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

[Dmitri Pal]

On 05/28/2013 07:50 AM, Alexander Bokovoy wrote:
> 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]])
>


Can you please explain how the older SSSD or other UNIX versions would
use Kerberos for authentication?
I thought we would have to issue some patch for client to switch the
clients from using Kerberos for authentication to using LDAP.
Alternatively we should modify KDC to use the new extop and go though
the SSSD on the IPA side but I do not see it mentioned anywhere.
Can we get a bit more clarity about the Kerberos authentication workflow?


-- 
Thank you,
Dmitri Pal

Sr. Engineering Manager for IdM portfolio
Red Hat Inc.


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