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V3/MultipleTrustServers


Overview

Ticket #2189;

Each FreeIPA server in the realm has potential to serve as domain controller in the cross-forest realm trust. This page outlines design for implementing multiple servers support in FreeIPA.

Use Cases

Once ipa-adtrust-install ran on the FreeIPA server, the server can handle requests from trusted domains by means of Samba project's smbd and winbindd daemons.

Hosts in FreeIPA realm may be enrolled against specific FreeIPA replica server. User from trusted domain can access these hosts and their identities will be resolved against the replica. However, if replica server does not have trust support configured, these identities will not be processed since running winbindd process is required to contact the trusted domain's domain controllers and Global Catalog servers.

Domain controllers are advertised to clients via SRV records in DNS. Since replica servers may be arranged in a specific topology, adding new domain controller would need to respect the topology design. It means priority/weight of the domain controller compared to other domain controllers should be adjustable. Prime use case for this is branch office deployments.

Design

  • Each domain controller uses separate identity and service key to talk to FreeIPA LDAP server. The identity is tied to the server hostname.
  • Service principal is cifs/hostname@REALM, identified in LDAP as krbprincipalname=cifs/hostname@REALM.
  • All identities are members of cn=adtrust agents,cn=sysaccounts,cn=etc,$SUFFIX. Thus, all replica servers can see what other servers are providing domain controller service.
  • Replica server only becomes domain controller when ipa-adtrust-install utility was executed on it. It means all DC setup is delivered via the ipa-adtrust-install.
  • ipa-adtrust-install should be able to detect other DCs by looking at existing identities as members of the cn=adtrust agents,cn=sysaccounts,cn=etc,$SUFFIX tree and modify list of SRV records under _msdcs and default site configuration if DNS is controlled by FreeIPA.
  • Domain Controller priority/weight can be modified at run time since it only affects SRV records in the DNS (if FreeIPA controls the DNS). Normal ipa dnsrecord-mod commands should be used for this purpose, operating on SRV records for _msdcs and default site configuration.
  • There are trust properties that are global for the realm. Some of them are modifiable, some not. Thus, ipa trustconfig-show and ipa trustconfig-mod should reflect both global and local settings (realm-wise and DC-wise).
  • Following properties of the trust are global for the realm:
    • NetBIOS domain name (read-only, affects existing trusts)
    • Domain name (read-only, affects existing trusts)
    • Domain GUID (read-only, informational)
    • Additional domain suffixes exposed to the trusted party, handled as black list against global list of additional domains associated with our or transitive realm, read/write
    • Fallback primary group (read-write)
  • Following properties of the trust are per Domain Controller:
    • priority of the DC and GC services (read-write, DNS SRV record)

Details on ipa trustconfig commands design are available at http://www.freeipa.org/page/V3/Trust_config_command Details on additional domain suffixes handling are available at http://www.freeipa.org/page/V3/Domain_suffixes

Implementation

Implementation-wise there are three parts:

  • ipa-adtrust-install:
    • Gather list of CIFS services that are also members of cn=adtrust agents and add SRV records for them to _msdcs in ipaserver/install/adtrustinstance.py:ADTrustInstance::__add_dns_service_records().
    • Once Global Catalog Server implementation will be ready, configure its use as one of ADTrustInstance setup steps.
  • IPA framework:
    • Add display and modification of trust properties as ipa trustconfig-* commands, including listing DC and GC servers.
    • NOTE: there is no need to modify trust creation procedure since it appears that AD DC assumes LSA CreateTrustedDomainEx2 call comes from the DC (in Windows the UI to establish trust is only on DC) and therefore does not do DNS discovery during validation step. Since smbd running on the same host that 'ipa trust-add' runs on will contact the same host's LDAP server, there is no issue in replication at this stage.
  • IPA Web UI
    • Add support for ipa trustconfig-* to Web UI.


Major configuration options and enablement

No additional options to ipa-adtrust-install

Replication

All data is already in a replicated namespace.

Updates and Upgrades

No changes to schema, no need to run anything additional on updates or upgrades

Dependencies

No additional dependencies beyond AD trusts support

External Impact

Once ipa-adtrust-install install ran on replica, the replica will be advertised via _msdcs SRV namespace as a domain controller.

RFE author

Alexander Bokovoy