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1.4. Understanding and configuring Samba ID mapping

Windows domains distinguish users and groups by unique Security Identifiers (SID). However, Linux requires unique UIDs and GIDs for each user and group. If you run Samba as a domain member, the winbindd service is responsible for providing information about domain users and groups to the operating system.

To enable the winbindd service to provide unique IDs for users and groups to Linux, you must configure ID mapping in the /etc/samba/smb.conf file for:

  • The local database (default domain)
  • The AD or NT4 domain the Samba server is a member of
  • Each trusted domain from which users must be able to access resources on this Samba server

Samba provides different ID mapping back ends for specific configurations. The most frequently used back ends are:

Expand
Back endUse case

tdb

The * default domain only

ad

AD domains only

rid

AD and NT4 domains

autorid

AD, NT4, and the * default domain

1.4.1. Planning Samba ID ranges
링크 복사

Regardless of whether you store the Linux UIDs and GIDs in AD or if you configure Samba to generate them, each domain configuration requires a unique ID range that must not overlap with any of the other domains.

주의

If you set overlapping ID ranges, Samba fails to work correctly.

예 1.1. Unique ID Ranges

The following shows non-overlapping ID mapping ranges for the default (*), AD-DOM, and the TRUST-DOM domains. 

[global]
...
idmap config * : backend = tdb
idmap config * : range = 10000-999999

idmap config AD-DOM:backend = rid
idmap config AD-DOM:range = 2000000-2999999

idmap config TRUST-DOM:backend = rid
idmap config TRUST-DOM:range = 4000000-4999999
중요

You can only assign one range per domain. Therefore, leave enough space between the domains ranges. This enables you to extend the range later if your domain grows.

If you later assign a different range to a domain, the ownership of files and directories previously created by these users and groups will be lost.

1.4.2. The * default domain
링크 복사

You can configure the default Samba ID mapping domain to ensure proper ID assignment for local users, groups, and built-in accounts. Select and manage appropriate back ends to maintain system security, scalability, and compliance in domain environments.

In a domain environment, you add one ID mapping configuration for each of the following:

  • The domain the Samba server is a member of
  • Each trusted domain that should be able to access the Samba server

However, for all other objects, Samba assigns IDs from the default domain. This includes:

  • Local Samba users and groups
  • Samba built-in accounts and groups, such as BUILTIN\Administrators
중요

You must configure the default domain as described to enable Samba to operate correctly.

The default domain back end must be writable to permanently store the assigned IDs.

For the default domain, you can use one of the following back ends:

tdb

When you configure the default domain to use the tdb back end, set an ID range that is big enough to include objects that will be created in the future and that are not part of a defined domain ID mapping configuration.

For example, set the following in the [global] section in the /etc/samba/smb.conf file: 

idmap config * : backend = tdb
idmap config * : range = 10000-999999

For further details, see Using the TDB ID mapping back end.

autorid

When you configure the default domain to use the autorid back end, adding additional ID mapping configurations for domains is optional.

참고

The range should be a multiple of the rangesize and must be at least twice its value to ensure sufficient id range space for the mandatory BUILTIN domain. With a default rangesize of 100000, the range must span at least 200000. For example, range = 100000 - 299999.

For example, set the following in the [global] section in the /etc/samba/smb.conf file: 

idmap config * : backend = autorid
idmap config * : range = 10000-999999

For further details, see Using the autorid ID mapping back end.

1.4.3. Using the tdb ID mapping back end
링크 복사

The winbindd service uses the writable tdb ID mapping back end by default to store Security Identifier (SID), UID, and GID mapping tables. This includes local users, groups, and built-in principals.

Use this back end only for the * default domain. For example: 

idmap config * : backend = tdb
idmap config * : range = 10000-999999

1.4.4. Using the ad ID mapping back end
링크 복사

You can configure a Samba AD member to use the ad ID mapping back end.

The ad ID mapping back end implements a read-only API to read account and group information from AD. This provides the following benefits:

  • All user and group settings are stored centrally in AD.
  • User and group IDs are consistent on all Samba servers that use this back end.
  • The IDs are not stored in a local database which can corrupt, and therefore file ownerships cannot be lost.
참고

The ad ID mapping back end does not support {AD} domains with one-way trusts. If you configure a domain member in an {AD} with one-way trusts, use instead one of the following ID mapping back ends: tdb, rid, or autorid.

The ad back end reads the following attributes from AD:

Expand
AD attribute nameObject typeMapped to

sAMAccountName

User and group

User or group name, depending on the object

uidNumber

User

User ID (UID)

gidNumber

Group

Group ID (GID)

loginShell [a]

User

Path to the shell of the user

unixHomeDirectory [a]

User

Path to the home directory of the user

primaryGroupID [b]

User

Primary group ID

[a] Samba only reads this attribute if you set idmap config DOMAIN:unix_nss_info = yes.
[b] Samba only reads this attribute if you set idmap config DOMAIN:unix_primary_group = yes.

Prerequisites

  • Both users and groups must have unique IDs set in AD, and the IDs must be within the range configured in the /etc/samba/smb.conf file. Objects whose IDs are outside of the range will not be available on the Samba server.
  • Users and groups must have all required attributes set in AD. If required attributes are missing, the user or group will not be available on the Samba server. The required attributes depend on your configuration.
  • You installed Samba.
  • The Samba configuration, except ID mapping, exists in the /etc/samba/smb.conf file.

Procedure

  1. Edit the [global] section in the /etc/samba/smb.conf file:

    1. Add an ID mapping configuration for the default domain (*) if it does not exist. For example: 

      idmap config * : backend = tdb
idmap config * : range = 10000-999999

    2. Enable the ad ID mapping back end for the AD domain: 

      idmap config DOMAIN : backend = ad

    3. Set the range of IDs that is assigned to users and groups in the AD domain. For example: 

      idmap config DOMAIN : range = 2000000-2999999
      중요

      The range must not overlap with any other domain configuration on this server. Additionally, the range must be set big enough to include all IDs assigned in the future. For further details, see Planning Samba ID ranges.

    4. Set that Samba uses the RFC 2307 schema when reading attributes from AD: 

      idmap config DOMAIN : schema_mode = rfc2307

    5. To enable Samba to read the login shell and the path to the users home directory from the corresponding AD attribute, set: 

      idmap config DOMAIN : unix_nss_info = yes

      Alternatively, you can set a uniform domain-wide home directory path and login shell that is applied to all users. For example: 

      template shell = /bin/bash
template homedir = /home/%U

    6. By default, Samba uses the primaryGroupID attribute of a user object as the user’s primary group on Linux. Alternatively, you can configure Samba to use the value set in the gidNumber attribute instead: 

      idmap config DOMAIN : unix_primary_group = yes

  2. Verify the /etc/samba/smb.conf file: 

    # testparm

  3. Reload the Samba configuration: 

    # smbcontrol all reload-config

1.4.5. Using the rid ID mapping back end
링크 복사

You can configure a Samba domain member to use the rid ID mapping back end.

Samba can use the relative identifier (RID) of a Windows SID to generate an ID on Red Hat Enterprise Linux.

참고

The RID is the last part of a SID. For example, if the SID of a user is S-1-5-21-5421822485-1151247151-421485315-30014, then 30014 is the corresponding RID.

The rid ID mapping back end implements a read-only API to calculate account and group information based on an algorithmic mapping scheme for AD and NT4 domains. When you configure the back end, you must set the lowest and highest RID in the idmap config DOMAIN : range parameter. Samba will not map users or groups with a lower or higher RID than set in this parameter.

중요

As a read-only back end, rid cannot assign new IDs, such as for BUILTIN groups. Therefore, do not use this back end for the * default domain.

Benefits of using the rid back end

  • All domain users and groups that have an RID within the configured range are automatically available on the domain member.
  • You do not need to manually assign IDs, home directories, and login shells.

Drawbacks of using the rid back end

  • All domain users get the same login shell and home directory assigned. However, you can use variables.
  • User and group IDs are only the same across Samba domain members if all use the rid back end with the same ID range settings.
  • You cannot exclude individual users or groups from being available on the domain member. Only users and groups outside of the configured range are excluded.
  • Based on the formulas the winbindd service uses to calculate the IDs, duplicate IDs can occur in multi-domain environments if objects in different domains have the same RID.

Prerequisites

  • You installed Samba.
  • The Samba configuration, except ID mapping, exists in the /etc/samba/smb.conf file.

Procedure

  1. Edit the [global] section in the /etc/samba/smb.conf file:

    1. Add an ID mapping configuration for the default domain (*) if it does not exist. For example: 

      idmap config * : backend = tdb
idmap config * : range = 10000-999999

    2. Enable the rid ID mapping back end for the domain: 

      idmap config DOMAIN : backend = rid

    3. Set a range that is big enough to include all RIDs that will be assigned in the future. For example: 

      idmap config DOMAIN : range = 2000000-2999999

      Samba ignores users and groups whose RIDs in this domain are not within the range.

      중요

      The range must not overlap with any other domain configuration on this server. Additionally, the range must be set big enough to include all IDs assigned in the future. For further details, see Planning Samba ID ranges.

    4. Set a shell and home directory path that will be assigned to all mapped users. For example: 

      template shell = /bin/bash
template homedir = /home/%U

  2. Verify the /etc/samba/smb.conf file: 

    # testparm

  3. Reload the Samba configuration: 

    # smbcontrol all reload-config

    For more information, see:

    • The * default domain
    • VARIABLE SUBSTITUTIONS section in the smb.conf(5) man page on your system
    • Calculation of the local ID from a RID, see the idmap_rid(8) man page on your system

1.4.6. Using the autorid ID mapping back end
링크 복사

You can configure a Samba domain member to use the autorid ID mapping back end.

The autorid back end works similar to the rid ID mapping back end, but can automatically assign IDs for different domains. This enables you to use the autorid back end in the following situations:

  • Only for the * default domain
  • For the * default domain and additional domains, without the need to create ID mapping configurations for each of the additional domains
  • Only for specific domains
참고

If you use autorid for the default domain, adding additional ID mapping configuration for domains is optional.

Parts of this section were adopted from the idmap config autorid documentation published in the Samba Wiki. License: CC BY 4.0. Authors and contributors: See the history tab on the Wiki page.

Benefits of using the autorid back end

  • All domain users and groups whose calculated UID and GID is within the configured range are automatically available on the domain member.
  • You do not need to manually assign IDs, home directories, and login shells.
  • No duplicate IDs, even if multiple objects in a multi-domain environment have the same RID.

Drawbacks

  • User and group IDs are not the same across Samba domain members.
  • All domain users get the same login shell and home directory assigned. However, you can use variables.
  • You cannot exclude individual users or groups from being available on the domain member. Only users and groups whose calculated UID or GID is outside of the configured range are excluded.

Prerequisites

  • You installed Samba.
  • The Samba configuration, except ID mapping, exists in the /etc/samba/smb.conf file.

Procedure

  1. Edit the [global] section in the /etc/samba/smb.conf file:

    1. Enable the autorid ID mapping back end for the * default domain: 

      idmap config * : backend = autorid

    2. Set a range that is big enough to assign IDs for all existing and future objects. For example: 

      idmap config * : range = 10000-999999

      Samba ignores users and groups whose calculated IDs in this domain are not within the range.

      주의

      After you set the range and Samba starts using it, you can only increase the upper limit of the range. Any other change to the range can result in new ID assignments, and thus in losing file ownerships.

    3. Optional: Set a range size. For example: 

      idmap config * : rangesize = 200000

      Samba assigns this number of continuous IDs for each domain’s object until all IDs from the range set in the idmap config * : range parameter are taken.

      참고

      If you set a rangesize, you need to adapt the range accordingly. The range needs to be a multiple of the rangesize.

    4. Set a shell and home directory path that will be assigned to all mapped users. For example: 

      template shell = /bin/bash
template homedir = /home/%U

    5. Optional: Add additional ID mapping configuration for domains. If no configuration for an individual domain is available, Samba calculates the ID using the autorid back end settings in the previously configured * default domain.

      중요

      The range must not overlap with any other domain configuration on this server. Additionally, the range must be set big enough to include all IDs assigned in the future. For further details, see Planning Samba ID ranges.

  2. Verify the /etc/samba/smb.conf file: 

    # testparm

  3. Reload the Samba configuration: 

    # smbcontrol all reload-config

    For more information, see:

    • THE MAPPING FORMULAS section in the idmap_autorid(8) man page on your system
    • rangesize parameter description in the idmap_autorid(8) man page on your system
    • VARIABLE SUBSTITUTIONS section in the smb.conf(5) man page on your system
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