[Freeipa-devel] [RFC] Serving legacy systems cliens for trusts

[Alexander Bokovoy]

Hi,


http://www.freeipa.org/page/V3/Serving_legacy_clients_for_trusts

= Overview =

Since version 3.0 FreeIPA supports cross-realm trusts with Active
Directory. In order to allow AD users to utilize services on IPA
clients, up to date version of SSSD should be configured at the IPA
client. In case it is not possible to install and configure SSSD > 1.09,
Active Directory users cannot access services on IPA clients.

This feature is designed to bridge the gap and provide minimal
compatibility level that allows to log-in to IPA clients for AD users.
IPA clients will be able to use any reasonable nss_ldap/pam_ldap/sssd
version. 

= Use Cases =

Access to IPA client machine resources for AD users in case IPA client
cannot utilize up to date version of SSSD with native support for IPA
cross-realm trusts.

= Design= 

Since IPA client is configured with the use of older SSSD or
nss_ldap/pam_ldap, all work should be performed at the IPA master.
Primary design decision is to provide a separate LDAP tree, similar to
compat tree, that has following features:

* information about both IPA and AD users can be queried;
* it ispossible to enumerate members of IPA and AD groups;
* authentication bind to IPA LDAP as AD users should automatically
* trigger obtaining ticket from AD DC; in case TGT is obtained,
* authentication bind should be treated as successful.

 From a client perspective, use of the separate LDAP tree is viewed as
traditional nss_ldap/pam_ldap configuration.

Proposed base for the LDAP tree:
'''cn=users,cn=trust-accounts,dc=example,dc=com'''

= Implementation =

# IPA server sets SSSD configuration to 'ipa_server_mode = true' on install or upgrade
# ipa-adtrust-install configures additional directory server plugin to serve trusted domains tree
# Directory server plugin uses  getpwnam_r(), getgrnam_r() and related calls to obtain information about AD user. For IPA users the information is fetched directly from the LDAP.
# IPA KDC database driver adds MS-PAC information into ticket granting ticket for host/fqdn at REALM principal of IPA master. This is required to allow SSSD on IPA master to authenticate against AD using host/fqdn at REALM principal.

For SSSD design see
https://fedorahosted.org/sssd/wiki/DesignDocs/IPAServerMode

= Feature Management =

=== UI ===

The feature is transparent and not exposed in UI

=== CLI ===

The feature is not directly exposed in CLI. 

IPA idrange management is expanded to specify idrange type (IPA local,
AD trust, AD with winsync, IPA trust, ..) to affect the way how AD users
SIDs are mapped to POSIX IDs.

= Major configuration options and enablement =

sssd.conf will have 'ipa_server_mode = true' set for IPA master.

= Replication =

No effect on replication. Since directory server plugin is only
configured when ipa-adtrust-install is run, IPA masters may opt out from
serving AD clients.

= Updates and Upgrades =

During upgrade of IPA master, sssd.conf should be updated to set
'ipa_server_mode = true'.

= Dependencies =

Depends on SSSD implementing IPA server mode (sssd 1.10.x)

= External Impact =

https://fedorahosted.org/sssd/wiki/DesignDocs/IPAServerMode

= Backup and Restore =

No external configuration files are affected

= Test Plan =

Testing the feature will require following:

# Configure IPA to serve AD trusts
# Establish trust with AD domain
# Configure a client to use nss_ldap/pam_ldap against AD-compatible tree
# Attempt to log-in to the client as AD user

= RFE Author =

[[User:Ab|ab]] ([[User talk:Ab|talk]])

-- 
/ Alexander Bokovoy