The System Security Services Daemon (SSSD) and Winbind are two complementing tools shipped in Red Hat Enterprise Linux for system integration with Microsoft Active Directory. Both tools provide ability to retrieve identity information about users and groups from Microsoft Active Directory forests, transform it for use in POSIX environment and present as system user and group identities for other applications.
A regular computer enrolled into Microsoft Active Directory is called a domain member. In a traditional domain member configuration, the member machine has no possession of a particular user credentials. Instead, it relies on its own connection to its own domain controller to identify a user and to proxy user’s authentication to the domain controller of the domain a user belongs to. In case a user is performing a remote authentication using Kerberos, a remote system has to present a Kerberos ticket to the domain member’s SMB service, like with any other Kerberos services.
A file server capabilities can be enabled on a domain member running under Red Hat Enterprise Linux with the help of Samba suite. Samba suite, when running as a domain member, starts two daemons:
smbd, the main process which handles network connections, file system operations, and remote procedure calls like LSA and NETLOGON. Each connection is handled by a separate smbd child;
winbindd, a process to perform identity resolution for all configured and known domains. Active connection to a domain is handled by a separate winbindd child. winbindd processes connect to domain controllers and perform required LSA and NETLOGON operations against them. Normally, authentication of a user from a trusted domain is delegated to the domain member`s own domain controller which then forwards it further.
Winbind uses a set of identity mapping modules collectively called
idmap modules in Samba terminology. Each idmap module represents a strategy to map security identifiers (SIDs) from Active Directory to corresponding POSIX IDs. Since SID namespace in Active Directory is common for all kinds of objects and POSIX ID name space is separate for users and groups, with both POSIX ID name spaces being smaller than a common SID name space, there exist multiple approaches to perform the translation. A choice of a translation method is tightly connected with a specific deployment configuration.
It is possible to configure Red Hat Enterprise Linux system to use winbindd for both system level POSIX IDs retrieval and file server operations in Samba suite. Thanks to winbindd
idmap modules support, it is also possible to configure it to look up system identities via SSSD. In the latter case winbindd daemon would still be running but will not provide NSS and PAM services.
In many deployments SSSD has already been configured for system-level authentication and authorization purposes. The
sssd-winbind-idmap package provides a winbind idmap module, called
idmap_sss which can be used by winbindd as an identity mapping module to leverage SSSD capabilities.
Since both winbindd and SSSD need to know domain member credentials when communicating with Microsoft Active Directory domain controller, they need to coordinate their operations on more than a configuration level.
There are a few limitations, though, when the
idmap_sss module is used with Red Hat Enterprise Linux 7. Please see Red Hat System Administrator Guide for more details. Therefore Red Hat currently does not recommend using the
idmap_sss module for Samba file server enrolled into an IdM or AD domain.
There are a few exceptions though:
- In cases where Red Hat Enterprise Linux 7.x or 8.0 is used and where Samba/Winbind has already been configured to use the
idmap_sssmodule and where the setup works as expected, Red Hat would still provide support for a single domain (e.g, Samba file server machine is enrolled in AD.COM and all users who want to access the Samba share are managed in this domain).
- In cases where Red Hat Enterprise Linux 7 or 8.0 is used and the same setup, as described above, doesn’t work anymore after an upgrade, Red Hat would still provide support to work on the regression.
- With the release of Red Hat Enterprise Linux 8.1, a technology preview for a Samba server to be set up on an IdM domain member. Further detail is available in section A Samba server, available to IdM and AD users logged into IdM hosts, can now be set up on an IdM domain member as a Technology Preview in RHEL-8.1 Release Notes.
- In a later release, Red Hat will also provide support for Samba file server on directly enrolled Active Directory member systems.
It is recommended to open a support case with Red Hat Global Support Services to provide your Samba use case details to allow improving overall coverage of supported Samba file server configurations. Please make sure to provide relevant debug data for the SSSD and Samba service to speed up the overall resolution time of your support request.