One-way trust with shared secret ================================ Overview -------- FreeIPA does support trust to an Active Directory forest. The trust can be established using administrative credentials from the forest root domain or using a so-called shared secret. In the latter case no administrative access is given to the remote side of the trust and each administrator performs their configuration separately: FreeIPA administrator configures IPA side, Active Directory administrator adds IPA forest as a trusted one on the Active Directory side. For trust to be active, one needs to validate it. Validation process includes a sequences of DCE RPC calls that force a domain controller on the trusted side to establish a so-called "secure channel" to a remote domain controller in the trusting domain. This is an administrative operation and requires administrative privileges to activate. If trust was established using a shared secret, IPA side will lack ability to initiate a validation process. At the same time, FreeIPA 4.6 or earlier versions do not include functionality to allow a remote validation from Active Directory to happen before trust objects are created and SSSD can retrieve information from the Active Directory side. Unfortunately, the latter is not possible until trust is validated. The purpose of this design is to extend FreeIPA setup to allow trust validation to be initiated from Windows UI in case a shared secret is used to create a trust agreement. Use Cases --------- As a FreeIPA administrator, I'd like to establish a trust between an Active Directory forest and a FreeIPA deployment using a shared secret. As FreeIPA administrator, I have no administrative access to Active Directory and would like to delegate the operation to create trust on Active Directory side to my counterpart in Active Directory forest. How to Use ---------- 1. Establish a one-way trust with a shared secret on IPA side: `ipa trust-add <ad-domain> --shared-secret` 2. On Windows side, open Active Directory Domain and Trusts tool * Open properties for the Windows forest * Choose 'Trusts' tab and press 'New trust' button there * Navigate through the trust wizard by entering: * IPA forest name, then 'next' * Choose 'Forest trust' on the Trust Type page * Choose 'One-way: incoming' on the Direction of Trust page * Choose 'This domain only' on the Sides of Trust page * Enter the same shared secret one was using in step (1) with 'ipa trust-add' * Complete trust wizard 3. Going back to the trust properties, one can now validate the trust from Windows side. Note, however, that retrieving forest trust information is not working due to Samba not having it implemented for non-Samba AD case. Design ------ There are two important parts of the solution. ### FreeIPA module for Samba passdb interface FreeIPA provides a special module for Samba, `ipasam`, that looks up information about trusted domains and user/group in FreeIPA LDAP. The module also maintains trust-related information when trust is created via DCE RPC interfaces. When trust is created, `ipasam` module needs to create a set of Kerberos principals to allow Kerberos KDC to issue cross-realm ticket granting tickets. These principals will have the same keys as trusted domain objects on Active Directory level. When a secure channel is established between two domain controllers from separate trusted domains, both DCs rely on the trusted domain object account credentials to be the same on both sides. However, since Samba has to perform SMB to POSIX translation when running in POSIX environment, it also needs to have a POSIX identity associated with the trusted domain object account. As result, `ipasam` module needs to maintain POSIX attributes for the trusted domain object account, along with Kerberos principals associated with the trust. ### SSSD When Windows successfully authenticates to Samba, Samba needs a POSIX identity to run `smbd` processes as the authenticated 'user'. `smbd` and `winbindd` processes use standard system calls to resolve authenticated user to a system one (`getpwnam_r()`) and if the call fails, whole Windows request is rejected. Given that trusted domain object accounts are associated with the cross-realm Kerberos principals, they are located in a special subtree in FreeIPA LDAP: `cn=trusts,$SUFFIX`. However, SSSD does not look by default in this subtree for users. By default, SSSD configuration for user accounts looks in `cn=users,cn=accounts,$SUFFIX` for `id_provider = ipa` and will not be able to see trusted domain object accounts. Thus, to allow Windows to successfully validate a one-way shared incoming trust to FreeIPA, SSSD needs to resolve trusted domain object accounts as POSIX users on IPA master side. Implementation -------------- ### FreeIPA `ipasam` module `ipasam` module needs to create and maintain POSIX identities of the trusted domain object accounts. Following objects and their aliases are created and maintained by `ipasam` module. In the description below `REMOTE` means Kerberos realm of the Active Directory forest's root domain (e.g. `AD.EXAMPLE.COM`), `REMOTE-FLAT` is NetBIOS name of the Active Directory forest's root domain (e.g. `AD`). Correspondingly, `LOCAL` means FreeIPA Kerberos realm (e.g. `IPA.EXAMPLE.COM`) and `LOCAL-FLAT` is the NetBIOS name of the FreeIPA primary domain (e.g. `IPA`). Principal | Description --------- | ----------- krbtgt/REMOTE@LOCAL | Cross-realm principal representing Active Directory forest root domain REMOTE-FLAT$@LOCAL | Trusted domain object account for the Active Directory forest root domain krbtgt/REMOTE-FLAT@LOCAL | Alias to REMOTE-FLAT$ TDO krbtgt/LOCAL@REMOTE | Cross-realm principal representing IPA domain in Active Directory forest to allow crross-realm TGT issuance from IPA KDC side LOCAL-FLAT$@REMOTE | Trusted domain object account for IPA domain in Active Directory forest krbtgt/LOCAL-FLAT@REMOTE | Alias to LOCAL-FLAT$ For inbound trust `ipasam` module creates following principals: * `krbtgt/LOCAL@REMOTE`, enabled by default * `LOCAL-FLAT$@REMOTE`, used by SSSD to talk to Active Directory domain controllers, with canonical name set to `LOCAL-FLAT$` because Kerberos KDC must use this salt when issuing tickets for this principal. The use of this principal is disabled on IPA side (IPA KDC does not issue tickets in this name) --- we only retrieve a keytab for the principal in SSSD. For outbound trust `ipasam` module creates following principals: * `krbtgt/REMOTE@LOCAL`, enabled by default. * `REMOTE-FLAT$@LOCAL@`, enabled by default. Used by a remote AD DC to authenticate against Samba on IPA master. This principal will have POSIX identity associated. ### SSSD In IPA master mode, SSSD needs to start look into `cn=trusts,$SUFFIX` subtree in addition to `cn=users,cn=accounts,$SUFFIX` subtree to find trusted domain object accounts. This can be achieved either by explicitly adding a second search base to `ldap_search_user_base` option in the `[domain/...]` section or by automatically expanding a list of search bases when running in the IPA master mode. Feature Management ------------------ We already allow to create a shared secret-based trust to Active Directory. No functional change is needed in either Web UI or CLI. Upgrade ------- During upgrade POSIX identities need to be created for existing trust agreements.