE
  X-ORIGIN 'IPA v4.9'
)
attributeTypes: (
  2.16.840.1.113730.3.8.23.10
  NAME 'ipaSubGidNumber'
  DESC 'Numerical subordinate group ID (range start value)'
  EQUALITY integerMatch ORDERING integerOrderingMatch
  SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE
  X-ORIGIN 'IPA v4.9'
)
attributeTypes: (
  2.16.840.1.113730.3.8.23.11
  NAME 'ipaSubGidCount'
  DESC 'Subordinate group ID count (range size)'
  EQUALITY integerMatch ORDERING integerOrderingMatch
  SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE
  X-ORIGIN 'IPA v4.9'
)

The ipaOwner attribute is a single-value DN attribute that refers to user entry that owns the subordinate ID entry. The proposal does not reuse any of the existing attributes like owner or member, because they are all multi-valued.

attributeTypes: (
  2.16.840.1.113730.3.8.23.13
  NAME 'ipaOwner'
  DESC 'Owner of an entry'
  SUP distinguishedName
  EQUALITY distinguishedNameMatch
  SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
  SINGLE-VALUE
  X-ORIGIN 'IPA v4.9')

The ipaSubordinateId object class is an auxiliary subclass of top and requires all four subordinate id attributes as well as ipaOwner attribute``

objectClasses: (
  2.16.840.1.113730.3.8.24.4
  NAME 'ipaSubordinateId'
  DESC 'Subordinate uid and gid for users'
  SUP top
  AUXILIARY
  MUST ( ipaOwner $ ipaSubUidNumber $ ipaSubUidCount $ ipaSubGidNumber $ ipaSubGidCount )
  X-ORIGIN 'IPA v4.9'
)

The ipaSubordinateGid and ipaSubordinateUid are defined for future use. IPA always assumes the presence of ipaSubordinateId.

objectClasses: (
  2.16.840.1.113730.3.8.24.2
  NAME 'ipaSubordinateUid'
  DESC 'Subordinate uids for users, see subuid(5)'
  SUP top
  AUXILIARY
  MUST ( ipaOwner $ ipaSubUidNumber $ ipaSubUidCount )
  X-ORIGIN 'IPA v4.9'
 )
objectClasses: (
  2.16.840.1.113730.3.8.24.3
  NAME 'ipaSubordinateGid'
  DESC 'Subordinate gids for users, see subgid(5)'
  SUP top
  AUXILIARY
  MUST ( ipaOwner $ ipaSubGidNumber $ ipaSubGidCount )
  X-ORIGIN 'IPA v4.9'
)

Subordinate id entries have the structural object class ipaSubordinateIdEntry and one or more of the auxiliary object classes ipaSubordinateId, ipaSubordinateGid, or ipaSubordinateUid. ipaUniqueId is used as a primary key (RDN).

objectClasses: (
  2.16.840.1.113730.3.8.24.5
  NAME 'ipaSubordinateIdEntry'
  DESC 'Subordinate uid and gid entry'
  SUP top
  STRUCTURAL
  MUST ( ipaUniqueId ) MAY ( description ) X-ORIGIN 'IPA v4.9'
)

Finally, setting ipaUserDefaultSubordinateId to TRUE will cause new user entries to gain subordinate id by default:

attributeTypes: (
  2.16.840.1.113730.3.8.23.14
  NAME 'ipaUserDefaultSubordinateId'
  DESC 'Enable adding user entries with subordinate id'
  SYNTAX 1.3.6.1.4.1.1466.115.121.1.7
  SINGLE-VALUE
  X-ORIGIN 'IPA v4.9'
)

cn=subids,cn=accounts,$SUFFIX

Subordinate ids and ACIs are stored in the new subtree cn=subids,cn=accounts,$SUFFIX.

The following is an example of a subordinate id entry:

dn: ipauniqueid=8a7c0808-375b-4643-9582-e9c851a4b45d,cn=subids,cn=accounts,dc=
 testrealm,dc=test
ipaOwner: uid=asmith-auto,cn=users,cn=accounts,dc=testrealm,dc=test
ipaUniqueID: 8a7c0808-375b-4643-9582-e9c851a4b45d
description: auto-assigned subid
objectClass: ipasubordinateidentry
objectClass: ipasubordinateuid
objectClass: ipasubordinateid
objectClass: ipasubordinategid
objectClass: top
ipaSubUidCount: 65536
ipaSubGidCount: 65536
ipaSubUidNumber: 2147745792
ipaSubGidNumber: 2147745792

where as can be seen, it shows the link to the ipa user asmith-auto throught the ipaOwner attribute:

ipaOwner: uid=asmith-auto,cn=users,cn=accounts,dc=testrealm,dc=test

Index, integrity, memberOf

The attributes ipaSubUidNumber and ipaSubGidNumber are index for pres and eq with nsMatchingRule: integerOrderingMatch to enable efficient =, >=, and <= searches.

The attribute ipaOwner is indexed for pres and eq. This DN attribute is also checked for referential integrity and uniqueness within the cn=subids,cn=accounts,$SUFFIX subtree. The memberOf plugin creates back-references for ipaOwner references.

Distributed numeric assignment (DNA) plug-in extension

Subordinate id auto-assignment requires an extension of 389-DS’ DNA plug-in. The DNA plug-in is responsible for safely assigning unique numeric ids across all replicas.

A new option dnaInterval will tell the DNA plug-in to use the value of entry attributes as interval size.

IPA plugins and commands

The config plugin has a new option to enable or disable generation of subordinate id entries for new users:

$ ipa config-mod --user-default-subid=true

Subordinate ids are managed by a new plugin class. The subid-add and subid-del commands are hidden from command line. New subordinate ids are generated and auto-assigned with subid-generate.

$ ipa help subid
Topic commands:
  subid-find      Search for subordinate id.
  subid-generate  Generate and auto-assign subuid and subgid range to user entry
  subid-match     Match users by any subordinate uid in their range
  subid-mod       Modify a subordinate id.
  subid-show      Display information about a subordinate id.
  subid-stats     Subordinate id statistics
$ ipa subid-generate --owner testuser
-----------------------------------------------------------
Added subordinate id "2cf2c0db-aa9d-45d8-acbd-118aba0c1db3"
-----------------------------------------------------------
  Unique ID: 2cf2c0db-aa9d-45d8-acbd-118aba0c1db3
  Description: auto-assigned subid
  Owner: testuser
  SubUID range start: 2147680256
  SubUID range size: 65536
  SubGID range start: 2147680256
  SubGID range size: 65536
$ ipa subid-find --owner testuser
------------------------
1 subordinate id matched
------------------------
  Unique ID: 2cf2c0db-aa9d-45d8-acbd-118aba0c1db3
  Owner: testuser
  SubUID range start: 2147680256
  SubUID range size: 65536
  SubGID range start: 2147680256
  SubGID range size: 65536
----------------------------
Number of entries returned 1
----------------------------
$ ipa -vv subid-stats
...
ipa: INFO: Response: {
    "error": null,
    "id": 0,
    "principal": "admin@TESTREALM.TEST",
    "result": {
        "result": {
            "assigned_subids": 1,
            "baseid": 2147483648,
            "dna_remaining": 65527,
            "rangesize": 2147352576,
            "remaining_subids": 0
        },
        "summary": "0 remaining subordinate id ranges"
    },
    "version": "4.9.9"
}
---------------------------------
0 remaining subordinate id ranges
---------------------------------

Permissions, Privileges, Roles

Self-servive RBAC

The self-service permission enables users to request auto-assignment of subordinate uid and gid ranges for themselves. Subordinate ids cannot be modified or deleted.

  • ACI: selfservice: Add subordinate id

  • Permission: Self-service subordinate ID

  • Privilege: Subordinate ID Selfservice User

  • Role: Subordinate ID Selfservice Users

  • role default member: n/a

Administrator RBAC

The administrator permission allows privileged users to auto-assign subordinate ids to users. Once assigned subordinate ids cannot be modified or deleted.

  • ACI: Add subordinate ids to any user

  • Permission: Manage subordinate ID

  • Privilege: Subordinate ID Administrators

  • default privilege role: User Administrator

Managed permissions

  • System: Read Subordinate Id Attributes (all authenticated users)

  • System: Read Subordinate Id Count (all authenticated usrs)

  • System: Manage Subordinate Ids (User Administrators)

  • System: Remove Subordinate Ids (User Administrators)

Workflows

In the default configuration of IPA, neither existing users nor new users will have subordinate ids assigned. There are a couple of ways to assign subordinate ids to users.

User administrator

Users with User Administrator role and members of the admins group have permission to auto-assign new subordinate ids to any user. Auto assignment can be performed with new subid-generate command on the command line or with the Auto assign subordinate ids action in the Actions drop-down menu in the web UI.

$ ipa subid-generate --owner myusername

Self-service for group members

Ordinary users cannot self-service subordinate ids by default. Admins can assign the new Subordinate ID Selfservice User to users group to enable self-service for members of the group.

For example to enable self-service for all members of the default user group ipausers, do:

$ ipa role-add-member "Subordinate ID Selfservice User" --groups=ipausers

This allows members of ipausers to request subordinate ids with the subid-generate command or the Auto assign subordinate ids action in the web UI. The command picks the name of the current user principal automatically.

$ ipa subid-generate

ipa-subid tool

Finally IPA includes a new tool for mass-assignment of subordinate ids. The command uses automatic LDAPI EXTERNAL bind when it’s executed as root user. Other it requires valid Kerberos TGT of an admin or user administrator.

# /usr/libexec/ipa/ipa-subids --help
Usage: ipa-subids

Mass-assign subordinate ids

Options:
  --version             show program's version number and exit
  -h, --help            show this help message and exit
  --group=GROUP         Filter by group membership
  --filter=USER_FILTER  Raw LDAP filter
  --dry-run             Dry run mode.
  --all-users           All users

  Logging and output options:
    -v, --verbose       print debugging information
    -d, --debug         alias for --verbose (deprecated)
    -q, --quiet         output only errors
    --log-file=FILE     log to the given file

# /usr/libexec/ipa/ipa-subids --group ipausers
Processing user 'testsubordinated1' (1/15)
Processing user 'testsubordinated2' (2/15)
Processing user 'testsubordinated3' (3/15)
Processing user 'testsubordinated4' (4/15)
Processing user 'testsubordinated5' (5/15)
Processing user 'testsubordinated6' (6/15)
Processing user 'testsubordinated7' (7/15)
Processing user 'testsubordinated8' (8/15)
Processing user 'testsubordinated9' (9/15)
Processing user 'testsubordinated10' (10/15)
Processing user 'testsubordinated11' (11/15)
Processing user 'testsubordinated12' (12/15)
Processing user 'testsubordinated13' (13/15)
Processing user 'testsubordinated14' (14/15)
Processing user 'testsubordinated15' (15/15)
Processed 15 user(s)
The ipa-subids command was successful

Find and match users by any subordinate id

The user-find command is now able to search for users matching a particular subordinate id when using --in-subids option:

$ ipa user-find --in-subids=8a7c0808-375b-4643-9582-e9c851a4b45d
--------------
1 user matched
--------------
  User login: asmith-auto
  First name: asmith
  Last name: auto
  Home directory: /home/asmith-auto
  Login shell: /bin/sh
  Principal name: asmith-auto@TESTREALM.TEST
  Principal alias: asmith-auto@TESTREALM.TEST
  Email address: asmith-auto@testrealm.test
  UID: 1242200006
  GID: 1242200006
  Account disabled: False
----------------------------
Number of entries returned 1
----------------------------

and search for users not matching a particular subordinate id when using --not-in-subids option:

$ ipa user-find --not-in-subids=8a7c0808-375b-4643-9582-e9c851a4b45d
--------------
1 user matched
--------------
  User login: bjones-auto
  First name: bjones
  Last name: auto
  Home directory: /home/bjones-auto
  Login shell: /bin/sh
  Principal name: bjones-auto@TESTREALM.TEST
  Principal alias: bjones-auto@TESTREALM.TEST
  Email address: bjones-auto@testrealm.test
  UID: 1242200007
  GID: 1242200007
  Account disabled: False

Additionally, the new command subid-match can be used to find a user by any subordinate id in their range when using the --subuid option:

$ ipa subid-match --subuid=2147745792
------------------------
1 subordinate id matched
------------------------
  Unique ID: 8a7c0808-375b-4643-9582-e9c851a4b45d
  Owner: asmith-auto
  SubUID range start: 2147745792
  SubUID range size: 65536
  SubGID range start: 2147745792
  SubGID range size: 65536
----------------------------
Number of entries returned 1
----------------------------

SSSD integration

  • search base: cn=subids,cn=accounts,$SUFFIX

  • scope: SCOPE_ONELEVEL (1)

  • filter: (objectClass=ipaSubordinateId)

  • attributes: ipaOwner ipaSubUidNumber ipaSubUidCount ipaSubGidNumber ipaSubGidCount

The attribute ipaSubUidNumber is always accompanied by ipaSubUidCount and ipaSubGidNumber is always accompanied by ipaSubGidCount. In revision 1 the presence of ipaSubUidNumber implies presence of the other three attributes. All four subordinate id attributes are single-value INTEGER types. Any value outside of range of uint32_t must treated as invalid. SSSD will never see the DNA magic value -1 in cn=accounts,$SUFFIX subtree. In revision 1 each user subordinate id entry is assigned to exactly one user and each user has either 0 or 1 subid.

IPA recommends that SSSD uses LDAP deref controls for ipaOwner attribute to fetch uidNumber from the user object.

Deref control example

$ ldapsearch -L -E '!deref=ipaOwner:uid,uidNumber' \
      -b 'cn=subids,cn=accounts,dc=ipasubid,dc=test' \
      '(ipaOwner=uid=testuser10,cn=users,cn=accounts,dc=ipasubid,dc=test)'

dn: ipauniqueid=35c02c93-3799-4551-a355-ebbf042e431c,cn=subids,cn=accounts,dc=ipasubid,dc=test
# control: 1.3.6.1.4.1.4203.666.5.16 false MIQAAABxMIQAAABrBAhpcGFPd25lcgQ3dWlk
 PXRlc3R1c2VyMTAsY249dXNlcnMsY249YWNjb3VudHMsZGM9aXBhc3ViaWQsZGM9dGVzdKCEAAAAIj
 CEAAAAHAQJdWlkTnVtYmVyMYQAAAALBAk2MjgwMDAwMTE=
# ipaOwner: <uidNumber=628000011>;uid=testuser10,cn=users,cn=accounts,dc=ipasubid,dc=test

ipaOwner: uid=testuser10,cn=users,cn=accounts,dc=ipasubid,dc=test
ipaUniqueID: 35c02c93-3799-4551-a355-ebbf042e431c
description: auto-assigned subid
objectClass: ipasubordinateidentry
objectClass: ipasubordinategid
objectClass: ipasubordinateuid
objectClass: ipasubordinateid
objectClass: top
ipaSubUidNumber: 3434020864
ipaSubGidNumber: 3434020864
ipaSubUidCount: 65536
ipaSubGidCount: 65536

Configure subid managed at IPA level

Starting in FreeIPA 4.10.0 release, ipa-client-install provides a new option that allows to configure the client subid managed at IPA level. This way, ipa-client-install can configure the sssd profile and customize /etc/nsswitch.conf in such a way the subid database relies on IPA instead of both of the local files /etc/subuid and /etc/subgid.

It is important to note that the default behavior remains unchanged and --subid option must be provided to the client, server and replica installers in order to have SSSD setup as a datasource for subid in /etc/nsswitch.conf

Implementation details

  • ipaSubordinateId object class does not subclass the other two object classes. LDAP supports SUP ( ipaSubordinateGid $ ipaSubordinateUid ) but 389-DS only auto-inherits from first object class.

  • The idrange entry $REALM_subid_range has preconfigured base RIDs and SID so idrange plug-in and sidgen task ignore the entry. It’s the simplest approach to ensure backwards compatibility with older IPA server versions that don’t know how to handle the new range. The SID is S-1-5-21-738065-838566-$DOMAIN_HASH. S-1-5-21 is the well-known SID prefix for domain SIDs. 738065-838566 is the decimal representation of the string IPA-SUB. DOMAIN_HASH is the MURMUR-3 hash of the domain name for key 0xdeadbeef. SSSD rejects SIDs unless they are prefixed with S-1-5-21 (see sss_idmap.c:is_domain_sid()).

  • The new $REALM_subid_range entry uses range type ipa-ad-trust instead of range type ipa-local-subid for backwards compatibility with older SSSD clients, see SSSD #5571.

  • Shared DNA configuration entries in cn=dna,cn=ipa,cn=etc,$SUFFIX are automatically removed by existing code. Server and replication plug-ins search and delete entries by dnaHostname attribute.

  • ipaSubordinateId entries no longer contains uidNumber attribute. I considered to use CoS plugin to provide uidNumber as virtual attribute. However it’s not possible to objectClass: cosIndirectDefinition with cosIndirectSpecifier: ipaOwner and cosAttribute: uidNumber override for the task. Indexes and searches don’t work with virtual attributes.

TODO

  • add custom range type for idranges and teach AD trust, sidgen, and range overlap check code to deal with new range type.

  • ipa subid-stats should include assigned_subids value as output, since the remaning_subids value is always seen as 0, its better to display assigned_subids as output for #ipa subid-stats which will be more useful (RFE#2063176)

  • user-del –preserve and user-undel. Preserving a user with ipa user-del --preserve is possible however it deletes the associated subid, meaning that ipa user-undel <ipauser> is not able to restore the subid of the deleted user (RFE#2063168).