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Supporting Local Users as members of LDAP Groups for RRFc2307 servers

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Problem Statement

SSSD Has been built around the concept of self-contained Identity Domains. Because of this all users of a domain must be present in the domain itself to be available as members of the domain groups.

Historically identity providers like nss_ldap has allowed to include local users in remote LDAP servers that use the RFC2307 (not bis) schema. With that schema group members are identified by the simple user name. So if a user by the same name happened to exist on the local workstation the LDAP group would end up being assigned to the user during operations like initgroups.

This is technically a violation of the Identity domain and works mostly by accident. However in order to keep compatibility with existing deployments it has been requested to allow sssd to honor initgroups request for local users that happen to be referenced in RFC2307 LDAP servers.

Solution

New Option

We introduce a new boolean option named ldap_rfc2307_fallback_to_local_users This option enables or disables the compatibility behavior. The option is set to ‘false’ by default.

Behavior

When the above option is enabled the LDAP provider will perform additional local lookups for users only if the schema in use is RFC2307. A simple getpwnam() or getpwuid() call is performed when looking up users if the LDAP server returns no entry. If the a local user by the same name or id exists it is stored in the cache like if it were an LDAP user. The same is done for initgroups calls.

Details

Calls like initgroups will not fail anymore if the user is not found in LDAP like they normally would do and groups this user ‘belongs to’ are returned. The groups returned are the ones found in LDAP that have this user’s name in the memberUid attribute.

SSSD backends disable by default recursion from nsswitch calls into SSSD itself. It is therefore safe to call functions like getpwnam() or getpwuid() from within a backend. These functions will not enter the nss client and will return all users from any other backend listed in nsswitch.conf for the ‘passwd’ database.