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External Users in IPA

Name: External Users in IPA
Incomplete.png Pending review
Last updated: 2015-05-5 by Mkosek

Important.png
Work in progress
This design is not complete yet. Initial discussion was held on freeipa-users list

Overview

External Collaboration Domains defines a use case where organizations host resources in a public-facing network space in support of cross-organization collaboration. A key requirement of that use case is that users be capable of logging into machines (i.e., via ssh), accessing file shares, and participating in POSIX groups for authorization and file permission purposes. This request proposes IPA support this use case via the ability to synthesize and harmonize POSIX attributes for external users without creating a new account in the collaboration domain.

There are two objectives to this enhancement:

  1. Ensure all users have the required POSIX attributes, that the POSIX attributes are unique, and that services in the domain have access to these attributes.
  2. Ensure that locally defined POSIX groups can add individual external users.

IPA uses Kerberos for authentication and LDAP to store user attributes. Users created within IPA are fully functional: they may log in to websites, perform console logins, and participate in group membership. External users, defined here as identities not allocated within the local IPA server, are not fully functional. In general, they may not participate in POSIX groups because they have no DN defined within the local LDAP server.

To address "participation in local POSIX groups", this RFE proposes creating and maintaining individual entries in the local LDAP server for every external user. This process should not create another password for the user to manage

Further, it is proposed that IPA centralize the functions of importing, harmonizing, and synthesizing POSIX attributes for the local domain. The explicit intent is that all services and hosts in the local realm obtain POSIX attributes from the local IPA.

These two actions are envisioned to happen automatically upon first encounter with an external user.

Use Cases

See External Collaboration Domains for the context driving this RFE.

Design

Overview

The crux of this proposal is the provisioning and storage of user attributes in LDAP for external users, while not creating a corresponding identity in the local Kerberos store. Automatic provisioning of user attributes on first access is desirable. Persistence of these attributes from login to login is required. It is also desirable to map and synchronize attributes from the user's home realm when they: a] exist in the user's home realm; and b] have not been overridden by the local IPA to prevent collisions.

Currently, sssd on hosts within the realm may be configured to recognize multiple domains. The intent is to allow each machine to authenticate directly against the user's home domain and receive user attribute information directly from the source. Each machine must be individually configured, the configuration must explicitly whitelist every permitted domain from which users may connect, and each machine's configuration must be individually maintained. sssd is capable of deriving POSIX-specific identity attributes from windows-specific ones, but is not currently capable of synthesizing POSIX attributes (or windows attributes) if they are completely absent. sssd, in its current state, is not a good solution for configuring an open, collaborative environment which accepts identities using a wide range of technologies and providers.

KrbUserAttributes.png

The above figure shows common configurations of realms which are designed to provide console logins and which utilize Kerberos as the authentication technology. Users have two identities with a mapping between them. The LDAP store represents the user's identity as a Distinguished Name (DN), which is essentially an absolute path to the user's identity entry from the root of the directory. The Kerberos store represents the user's identity as "name@realm". The mapping between the two identities may be stored in the user's entry in the LDAP store using the krbPrincipalName attribute. This attribute is defined in the expired internet draft draft-skibbie-krb-kdc-ldap-schema-02. The relationship between the two identities may also be embodied in the way configuration of hosts is managed in the source realm.

Provisioning External Users in LDAP

For a stand-alone domain which allows no cross-realm operation, there is a 1:1 relationship between the users defined in the LDAP store and the users defined in Kerberos. For reliable and robust cross realm operation which is centrally managed for the local realm, it is necessary to add an external entry to LDAP without adding the corresponding entry to the local KDC. Two identities are still in play: the user still has a Kerberos identity, it's just not a local identity. The mapping still needs to be maintained, and should be made explicit using the krbPrincipalName LDAP attribute.

KrbExtUserAttributes.png

Due to the fact that IPA would require substantial work to allocate external users in a separate OU, no such provision is specified in this RFE. Directory entries for external users will be allocated in the same OU as entries for users created by the IPA realm. This feature, however, does impose the constraint that directory entries for external users have a globally unique RDN. In other words, the RDN must include both the name and realm part of the external user's kerberos identity. Failure to do so would result in collisions between principals having the same name, but from two different realms.

It is necessary that hosts and services within the realm be readily configured to understand the mapping between LDAP and Kerberos user identities.


Attributes from the realm of origin

The focus of this design is to consolidate the formerly distributed sssd configuration to provide equivalent functionality by pointing all hosts and services in the local realm at the IPA server. While the Kerberos server orchestrates which external principals are permitted to authenticate to services within the local realm, the LDAP server should provide the local realm's clients access to as much information about those Kerberos principals as it can. Administrators should be able to configure the hostname of remote LDAP servers as well as a mapping from the remote schema to the IPA schema.

User attributes should be classified as being under local control or remote control. IPA takes local control of an attribute for all identities connecting to the local realm when there is a potential for conflict. Candidates for local control are the POSIX UID, and GID. The Windows SID is also a candidate for local control, particularly when the realm of origin does not provide such a value (i.e., is a POSIX domain or web-oriented IdP.) IPA may also take local control over an attribute if the value defined in the realm of origin is irrelevant to the local realm: such as homeDirectory.

Group information from the realm(s) of origin are potentially interesting. However, it is more likely that new groups relevant to specific collaborative partnerships will be more interesting. These groups may be composed within the local IPA realm using identities from any available source without regard to whether the contributing identity sources have a direct relationship with one another. Harvesting group information from donor realms is explicitly not a goal of this RFE.

LDAP requests for information about external identities may be filled by relaying the request to the realm of origin (if configured) and cached in the local LDAP server. Periodically, the cache should be refreshed in order to pick up changes. This can be configured to happen on a "just in time" basis (i.e., when a new request for old cached data is received), or it can be configured to happen on a schedule (nightly or weekly). Just as group information from donor realms is not obviously critical, synchronizing all identities from every potential donor realm is unlikely to serve a useful purpose. The only identities important to the local realm are those identities which access services within the realm.

External User participation in POSIX groups

Automatically provisioning external users in LDAP provides the local DN required to directly add them to POSIX groups. In the case of AD trusts, it will no longer be necessary to manually mirror AD groups to IPA and then add them as a member to POSIX groups. However, while this method offers administrators another method of handling users from a tightly coordinated AD domain, it does not conflict with the current way of integrating AD users into an IPA domain.

The limitation on this approach is that the external user must login to the domain once (to create their user attribute entry in the LDAP server) prior to being added to a group.

Implementation

Any additional requirements or changes discovered during the implementation phase.

TBD

Feature Management

UI

How the feature will be manged via the UI

TBD

CLI

Overview of the CLI commands

TBD

Major configuration options and enablement

  • Globally enable or disable cross-realm operation.
  • Allow admin to provide a set of default values for automatically provisioned external users.
  • Allow admin to configure the identity server in the realm of origin when known. This should include both the hostname and a mapping from the original schema to IPA's schema. It should be possible to save/load mappings to a file to share them with other admins.
  • Allow admin to select an allocation strategy for POSIX IDs within the realm (one pool of external IDs for the realm? one pool per donor realm?)
  • Allow control over which external Kerberos realms may provide identities for use in this realm. This may consist of a whitelist, blacklist, or both.

Replication

New configuration must be replicated.

"external users" exist only in LDAP and should leverage the existing replication mechanism.

Updates and Upgrades

When users upgrade to IPA with this feature, the default configuration should be to have cross-realm operation turned off.

Dependencies

no change

External Impact

This work should make it possible for Ipsilon to adapt external users into a form that the local IPA realm can transparently use.

The production of certificates by FreeIPA is not currently compatible with PKINIT usage. FreeIPA, DogTag, and nss need to be modified to provide this support (Ticket #521). This functionality is required for the OTP effort. This RFE describes the complementary operation which could consume kx509 tickets, should they become available. As such, implementation of this RFE need not depend on completion of kx509 certificate support.

Backup and Restore

If the extra configuration creates new configuration files, these will need to be included in a backup/restore scheme.

External user attribute entries in LDAP should be covered by existing backup/restore strategies.

Test Plan

  • Verify the KDB plugin correctly audits PKINIT transactions from the AS.
  • Verify the KDB plugin correctly audits requests using cross realm TGTs in the TGS.
  • Verify that IPA can automatically provision an external user account given no information other than cname and crealm. This should respect the default values provided by the admin.
  • Verify that IPA's attribute mapper can adapt remote user attribute schemas to the local IPA schema.
  • Verify that upstream changes to a user attribute in the realm of origin are reflected in the local realm.

RFE Author

bnordgren@fs.fed.us