Access control in SPS is based on groups. Whenever a user needs to access a protected resource, like navigating to a configuration page on the SPS web interface, or opening a channel in a connection, SPS checks the access control list associated with the resource in question.
The access control lists grant access to groups. Therefore, SPS needs to determine which groups the user is a member of to evaluate the access rules.
When you configure SPS to use an LDAP backend, SPS will:
Identify the user. For more information, see User identification below.
Determine the relevant groups the user is a member of. For more information, see Group membership resolution below.
SPS works with plain usernames, for example, administrator. This must be unambiguously resolved to an LDAP user object in order to determine the user’s groups. If a user identification returns multiple results, SPS treats this as an error, and access to the user in question is denied.
Only the user object returned in this phase is used for group membership checks, and not the original plain username.
User resolution depends on the type of the backend (POSIX or Active Directory).
For more information, see the backend-specific sections below.
SPS works with plain group names, for example, superusers. For group membership checks, SPS looks up a relevant group object in LDAP and checks if the user object returned during user identification is a member of that group. Since some of the group object’s attributes are always used for group membership checks, the group object must also exist in LDAP.
Group membership resolution depends on the LDAP backend type.
For more information, see the backend-specific sections below.
All backends have configurable parameters relevant for user identification and group membership:
bind_dn and bind_password: Bind DN and Bind password are used for user identification and group membership check during authentication to the LDAP database. If you leave it empty, SPS will try to bind anonymously.
user_base_dn: User Base DN is where SPS searches for users.
group_base_dn: Group Base DN is where SPS searches for groups. Only groups under this base are considered for membership.
memberof_check: the Enable checking for group DNs in user objects setting allows checking a configurable attribute in the user object. This attribute contains a list of group DNs the user is additionally a member of. This user attribute is usually memberOf. For more information, see the backend-specific sections below.
user_dn_in_groups: Check the user DN in these groups is a list of additional group object classes and their respective attributes where SPS will look for member user DNs. For more information, see the backend-specific sections below.
All comparisons and searches are done by SPS in a way that plain user and group names are matched with attribute values by the LDAP server. As a result, user and group names are case insensitive if and only if the matching rule for the attribute in question is case insensitive in the LDAP database.
In addition to the common parameters, the Active Directory (AD) backend has the following additional configurable parameters:
membership_check: Enable AD group membership check enables AD specific non-primary group membership checking.
NOTE: The AD user’s primary group is always checked regardless of this setting.
nested_groups: Enable nested groups allows AD nested group support. See below for details.
Additionally, AD supports case and accent insensitive matching in many of the user and group name attributes. Since SPS relies on the server to perform comparisons, case and accent insensitive user and group name support depends solely on the server configuration.
To determine the user entry for a given plain username, SPS performs a search under user_base_dn for objects having either the sAMAccountName or the userPrincipalName equal to the plain username of the user. The objectClass of the user object is not restricted.
NOTE: Although userPrincipalName in AD is a Internet-style name like user@example.com, it matches simple names like user.
Only the user object returned here is used for group membership checks.
For all group membership checks, only the LDAP user object returned during user identification phase is used.
The plain group name is always compared to the cn attribute of the group object.
A user is treated as a member of a group if both the group object’s objectClass and objectCategory is group, and any of the following is true:
The group is the user’s primary group. That is, the objectSID attribute of the group matches the Security Identifier calculated from the user object’s objectSID and primaryGroupID attributes, as described in the Microsoft Support article How to use the PrimaryGroupID attribute to find the primary group for a user.
NOTE: When using the AD backend, this check is always performed, even if the membership_check option is disabled. However, it is OK for the user to have no primary group.
The group lists the user’s short username. That is, the group’s memberUid attribute contains the short username from the user object.
This check is performed only when the membership_check option is enabled for AD.
NOTE: For the purpose of this check, the user’s short username is retrieved from the user object’s sAMAccountName attribute only, which is a single-valued attribute in AD. This is a known limitation.
It is OK for the sAMAccountName attribute to be missing, in which case this check will be skipped.
The group lists the user’s dn. That is, the group object’s member attribute contains the user’s dn.
This check is performed only when the membership_check option is enabled for AD.
This is the only place where nested groups are supported. When the nested_groups setting is enabled in the configuration, SPS will also find groups which do not directly contain the user’s dn in their member attribute, but do contain an intermediate group’s dn, which in turn contains the user dn in its member attribute. This nesting can be arbitrarily deep, limited only by AD.
NOTE: Due to the nature of the way AD resolves the nested group chain, intermediate groups might be outside the configured group_base_dn.
NOTE: Although an objectCategory in AD is a DN-valued attribute, it does match simple names like group.
Additionally, a user is treated as a member of a group if:
The group lists the user’s dn in any of the additional group objects configured in user_dn_in_groups.
For example, if a row is added with objectClass set to groupOfNames and attribute set to member, SPS will treat the user as a member of all groups where the group is a groupOfNames, and the group’s member attribute contains the user’s dn.
NOTE: There is no additional restriction on the group’s objectClass in this case.
The user lists the group’s dn. That is, the user’s memberof_user_attribute contains the dn of the group, and the objectClass of the referred group is group.
This check is performed only when the memberof_check option is enabled for AD.
NOTE: SPS compares the dn stored in the memberof_user_attribute to the dn of the group object itself in a strict stringwise manner. Therefore, this user attribute must contain the group DN exactly as it would be returned by the LDAP server. No case or accent differences are allowed.
In addition to the common parameters, the POSIX backend has the following configurable parameters:
username_attribute: Username (user ID) attribute name is the name of the attribute in the user object, which contains the user’s plain username.
membership_check: Enable POSIX group membership check enables POSIX primary and supplementary group membership checking. When enabled, it has the following configurable parameter:
member_uid_attribute: the optional POSIX group membership attribute name is the name of the attribute in a posixGroup group object, which lists the plain usernames that are members of the group. These groups are usually referred to as supplementary groups of the referred user.
To determine the user entry for a given plain username, SPS performs a search under user_base_dn for objects having the username_attribute equal to the plain username of the user. The objectClass of the user object is not restricted.
The user object returned here is used for group membership checks.
For all group membership checks, only the LDAP user object returned during user identification phase is used.
The plain group name is always compared to the cn attribute of the group object.
A user is treated as a member of a group given by its plain group name if the plain group name matches the cn attribute of the group object, and any of the following is true:
The group is the user’s primary group. That is, the group is a posixGroup, and the user’s gidNumber attribute is equal to the group’s gidNumber attribute.
This check is performed only when the membership_check option is enabled for POSIX.
NOTE: It is OK for the user to have no gidNumber attribute, in which case this check will be skipped.
The group lists the user’s short username. That is, the group is a posixGroup, and it’s member_uid_attribute contains the short username from the user object.
This check is performed only when the membership_check option is enabled, and the member_uid_attribute is configured.
NOTE: For the purpose of this check, the user’s short username is retrieved from the user object’s username_attribute. Currently, this attribute should only contain a single username. A warning will appear in the logs if this is not the case, and the first value of the attribute will be used as returned by the server. This is a known limitation.
The group lists the user’s dn in any of the additional group objects configured in user_dn_in_groups.
For example, if a row is added with objectClass set to groupOfNames and attribute set to member, SPS will treat the user as a member of all groups where the group is a groupOfNames, and the group’s member attribute contains the user’s dn.
The user lists the group’s dn. That is, the user’s memberof_user_attribute contains the dn of the group, and the objectClass of the referred group is memberof_group_objectclass.
This check is performed only when the memberof_check option is enabled for POSIX.
NOTE: SPS compares the dn stored in the memberof_user_attribute to the dn of the group object itself in a strict stringwise manner. Therefore, the user attribute must contain the group DN exactly as it would be returned by the LDAP server. No case or accent differences are allowed.
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