Identity Mapping#

In FreeIPA deployments user and group objects get their POSIX identities (IDs) assigned and managed in an automated way. This document describes how identity mapping is performed and enforced in FreeIPA deployments.


FreeIPA deployment has three major identity mapping elements:

  • automated identifier assignment through LDAP

  • automated identifier assignment through SSSD

  • identity information consistency enforcement in LDAP and Kerberos KDC

POSIX identities are allocated to objects that need to be visible at run-time in POSIX environment. These are typically POSIX users and groups but there are some exceptions to these objects, to accomodate specialized use cases.

In POSIX environment user and group identities are properties of several different objects. When login session is initiated, the login process is started as root (ID 0) and during login flow at some point a switch to a certain user ID is performed. User’s primary group is associated with a process and information about other groups a user is a member of is populated as ‘secondary groups’ of the process. During lifetime of the process these lists are typically left intact and cannot be modified unless a process possesses certain administrative capabilities.

Both user and group IDs in POSIX environment are represented by unsigned integers. In Linux environment user and group ID spaces are separate and fit into 32-bit unsigned integer since 2001. Due to historical reasons, some of file formats might not support more than 16- or 15-bit unsigned integer spaces for UID and GID values. FreeIPA environment assumes operations in a contemporary environment, with 32-bit user and group IDs.

Linux Standard Base Core Specification defines certain reserved ID ranges. Some Linux distributions or projects go beyond that and define own ID ranges. To avoid conflicts with these ranges, FreeIPA defines its own default ID ranges above 16-bit space.

For interoperability purposes a mapping between POSIX and non-POSIX identities has to be established. The only supported non-POSIX identities for which there exists a well defined mapping mechanism are identities represented with the help of Security IDentifiers (SIDs) from Active Directory.

Identity ranges#

Each POSIX identity issued by the FreeIPA server is generated within a certain identity range. ID ranges serve several purposes: they help multiple FreeIPA components to coordinate identity allocation and convey details of the purpose of the allocated identity ranges.

ID ranges are stored in LDAP, under cn=ranges,cn=etc,$BASEDN subtree. LDAP objects for ID ranges are based on the ipaidrange objectclass. Depending on the type of the range, other objectclasses might be added to express additional attributes.

As of FreeIPA 4.11, there are following ID range categories in FreeIPA:

  • local FreeIPA POSIX ID range (ipa-local ID range type)

  • POSIX ID range for local subordinate identities (ipa-ad-trust)

  • trusted domain ID range using automated allocation based on SID of an object (ipa-ad-trust)

  • trusted domain ID range using explicit allocation for POSIX identities (ipa-ad-trust-posix)

These four categories expressed using three ID range types for technical reasons.

More ID ranges can be added through the lifetime of the deployment to accomodate administrative needs. The main requirement for these additional ranges is to not overlap with existing ranges. Range consistency is controlled with the help of a specialized plugin to 389-ds LDAP server, ipa-range-check. When a new range conflicts with existing configuration, its acceptance will be refused and an LDAP error LDAP_CONSTRAINT_VIOLATION will be returned. This ensures all ranges are in consistent state.

Trusted domain ID ranges have information about the domain SID of the associated domain stored directly in the range LDAP object. For the local domain ID ranges this information is stored elsewhere because only a single organizational domain (as opposed to DNS domains) can be present in IPA deployment.

Information about SID and other parameters of the local domain is stored in the LDAP object cn=<domain>,cn=ad,cn=etc,$SUFFIX. This information is used for ID mapping enforcement purposes by SID generation plugin and Kerberos KDC driver.

Information about SID and other parameters of a trusted domain is stored in the LDAP object cn=<domain>,cn=ad,cn=trusts,$SUFFIX. It is used by the Kerberos KDC driver when processing Kerberos requests.

Local FreeIPA POSIX ID range#

Each FreeIPA deployment has its own primary (or local) POSIX ID range. The range is chosen during an initial server deployment and cannot be changed. Initial admin user and associated groups (e.g. admins) are all allocated from this range. The range size defaults to 200000 identities but both starting point and the size of the range can be modified with ipa-server-install options.

The primary ID range corresponds to the DNA range also created during the initial IPA server deployment. When replica is deployed, this ID range already exists in the topology and is visible to the replica. However, a corresponding slice of the DNA range will not be created on the replica until the DNA plugin on the replica is not asked to allocate an ID. See Automated Identifier Assignment Through LDAP section for details how DNA ranges are used.

POSIX ID range for local subordinate identities#

Subordinate identities 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. More details about subordinate ranges can be found in “Subordinate IDs” design page.

Due to a technical use of a trusted domain type to represent ID range of the local subordinate identities, the ID range has a domain SID associated with it. The SID has a special structure: 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 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. This SID is never used for any SID generation.

Trusted domain ID range using automated allocation based on SID of an object#

Active Directory environment does not use itself any POSIX identities because they have no meaning within the context of Windows operating system. Each object in Active Directory has associated security identifier (SID). SIDs stay the same for the lifetime of the object and never re-used by other objects. SID can be used to map an object in Active Directory to POSIX environment.

There are several methods to map SIDs to POSIX IDs. FreeIPA relies on an algorithmic method provided by SSSD. The algorithm is described in sssd-ad(5) manual page in the section “Mapping algorithm”.

On start-up, SSSD looks up all ID ranges from the active IPA server and uses information about trusted domains to map between SIDs and POSIX IDs. This avoids static allocation of identities in LDAP. Instead, ID ranges in FreeIPA serve as fences, to prevent other allocators from using these ranges for static allocation.

For each trusted domain, a separate ID range is created since every trusted domain has unique domain SID. Each ID range will have own POSIX ID range allocation. These allocations can be created automatically during ipa trust-add operation or ID ranges can be pre-created in advance with ipa idrange-add operation. The latter is useful when there is a need to explicitly define specific POSIX ID range boundaries as ipa trust-add command only allows to define a new range’s size.

Trusted domain ID range using explicit allocation for POSIX identities#

In case there is already existing static mapping of POSIX identities in Active Directory, one can explicitly define ID range assocated with the trust to Active Directory to handle explicit allocation. In this case SSSD will not use algorithmic method but instead will look up identities from LDAP entry (e.g. uidNumber and gidNumber attributes).

In case of explicit allocation, ID range associated with the trusted forest Active Directory root domain must have ID range type of ipa-ad-trust-posix, as well as all other domains visible through the trust link.

ID range allocation#

During deployment FreeIPA takes a random ID range slice from (1,10000) and multiplies the base offset by 200000. This base becomes a starting point for the deployment’s ID range.

Both starting point and size of the range can be overridden by an administrator who deploys the initial server. The minimal value for the initial ID range start cannot be less than UID_MAX or GID_MAX from /etc/login.defs. This range is then used by all systems enrolled into the domain managed by this initial server.

Thus, a default range that FreeIPA chooses the deployment ID range from is 200000…2000000000. If this is a single domain and amount of users in it is not going to be above 200K, then FreeIPA administrators may never need to allocate another ID range within 200000…2000000000 and that original ID range stays fixed forever.

There are two other use cases:

  • trusting another IPA or AD deployment, and

  • migrating pre-existing deployment not based on IPA.

When there are multiple organizational domains around and they need to interoperate (establish trust between them), additional ID ranges get added on the consuming side to represent trusted domain(s). For a trusted domain that is an Active Directory, there would be two possibilities, managed within SSSD and IPA:

  • an ID range is implicit, defined through a murmurhash3 hash of the domain SID,

  • an ID range is explicit, defined by the admin as UID/GID values stored in AD LDAP user entries for each user. The explicitly defined ID ranges most likely are part of a legacy setup and most likely are within UID_MAX and GID_MAX values from /etc/login.defs. There are exceptions, of course.

An implicit ID range derivation by SSSD is described in sssd-ad(5), section ‘ID Mapping’. Samba has own way to derive similar ID ranges based on different properties of the domain SID, handled by individual idmap modules but conceptually it is similar: a rule is chosen to map those properties to POSIX IDs and a map is maintained, algorithmically or based on a stored mapping in Samba’s own ID database.

With Active Directory environments all domain objects have their own unique identifier, in the form of a SID. RID value of the SID is never reused when objects get deleted and added. This means that for a company with an Active Directory domain of ~20-25 years and large churn of employees over that time it is not uncommon to see RIDs above 200K. For such deployments an ID range SSSD and IPA would need to synthesize on Linux side would have size larger than 200K.

Thus ID range landscape is ‘dynamic’. At a given time SSSD on the client will be able to see all the ID ranges defined within the domain they enrolled into. When a new trust is established and a new ID range for that trusted domain is added, SSSD on the client will see this new ID range and will be able to derive user/group IDs automatically for them. Most likely there are no existing files on the client with ownership for these IDs yet – except networking file systems but for those to be usable at least one system should be able to create those files with those POSIX IDs on a compatible client.

Existing ranges never change, LDAP plugins provided by FreeIPA prevent modifying existing ranges in IPA or delete them if there are users/groups with IDs assigned from those ID ranges.

Automated identifier assignment through LDAP#

When algorithmic ID range is used, SSSD only maps existing users and groups coming from trusted Active Directory domains and those so based on their SID values. For all other range types allocation of POSIX IDs is done in advance, typically when user or group is created or moved from staged/preserved state.

This automated allocation is done with the help of Distributed Numeric Assignment (DNA) plugin in 389-ds LDAP server. DNA plugin has its own identity ranges and provides a generic mechanism to assign identities to various objects across the whole LDAP topology. During installation of the original IPA server a DNA range is created to match the primary IPA ID range. When new replicas added to IPA topology, their DNA plugins become aware of the overal DNA range and may ask for a sub-set of it for own needs. This sub-allocation only happens when DNA plugin needs a range to allocate an ID. For example, when a user or a group is created on that replica.

Since DNA ranges only get splitted and not extended automatically, IPA ID ranges aren’t tracking changes in DNA ranges. However, if somebody adds new DNA ranges outside of the original primary IPA ID range, then a corresponding IPA ID range needs to be created manually. This affects a lot of legacy deployments where no real control over use of POSIX IDs through static allocation was done.

Another automatic ID assignment happens for SIDs. For each object in LDAP that has POSIX attributes and ipaNTUserAttrs or ipaNTGroupAttrs object classes upon creation a SID is generated by the ipa-sidgen 389-ds plugin. The SID is stored in ipaNTSecurityIdentifier attribute. There is also another plugin, ipa-sidgen-task, which handles a task of generating of SIDs for existing objects. Both plugins issue error messages in the 389-ds error log in case they were unable to map POSIX ID to a SID.

ID mapping adjustments using ID overrides#

On top of automatically or algorithmically allocated POSIX IDs, administrators have possiblity to adjust POSIX IDs locally for users and groups coming from trusted Active Directory domains. The same mechanism can also be used for adjustments of POSIX IDs on individual IPA clients (or groups of IPA clients) for legacy compatibility. These mechanisms should not be used for IPA users and groups in general case because they might break certain assumptions in SSSD and Kerberos KDC driver.

For users and groups from trusted Active Directory domains, ID overrides in “Default Trust View” are used by SSSD on IPA server to override any ID retrieved from the trusted domain controler side or generated algorithmically. This mechanism allows to migrate pre-defined POSIX IDs from Active Directory by creating ID overrides for individual users and groups.

From SSSD point of view, these ID overrides represent uidNumber and gidNumber attributes as if they were specified in the original LDAP object entry.

IPA users and groups cannot have ID overrides in the “Default Trust View”. Their POSIX IDs have to be present in their own LDAP object entries.

Security IDentifiers#

While primary goal of FreeIPA is to serve identities for POSIX environment, a large effort is put into making interoperability with other identity management systems, out of which Active Directory is the most important one. Active Directory is using a different method to control access to their resources than POSIX environment and this method extends from identity management to authorization mechanisms. It is also has visible effects on various protocols through which FreeIPA interoperability with Active Directory is achieved.

One of the corner stones of Active Directory security model is the security identifier (SID) assignment to each object in Active Directory that can be used for security evaluation. Objects grouped by Active Directory domains, each domain having a domain SID and all objects within the same domain have their SIDs starting with the SID of the domain. The relative identifier (RID) is the unique identity of an object within the domain scope. A full object SID, thus, is a combination of the <domain-SID>-<object RID>.

Since objects are unique within the domain, RIDs can be reused in different domains. There are common, so-called ‘well-known’ identifiers, which express the same functional objects in different domains. These well-known objects have the same RID values but still be distinct: Objects from domain1.test and domain2.test with the same RID values will be different. For example, domain administrator user Administrator has RID value 500. Administrator user from domain1.test would be different from Administrator user from domain2.test, even though their individual RIDs (500) would be the same. If domain1.test has SID S-1-5-21-123-45-6789 and domain2.test has SID S-1-5-21-54-321-6789 then Administrator user from domain1.test would have SID S-1-5-21-123-45-6789-500 and Administrator user from domain2.test would have SID S-1-5-21-54-321-6789-500.

Internally Active Directory does only use user and group names to perform translation to a SID. Once SID of the object who authenticated the connection is established, names don’t matter anymore. For each authenticated object within a specific connection a special security token is built which contains this object’s membership information in form of SIDs of groups it belongs to. Access control lists attached to Active Directory objects contain SIDs as well, making access control evaluation straightforward: security token SID details evaluated against access control lists to grant or deny access.

This model is loosely similar to how processes in POSIX environment get evaluated when accessing resources. Each POSIX process has user identity it is running under, with a set of primary and secondary groups associated with the identity as part of the process description. The difference is that POSIX UID and GID values exist only on a single machine and a care to avoid conflicts between multiple systems should be taken. In Active Directory model the SIDs are already bearing domain-specific information, hence allowing to distinguish objects belonging to different systems. The latter means for both networking and local resources such as file systems a single access control list mechanism can be used to address both remote and local identities.

Since SIDs do not exist in POSIX environment, mapping of objects from trusted Active Directory domains to POSIX environment within IPA deployment is based on other properties. In particular, a fully-qualified user or group name is used to represent the user (or group): a user Administrator from domain1.test would have a fully-qualified name Administrator@domain1.test, different from the user Administrator of the domain domain2.test which would have a fully-qualified name of Administrator@domain2.test.

Above description uses so-called user principal name (UPN) notation to describe user names. In this notation a user name corresponds to a (case-insensitive) Kerberos principal name. Since each Active Directory domain is a Kerberos realm, this allows to establish mapping between identities on POSIX and Active Directory levels through Kerberos authorization mechanism.

Active Directory has extended Kerberos protocol by adding a privilege attributes certificate (PAC) information to the Kerberos ticket issued by Active Directory domain controllers. The PAC part contains the security token details of the authenticated object represented by the Kerberos principal. These details are protected by a set of security checksums that prevent external modifications to the PAC content. This information is detailed in the MS-PAC and MS-KILE specifications Microsoft has published and Open Source projects such as MIT Kerberos, Heimdal Kerberos, Samba, and FreeIPA have since implemented.

Enforcement of ID mapping#

For trusted domains using Kerberos protocol to establish trust, a duty to validate information falls onto Kerberos KDCs of each realm. FreeIPA KDCs re-assess content of the presented Kerberos tickets and validate PAC issued by a trusted domain’s domain controller. This validation includes checks to make sure only SIDs from trusted domains can be present there and a trusted domain’s KDC cannot inject SIDs that belong to IPA domain.

The latter is needed because both sides of the trust possess a key that represents a cross-realm service principal, krbtgt/DOMAIN1.TEST@DOMAIN2.TEST. This key allows KDC from DOMAIN1.TEST realm to issue a service ticket that a client from DOMAIN1.TEST can present to a KDC from DOMAIN2.TEST to request a service ticket for a service from DOMAIN2.TEST. If a KDC from DOMAIN1.TEST goes rogue and decides to inject SIDs of groups from DOMAIN2.TEST into a security token of the user from DOMAIN1.TEST, nothing can stop it. Thus, a KDC from DOMAIN2.TEST must validate that a token encoded in the PAC buffer of the Kerberos ticket coming from the DOMAIN1.TEST is ‘sane’.

Sanity checks performed by the KDC in FreeIPA include multiple steps. For each trusted domain FreeIPA records a list of SIDs that must be filtered out unconditionally. This list is based on the section ‘ SID Filtering and Claims Transformation’ of the MS-PAC specification. The list can be extended for each specific domain using ipa trust-mod command for both incoming (--sid-blacklist-incoming) and outgoing (--sid-blacklist-outgoing) directions.

Additionally, KDC verifies that a PAC issued by a trusted domain’s KDC does not contain SIDs from IPA domain. In such case the ticket issuance will be rejected.

For each ticket issued, KDC adds information about the requestor’s SID in a separate PAC buffer. During processing of the consequent requests KDC does validate that a requestor SID is the same as the SID of the identity of the security token in PAC buffer. This information is then cross-verified against the list of known trusted domains to avoid cases of impersonation exploited through CVE-2020-25721.

As a consequence of these checks, FreeIPA Kerberos KDC only issues initial Kerberos tickets for principals which have SID assigned. The SID assignment in FreeIPA is tied to presence of an ID range that covers both uidNumber and gidNumber of the LDAP object representing the principal. If there is no such range can be found, SID will not be issued and a Kerberos principal associated with this LDAP object will be used for authentication. Thus, SID and POSIX attributes are tied together.