# 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 engies 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 * [FreeIPA feature request #8361](https://pagure.io/freeipa/issue/8361) * [SSSD feature request #5197](https://github.com/SSSD/sssd/issues/5197) * [shadow-util feature request #154](https://github.com/shadow-maint/shadow/issues/154) * [389-DS RFE for DNA plugin rhbz#1938239](https://bugzilla.redhat.com/show_bug.cgi?id=1938239) Man pages * [man subuid(5)](https://man7.org/linux/man-pages/man5/subuid.5.html) * [man subgid(5)](https://man7.org/linux/man-pages/man5/subgid.5.html) * [man user_namespaces(7)](https://man7.org/linux/man-pages/man7/user_namespaces.7.html) * [man newuidmap(1)](https://man7.org/linux/man-pages/man1/newuidmap.1.html) Articles / blog posts * [Basic Setup and Use of Podman in a Rootless environment](https://github.com/containers/podman/blob/master/docs/tutorials/rootless_tutorial.md) * [How does rootless Podman work](https://opensource.com/article/19/2/how-does-rootless-podman-work) ## 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. Contrary to ``/etc/subuid``, users are limited to one set of subordinate ids. * subids are auto-assigned. Auto-assignment is currently emulated until 389-DS has been extended to support DNA with step interval. * 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. ### 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](https://directory.fedoraproject.org/docs/389ds/design/dna-plugin.html) 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 four required attributes ``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``. All four 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. ```raw attributeTypes: ( 2.16.840.1.113730.3.8.23.6 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: ( 2.16.840.1.113730.3.8.23.7 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: ( 2.16.840.1.113730.3.8.23.9 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: ( 2.16.840.1.113730.3.8.23.10 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 ``ipaSubordinateId`` object class is an auxiliar subclass of ``top`` and requires all four subordinate id attributes as well as ``uidNumber``. It does not subclass ``posixAccount`` to make the class reusable in idview overrides later. ```raw objectClasses: ( 2.16.840.1.113730.3.8.24.4 NAME 'ipaSubordinateId' DESC 'Subordinate uid and gid for users' SUP top AUXILIARY MUST ( uidNumber $ 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`` and does not use these object classes. ```raw objectClasses: ( 2.16.840.1.113730.3.8.24.2 NAME 'ipaSubordinateUid' DESC 'Subordinate uids for users, see subuid(5)' SUP top AUXILIARY MUST ( uidNumber $ 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 ( uidNumber $ ipaSubGidNumber $ ipaSubGidCount ) X-ORIGIN 'IPA v4.9' ) ``` ### Index The attributes ``ipaSubUidNumber`` and ``ipaSubGidNumber`` are index for ``pres`` and ``eq`` with ``nsMatchingRule: integerOrderingMatch`` to enable efficient ``=``, ``>=``, and ``<=`` searches. ### Distributed numeric assignment (DNA) plug-in extension Subordinate id auto-assignment requires an extension of 389-DS' [DNA](https://directory.fedoraproject.org/docs/389ds/design/dna-plugin.html) plug-in. The DNA plug-in is responsible for safely assigning unique numeric ids across all replicas. Currently the DNA plug-in only supports a step size of ``1``. A new option ``dnaStepAttr`` (name is tentative) will tell the DNA plug-in to use the value of entry attributes as step size. ## 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* ## 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 ``user-auto-subid`` command on the command line or with the *Auto assign subordinate ids* action in the *Actions* drop-down menu in the web UI. ```shell $ ipa user-auto-subid someusername ``` ### 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: ```shell $ ipa role-add-member "Subordinate ID Selfservice User" --groups=ipausers ``` This allows members of ``ipausers`` to request subordinate ids with the ``user-auto-subid`` command or the *Auto assign subordinate ids* action in the web UI. ```shell $ ipa user-auto-subid myusername ``` ### Auto assignment with user default object class Admins can also enable auto-assignment of subordinate ids for all new users by adding ``ipasubordinateid`` as a default user objectclass. This can be accomplished in the web UI under "IPA Server" / "Configuration" / "Default user objectclasses" or on the command line with: ```shell $ ipa config-mod --addattr="ipaUserObjectClasses=ipasubordinateid" ``` **NOTE:** The objectclass must be written all lower case. ### 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. ```raw # /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 search by start value of subordinate uid and gid range. The new command ``user-match-subid`` can be used to find a user by any subordinate id in their range. ```raw $ ipa user-match-subid --subuid=2153185287 User login: asmith First name: Alice Last name: Smith ... SubUID range start: 2153185280 SubUID range size: 65536 SubGID range start: 2153185280 SubGID range size: 65536 ---------------------------- Number of entries returned 1 ---------------------------- $ ipa user-match-subid --subuid=2153185279 User login: bjones First name: Bob Last name: Jones ... SubUID range start: 2153119744 SubUID range size: 65536 SubGID range start: 2153119744 SubGID range size: 65536 ---------------------------- Number of entries returned 1 ---------------------------- ``` ## SSSD integration * base: ``cn=accounts,$SUFFIX`` / ``cn=users,cn=accounts,$SUFFIX`` * scope: ``SCOPE_SUBTREE`` (2) / ``SCOPE_ONELEVEL`` (1) * user filter: should include ``(objectClass=posixAccount)`` * attributes: ``uidNumber ipaSubUidNumber ipaSubUidCount ipaSubGidNumber ipaSubGidCount`` SSSD can safely assume that only *user accounts* of type ``posixAccount`` have subordinate ids. In the first revision there are no other entries with subordinate ids. The ``posixAccount`` object class has ``uid`` (user login name) and ``uidNumber`` (numeric user id) as mandatory attributes. The ``uid`` attribute is guaranteed to be unique across all user accounts in an IPA domain. The ``uidNumber`` attribute is commonly unique, too. However it's technically possible that an administrator has assigned the same numeric user id to multiple users. Automatically assigned uid numbers don't conflict. SSSD should treat multiple users with same numeric user id as an error. 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 and ``uidNumber`` 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,$SUFFIX`` subtree. IPA recommends that SSSD simply extends its existing query for user accounts and requests the four subordinate attributes additionally to RFC 2307 attributes ``rfc2307_user_map``. SSSD can directly take the values and return them without further processing, e.g. ``uidNumber:ipaSubUidNumber:ipaSubUidCount`` for ``/etc/subuid``. Filters for additional cases: * subuid filter (find user with subuid by numeric uid): ``&((objectClass=posixAccount)(ipaSubUidNumber=*)(uidNumber=$UID))``, ``(&(objectClass=ipaSubordinateId)(uidNumber=$UID))``, or similar * subuid enumeration filter: ``&((objectClass=posixAccount)(ipaSubUidNumber=*)(uidNumber=*))``, ``(objectClass=ipaSubordinateId)``, or similar * subgid filter (find user with subgid by numeric uid): ``&((objectClass=posixAccount)(ipaSubGidNumber=*)(uidNumber=$UID))``, ``(&(objectClass=ipaSubordinateId)(uidNumber=$UID))``, or similar * subgid enumeration filter: ``&((objectClass=posixAccount)(ipaSubGidNumber=*)(uidNumber=*))``, ``(objectClass=ipaSubordinateId)``, or similar ## Implementation details * The four subid attributes are not included in ``baseuser.default_attributes`` on purpose. The ``config-mod`` command does not permit removal of a user default objectclasses when the class is the last provider of an attribute in ``default_attributes``. * ``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 ``$REALM_subid_range`` entry uses range type ``ipa-ad-trust`` instead of range type ``ipa-local-subid`` for backwards compatibility with older SSSD clients, see [SSSD #5571](https://github.com/SSSD/sssd/issues/5571). * Shared DNA configuration entries in ``cn=dna,cn=ipa,cn=etc,$SUFFIX`` are automatically removed by existing code. Server and replication plug-ins search and delete entries by ``dnaHostname`` attribute. ### TODO * enable configuration for ``dnaStepAttr`` * remove ``fake_dna_plugin`` hack from ``baseuser`` plug-in. * add custom range type for idranges and teach AD trust, sidgen, and range overlap check code to deal with new range type.