Hi,

http://www.freeipa.org/page/V4/Support_of_UPN_for_trusted_domains
describes a design page to support name suffixes from trusted Active Directory domains.


A prototype code exists (written by me and Sumit) and was tested by Sumit
against recent releases of SSSD.

Text is provided below for easier commenting.
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{{Feature|version=TODO|ticket=TODO|author=Ab}}

== Overview ==
User principal name (UPN) in Active Directory is the primary form of
addressing users. UPN has structure of 'user name@suffix' where both
user name and suffix parts may vary. By default the suffix is the same
as the Active Directory domain name but AD administrators may create
additional name suffixes and associate them with specific users. These
additional UPNs for users may then be used for Kerberos authentication
against Active Directory domains.

Alternative UPNs are often used when several companies with Active
Directory deployments merge and want to provide unified logon namespace.

The purpose of this feature is to allow using alternative UPNs
associated with the Active Directory users when accessing resources in
FreeIPA domain.

== Use Cases ==

As an Active Directory user, I want to login using my user@EXAMPLE user
principal name even if my Active Directory domain is named
REGION.EXAMPLE.COM.
== Design==
Support for UPNs is split to three different components:
;Client-side
: SSSD already supports logon with UPN by asking a KDC to accept
enterprise logon names.  By default, the use of enterprise principals is
disabled, therefore, <code>krb5_use_enterprise_principal = True</code>
needs to be added to sssd.conf to enable it.

;KDC
: IPA KDC does understand multiple domains associated with the trusted
AD forest. However, since no information about name suffixes associated
with the forest is available, it cannot take them into account when
processing enteprise logon names to issue referrals to the correct
realm. Support needs to be added to allow IPA KDC to look up name
suffixes associated with a trusted forest.

; IPA framework
: Changes needed on IPA framework side to fetch from Active Directory a
list of name suffixes and store them in the trusted domain objects.

== Implementation ==
For retrieving name suffixes, IPA framework needs to move to use
NETLOGON netr_DsRGetForestTrustInformation function instead of
netr_DsrEnumerateDomainTrusts. This allows to retrieve both domains and
top level names associated with the forest.

As top level names (TLNs) have only a single string as a name suffix,
they cannot be stored as trusted domains (they lack SID and NetBIOS
name). Thus, either IPA KDB driver needs to be extended to understand
trusted domains without SID and NetBIOS name, or TLNs need to be stored
as a property of tree root domains of the forest.

== Feature Management ==

=== UI ===
If TLNs are added as a property of tree root domains of the forest,
appropriate panel needs to be extended to display them.

=== CLI ===
If TLNs are added as a property of tree root domains of the forest,
appropriate attribute need to be handled by '''trust-show''' command. If
TLNs represented as separate 'trusted domains' of the trusted forest, no
work is needed on CLI other than being able to support 'trusted domains'
without SID and NetBIOS name.

=== Configuration ===
No configuration options.

== Upgrade ==
No impact to upgrade.

== How to Test ==
In order to test UPN-based logons, create additional name suffixes in
Active Directory and establish trust to it. After trust is established,
the name suffixes should be usable when trying to kinit as enterprise
principal.

== Test Plan ==

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--
/ Alexander Bokovoy

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