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V4/One-way trust

Name: V4/One-way trust
Ticket: #4546
Target version: 4.2.0
Author: Ab
Green tick.png Reviewed by: Sbose
Last updated: 2016-04-25 by Alich

Overview

Active Directory implementation of a trust between domains and forests uses credentials of a trust domain object (TDO) to communicate across the trust boundary. This is made possible on AD side because whole domain controller implementation is seen as a monolith that doesn't pass around the credentials for the trust domain object. This is purely implementation detail though important one.

In early stages of a trust feature development FreeIPA also used trust domain object to directly authenticate against Active Directory services. However, as IPA is a combination of several loosely coupled services, access to the trust domain object is highly guarded to prevent unwanted elevation of privileges across the trust boundary. If FreeIPA was to use TDO's credentials everywhere, it would mean most of trust-related functionality would be limited to IPA admins or TDO object in LDAP would have to be more accessible. Given that TDO credentials can be used to compromise access to our domain, it is not advisable to give a wider access to them.

As a side-effect of reducing exposure of TDO credentials, FreeIPA lost ability to establish and use one-way trust to Active Directory. The purpose of this feature is to regain the one-way trust support, yet without giving an elevated access to TDO credentials.

Use cases

A primary use case is the following one:

  • One-way trust to Active Directory where FreeIPA realm trusts Active Directory forest using cross-forest trust feature of AD but the AD forest does not trust FreeIPA realm. Users from AD forest can access resources in FreeIPA realm.

No other use cases exist at the moment.

Design

The one-way feature relies on an implementation of FreeIPA trust to AD feature as released in FreeIPA v3.3. The difference between FreeIPA v3.3 and v3.0 is in the way how credentials to access information from a trusted forest are used.

FreeIPA v3.0 and v3.3

In FreeIPA v3.0 each IPA master initialized with ipa-adtrust-install command was running Samba suite: smbd and winbindd daemons were used to provide both capabilities to resolve AD users from trusted forests, to manage trust forest topology, and to respond on NETLOGON interfaces as Active Directory Domain Controllers expect to complete the sequence of establishing trust relationships. The rest of clients in FreeIPA were connecting to IPA masters through SSSD by means of an extended LDAP control to resolve AD users and groups. FreeIPA LDAP server's plugin which implemented the extended LDAP control was, in turn, talking to winbindd daemon to complete the resolution of AD users and groups.

Additionally, in early FreeIPA v3.0 versions a management framework (both CLI and web UI) was using credentials of TDO to directly resolve AD users and groups against Active Directory Domain Controllers. The consequence of this was that only IPA admins were able to map users and groups from trusted Active Directory forests to local groups.

The trust by AD DCs means that FreeIPA framework can utilize existing Kerberos service ticket it has (HTTP/ipa.master@IPA.REALM) to authenticate to AD LDAP servers. AD LDAP servers allow access to its information only to authenticated clients but the clients can provide any proof of authenticity allowed by Active Directory. In the case of cross-forest trust in AD, a properly issued Kerberos ticket from a trusted forest is enough. In order to issue such ticket, FreeIPA KDC does generate privilege attribute certificate data (MS-PAC) as required by Microsoft's specification [MS-PAC]. In order to limit which Kerberos services are allowed to authenticate against services in a trusting AD forest, only HTTP/ipa.master@IPA.REALM, cifs/ipa.master@IPA.REALM, and host/ipa.master@IPA.REALM are given the MS-PAC in their TGT tickets where the services are presented as members of a virtual Domain Controllers group in FreeIPA domain.

FreeIPA v3.0 management framework was switched to use HTTP/ipa.master@IPA.REALM Kerberos ticket with attached MS-PAC information to directly resolve AD users and groups.

In FreeIPA v3.3 each IPA master initialized with ipa-adtrust-install command still runs Samba suite: smbd and winbindd daemons. They are used to respond on NETLOGON interfaces as Active Directory Domain Controllers expect them, and to manage trust forest topology. However, users and groups from trusted Active Directory forests are now resolved by SSSD running on the IPA masters. SSSD has gained a so-called "IPA server mode" which means the requests to resolve AD users and groups will go directly to Active Directory Domain Controllers. The rest of clients in FreeIPA are connecting to IPA masters through SSSD by means of the same extended LDAP control to resolve AD users and groups. However, FreeIPA LDAP server's plugin which implements the extended LDAP control now talks to SSSD on the IPA master instead of winbindd daemon to complete the resolution of AD users and groups. Finally, a management framework also relies on the SSSD on IPA master to resolve users and groups from trusted Active Directory forests. As a consequence of that, IPA admins can delegate rights to map AD users and groups without giving access to TDO credentials anymore.

In its own turn, SSSD 1.11 as used by FreeIPA v3.3, relies on the fact that Active Directory Domain Controllers do trust FreeIPA realm and uses host/ipa.master@IPA.REALM Kerberos ticket to directly resolve AD users and groups.

Security of two-way trust solution

It should be noted that existing two-way trust solution as implemented by IPA is not giving any additional rights compared to one-way trust as implemented by Active Directory. In fact, it is as secure, as one-way trust on AD side, thanks to cross-forest trust SID filtering defaults.

While the second leg of the trust is used to provide SSSD ways to look up users and groups in AD LDAP, thanks to ability to authenticate with cross-realm ticket, this does not allow to elevate permissions of IPA users and services in their access to AD resources. The only rights they get are the ones given by AD administrators by default to 'all authenticated users". An attempt to elevate rights would require IPA to provide Global Catalog so that AD DCs would be able to resolve IPA users and groups SIDs to their names. As IPA has no Global Catalog service, it is not possible to assign additional rights to IPA users and groups access the Active Directory resources, like logging into the Windows machines or change any files other than those assigned for "all authenticated users".

Note also, that with cross-forest trust, there are explicit SID filters set up by both Active Directory and IPA. These filters disallow IPA users and services to have MS-PAC record which includes SIDs from the Active Directory domains. Even if an attacker would fake MS-PAC record (to do so, it needs to completely own IPA KDC), it would not be able to add the SIDs from Active Directory domains to the MS-PAC -- such MS-PAC will be refused by Active Directory Domain Controllers at the forest boundary.

Final consequence of v3.0 and v3.3 trust feature designs was the fact that if IPA clients needed to resolve AD users and groups, they needed to be enrolled to an IPA master which has been initialized with ipa-adtrust-install. As a result, in major deployments all of IPA masters had to run smbd and winbindd processes. While this is an improvement over requirement to run winbindd to every single client as with some other solutions, it is still too fragile for production. This shortcoming is addressed by V4/Trust agents feature design.

New design

In order to support one-way trust to Active Directory, we need to switch SSSD in IPA master mode to use TDO credentials when resolving AD users and groups. This is a high level description of the design, and majority of work to allow the switch will be done by SSSD team. Corresponding ticket tracker on SSSD side is ticket 2579, the text below is an overview of the design.

On each IPA master SSSD runs in "IPA master mode". This mode means that in case of existing trust to AD forest, SSSD will directly resolve AD users and groups against Active Directory Domain Controllers. To perform user/group resolution, SSSD needs to authenticate against AD LDAP servers and it does so using Kerberos authentication based on a host/ipa.master@IPA.REALM service ticket. The ticket towards AD LDAP services is issued by FreeIPA KDC with the help of cross-realm trust credentials.

For one-way trust SSSD cannot use this approach because Active Directory Domain Controllers do not trust FreeIPA realm and, therefore, no cross-realm trust credentials exist in AD for FreeIPA realm. However, SSSD can use TDO object which always exists in AD for the trusting domain (cross-forest trust is done by forest root domains' trust). This means the ticket SSSD would need to request belongs to a different realm (AD forest root realm) rather than to FreeIPA realm.

As FreeIPA supports multiple trusts to separate Active Directory forests, a support for multiple separate tickets is required. SSSD will need to gain ability to use different credentials caches to store TDO tickets and use different keytabs with TDO credentials to obtain the ticket from an Active Directory Domain Controllers.

In order to separate privilege access, FreeIPA masters have to provide keytabs for SSSD running on IPA masters, one keytab per trusted AD forest, so that SSSD could request the keys when required.

Additionally, FreeIPA management framework will need to change its defaults from producing a two-way trust to a one-way trust. Two-way trust will be added back when support for Global Catalog service will be added so that Active Directory resources could be properly accessed and access to them discretionally granted to FreeIPA users and groups.

Implementation

Following changes will need to be done on FreeIPA side in order to support one-way trust:

  1. Switch two-way trust creation in ipaserver/dcerpc.py to one-way by default.
    1. The code needs to be changed to allow specifying either one- or two-way trust and should manipulate trust_direction property (by setting lsa.LSA_TRUST_DIRECTION_OUTBOUND or a combination of lsa.LSA_TRUST_DIRECTION_INBOUND and lsa.LSA_TRUST_DIRECTION_OUTBOUND) in TrustDomainInstance.establish_trust() method.
    2. One-way trust can be created with full AD administrator credentials too, while shared secret method will rely on the AD administrator creating the remote part of it in AD.
  2. Make sure ipalib/plugins/trust.py passes properly a flag to enable two-way trust.
  3. Change ipasam to create additional principal named IPA$@AD.REALM form when creating TDO object for AD.REALM forest trust. This principal has to be disabled so that KDC cannot use it to issue tickets.
  4. Swtich IPA framework to perform out of band DBus call to external script that would use TDO credentials to populate information about trusted domains

Details about oddjobd-triggered script

Access to trusted domain object (TDO) is highly regulated (by us) because possession of the TDO credentials impersonates whole trust link. Thus, we want to avoid authenticating as TDO within Apache process.

To achieve this a scheme similar to oddjob-mkhomedir is used, by providing a helper script which is executed by oddjobd on request from Apache:

Apache process sends DBus request to oddjobd daemon. Oddjobd daemon executes an IPA helper. IPA helper accesses /etc/samba/samba.keytab and authenticates as cifs/ipa.master@IPA.REALM. It then fetches TDO credentials from IPA LDAP and authenticates with them to AD DC. Once operation is performed, it connects again to IPA LDAP and updates it.

There are several moving parts here:

  1. /etc/samba/samba.keytab is root:root, 0600, unconfined_u:object_r:samba_etc_t:s0. It is created by /usr/sbin/ipa-adtrust-install
  2. /var/lib/sss/keytabs/ad.test.keytab is sssd:sssd (or root:root), 0600, unconfined_u:object_r:sssd_var_lib_t:s0. It can be created by IPA helper or by SSSD, whoever runs into need of the keytab first. The name is dependent on the AD forest root name (ad.test in for example). The ownership of the keytabs depends on the way SSSD runs --- either privileged (as root) or non-privileged (as sssd user).
  3. /usr/libexec/ipa/com.redhat.idm.trust-fetch-domains is root:root, 0755, system_u:object_r:ipa_helper_exec_t:s0 label. It is the IPA helper oddjobd daemon will be calling in response to Apache request. The helper is written in Python.
  4. /var/run/ipa/krb5cc_oddjob_trusts{,_fetch} -- credential caches used by the helper. They are root:root, 0600, system_u:object_r:ipa_var_run_t:s0 label.
  5. oddjobd daemon runs under system_u:system_r:oddjob_t:s0-s0:c0.c1023 context.

Feature management

CLI

The newly created trust will become one-way only, no additional options will be needed. To force creating a two-way trust, pass --two-way option to ipa trust-add.

ipa-adtrust-install utility gained --add-agents option. This option adds IPA masters to the list that allows to serve information about users from trusted forests. Starting with FreeIPA 4.2, a regular IPA master can provide this information to SSSD clients. IPA masters aren't added to the list automatically as restart of the LDAP service on each of them is required. The host where ipa-adtrust-install is being run is added automatically.

Note that IPA masters where ipa-adtrust-install wasn't run, can serve information about users from trusted forests only if they are enabled via ipa-adtrust-install run on any other IPA master. At least SSSD version 1.13 on IPA master is required to be able to perform as a trust agent.

Web UI

No changes in Web UI are required if we wouldn't expose two-way trust option.

Replication

Trust-related information is in the replicated subtree already.

Upgrades

On upgrade sidgen and extdom plugins get enabled by default on all IPA masters to help with AD trust agent mode.

SSSD will use ipa-getkeytab to obtain the keytabs if keytab is missing.

Test Plan

http://www.freeipa.org/page/V4/One-way_trust/Test_Plan