<refsect1 id='idmap'>

<title>ID MAPPING</title>

<para>

The ID-mapping feature allows SSSD to act as a client of Active

Directory without requiring administrators to extend user attributes

to support POSIX attributes for user and group identifiers.

</para>

<para>

NOTE: When ID-mapping is enabled, the uidNumber and gidNumber

attributes are ignored. This is to avoid the possibility of conflicts

between automatically-assigned and manually-assigned values. If you

need to use manually-assigned values, ALL values must be

manually-assigned.

</para>

<refsect2 id='idmap_algorithm'>

<title>Mapping Algorithm</title>

<para>

Active Directory provides an objectSID for every user and group

object in the directory. This objectSID can be broken up into

components that represent the Active Directory domain identity and

the relative identifier (RID) of the user or group object.

</para>

<para>

The SSSD ID-mapping algorithm takes a range of available UIDs and

divides it into equally-sized component sections - called

"slices"-. Each slice represents the space available to an Active

Directory domain.

</para>

<para>

When a user or group entry for a particular domain is encountered

for the first time, the SSSD allocates one of the available slices

for that domain. In order to make this slice-assignment repeatable

on different client machines, we select the slice based on the

following algorithm:

</para>

<para>

The SID string is passed through the murmurhash3 algorithm to

convert it to a 32-bit hashed value. We then take the modulus of

this value with the total number of available slices to pick the

slice.

</para>

<para>

NOTE: It is possible to encounter collisions in the hash and

subsequent modulus. In these situations, we will select the next

available slice, but it may not be possible to reproduce the same

exact set of slices on other machines (since the order that they

are encountered will determine their slice). In this situation, it

is recommended to either switch to using explicit POSIX attributes

in Active Directory (disabling ID-mapping) or configure a default

domain to guarantee that at least one is always consistent. See

<quote>Configuration</quote> for details.

</para>

</refsect2>

<refsect2 id='idmap_config'>

<title>Configuration</title>

<para>

Minimum configuration (in the <quote>[domain/DOMAINNAME]</quote>

section):

</para>

<para>

<programlisting>

ldap_id_mapping = True

ldap_schema = ad

</programlisting>

</para>

<para>

The default configuration results in configuring 10,000 slices,

each capable of holding up to 200,000 IDs, starting from 10,001

and going up to 2,000,100,000. This should be sufficient for

most deployments.

</para>

<refsect3 id='idmap_advanced_config'>

<title>Advanced Configuration</title>

<variablelist>

<varlistentry>

<term>ldap_idmap_range_min (integer)</term>

<listitem>

<para>

Specifies the lower bound of the range of POSIX IDs to

use for mapping Active Directory user and group SIDs.

</para>

<para>

NOTE: This option is different from

<quote>min_id</quote> in that <quote>min_id</quote>

acts to filter the output of requests to this domain,

whereas this option controls the range of ID

assignment. This is a subtle distinction, but the

good general advice would be to have

<quote>min_id</quote> be less-than or equal to

<quote>ldap_idmap_range_min</quote>

</para>

<para>

Default: 200000

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>ldap_idmap_range_max (integer)</term>

<listitem>

<para>

Specifies the upper bound of the range of POSIX IDs to

use for mapping Active Directory user and group SIDs.

</para>

<para>

NOTE: This option is different from

<quote>max_id</quote> in that <quote>max_id</quote>

acts to filter the output of requests to this domain,

whereas this option controls the range of ID

assignment. This is a subtle distinction, but the

good general advice would be to have

<quote>max_id</quote> be greater-than or equal to

<quote>ldap_idmap_range_max</quote>

</para>

<para>

Default: 2000200000

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>ldap_idmap_range_size (integer)</term>

<listitem>

<para>

Specifies the number of IDs available for each slice.

If the range size does not divide evenly into the min

and max values, it will create as many complete slices

as it can.

</para>

<para>

Default: 200000

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>ldap_idmap_default_domain_sid (string)</term>

<listitem>

<para>

Specify the domain SID of the default domain. This

will guarantee that this domain will always be

assigned to slice zero in the ID map, bypassing

the murmurhash algorithm described above.

</para>

<para>

Default: not set

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>ldap_idmap_default_domain (string)</term>

<listitem>

<para>

Specify the name of the default domain.

</para>

<para>

Default: not set

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>ldap_idmap_autorid_compat (boolean)</term>

<listitem>

<para>

Changes the behavior of the ID-mapping algorithm

to behave more similarly to winbind's

<quote>idmap_autorid</quote> algorithm.

</para>

<para>

When this option is configured, domains will be

allocated starting with slice zero and increasing

monatomically with each additional domain.

</para>

<para>

NOTE: This algorithm is non-deterministic (it

depends on the order that users and groups are

requested). If this mode is required for

compatibility with machines running winbind, it

is recommended to also use the

<quote>ldap_idmap_default_domain_sid</quote>

option to guarantee that at least one domain is

consistently allocated to slice zero.

</para>

<para>

Default: False

</para>

</listitem>

</varlistentry>

</variablelist>

</refsect3>

</refsect2>

</refsect1>