The rid idmap backend

Note:
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The rid idmap backend provides an algorithmic mapping between Linux uids/gids and Active Directory SIDs. That means that a given SID will always map to the same uid/gid, and vice-versa, within the same domain.

To use this backend, we have to choose two or more ID ranges:

a range for the domain we are joining
another range to serve as a “catch all”, which will store mappings for users and groups that might come from other domains, as well as the default built-in entries

Let’s analyse an example configuration:

[global]
    ...
    security = ads
    realm = EXAMPLE.INTERNAL
    workgroup = EXAMPLE
    ...
    idmap config * : backend       = tdb
    idmap config * : range         = 100000 - 199999

    idmap config EXAMPLE : backend = rid
    idmap config EXAMPLE : range   = 1000000 - 1999999

Note:
This is not yet a complete configuration file, and is just illustrating the idmap configuration. More is needed to join an Active Directory domain.

This configuration is using two idmap backends, and carving out two ranges:

* domain, or “default domain”: any SID that is not mapped via another more specific idmap configuration will use this backend. Since this mapping is not deterministic, a database is needed to keep a record, hence the tdb backend is used.
EXAMPLE domain: uses the rid idmap backend, and users from the EXAMPLE domain will be allocated IDs in the range of 1.000.000 to 1.999.999, that is, there is space for 1 million IDs. Since the mapping is deterministic, there is no need for a database.

simple-rid-ranges

Important:
Planning a range of IDs to be used for the mapping critically important. Such a range can never be reduced, just expanded (carefully!), and it must NEVER overlap with another allocated range.

Once this system is joined to the EXAMPLE.INTERNAL domain, users from that domain will be allocated corresponding Linux uids and gids from the specified range in a deterministic way, following a formula. As long as the above configuration is used in all Ubuntu systems joined to the domain, the same Active Directory user will always get the same Linux IDs in all those systems.

Things start to get more complicated if the EXAMPLE.INTERNAL domain establishes a trust relationship with another Active Directory domain. The correct way to handle this is to, before, add a new idmap configuration for that domain. For example:

[global]
    ...
    security = ads
    realm = EXAMPLE.INTERNAL
    workgroup = EXAMPLE
    ...
    idmap config * : backend       = tdb
    idmap config * : range         = 100000 - 199999

    idmap config EXAMPLE : backend = rid
    idmap config EXAMPLE : range   = 1000000 - 1999999

    idmap config COMPANY : backend = rid
    idmap config COMPANY : range   = 2000000 - 2999999

This change is allocating a new range for the new COMPANY domain. Then, when the domain trust relationship is established between the Active Directory domains, the Ubuntu systems with this extra idmap configuration will know that users from the COMPANY belong to the range 2000000 - 2999999.

If, however, the domain trust relationship is established between EXAMPLE and COMPANY before the new idmap range is configured, then the users and groups from COMPANY will have their ID allocations taken from the default domain “*”, which is NOT deterministic and is done on a first come, first serve, basis. This means that the same Active Directory user from domain COMPANY connecting to different Ubuntu systems will likely get a different Linux ID.

Pros and Cons of the rid backend

We now have enough information to understand the pros and cons of the idmap_rid backend, and which scenarios are a better fit for it.

Pros:

Stable mapping of SIDs to IDs within a single domain: all Ubuntu systems sharing the same configuration will arrive at the same mapping for Active Directory users.

Cons:

extra config for trusted domains: you must add idmap config entries for all trusted domains, and deploy these changes to all joined systems before the domains establish a new trust relationship, or else you risk having users of the new trusted domain be allocated IDs from the default backend (“*”) range.

With that in mind, idmap_rid is best used in the following scenarios:

Single domain with no trust relationships
If there are trust relationships, they are fairly static, and well planned in advance, and there is a configuration file management system in place to easily update the smb.conf config file with the new idmap config lines across all joined systems.
Stability of Linux IDs across multiple joined systems is important. For example, NFSv3 is being used.

The autorid idmap backend

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