6.9. Prevent reuse of user property values#

New in version 5.0-6-erratum-974: Since UCS 5.0 erratum 974, UCS supports block lists to prevent the reuse of user or group property values.

Block lists is a module in UDM. It allows to configure block lists for UDM properties. When an administrator or software modifies or removes a UDM property on a UDM object, the block list automatically adds an entry about this property with its value to the block list. The entry in the block list prevents that another UDM object can use the same value of the UDM property. Block lists operate on the UDM level.

For example, you want to prevent that UCS reuses values of the UDM property mailPrimaryAddress of the UDM objects user. You configure a block list for the UDM property mailPrimaryAddress. If you then remove the value chef@example.com for the UDM property mailPrimaryAddress from a UDM user object, the UDM block list creates an entry for that value. If you change the value from james@example.com to john@example.com for the UDM property mailPrimaryAddress, the UDM block list creates another entries for james@example.com.

UDM block lists now prevents reusing the values chef@example.com and james@example.com. You can’t use them on other UDM user objects for the UDM property mailPrimaryAddress.

6.9.1. Activate block lists#

Before you can activate the block lists, you first need to update the UCS systems, where you manage UDM objects, to at least UCS 5.0 erratum 974.

Second, you need to set the Univention Configuration Registry Variable directory/manager/blocklist/enabled to true with ucr set on all UCS systems, where you manage UDM objects.

6.9.2. Configure block lists#

You can create, list, and remove block list entries in the UMC module Blocklists, or through the command line tool udm blocklists/list.

On every block list you need to define the following properties:


Provides a human-readable name for the block list for later identification.

Retention time

Defines the retention time for entries in this block list. The retention time is the time period that needs to expire to automatically remove entries from the block list. For example 1m 20d which results in one month and twenty days.

Properties to block

Defines the UDM modules and their properties that the block list prevents from reuse.

The following example for udm blocklists/list shows how to create a block list through the command line. The block list prevents the reuse of the UDM property mailPrimaryAddress for users/user objects and the UDM property mailAddress for groups/group objects.

$ udm blocklists/list create \
  --set name=user-and-group-emails \
  --set retentionTime=40d \
  --append blockingProperties="users/user mailPrimaryAddress" \
  --append blockingProperties="groups/group mailAddress"

6.9.3. Manage block list entries#

You can manage block list entries in the UMC module Blocklists, or through the command line tool udm blocklists/list.

When you activated block lists, UDM automatically creates entries in the configured block list, when you remove a value from a UDM property of a UDM object. UDM automatically deletes expired entries from the block list.

Every block list entry has the following properties:


A SHA-256 hash representing the value that the block list is blocking from reuse. The UDM property value is a clear text value. Before UDM creates the block list entry, it converts the value to lowercase text. All uppercase and lowercase variants of the value then match the block list entry when validated by UDM.

Blocked until

The timestamp when the block list entry expires. It uses the GeneralizedTime-LDAP-Syntax format.

When UDM creates a block list entry, it takes the current date and time, adds the configured retention time of the corresponding block list and writes the result to Blocked until.

Changing the retention time of the block list doesn’t update the Blocked until property of the block list entry.

Origin ID

The ID of the UDM object that caused the block list entry. You can still use the value of the block list entry on this UDM object.


Listing block list entries gives you only the hashes of the blocked values.

Nevertheless, you can search for the clear text value of a particular entry, for example, in case you want to delete that entry.

$ udm blocklists/entry list
DN: cn=sha256:a859cd5964b6ac...,cn=emails,cn=blocklists
DN: cn=sha256:b859cd5964b6ac...,cn=emails,cn=blocklists
DN: cn=sha256:c859cd5964b6ac...,cn=emails,cn=blocklists

$ udm blocklists/entry list --filter value=blocked_email@example.com
DN: cn=sha256:c859cd5964b6ac...,cn=emails,cn=blocklists

6.9.4. Expired block list entries#

Every entry in a block list has a Blocked until property. Block list entries expire after the timestamp passed. A cron job on the Primary Directory Node deletes expired block list entries.

You can configure how often cron runs the job with the Univention Configuration Registry Variable directory/manager/blocklist/cleanup/cron. The log file /var/log/univention/blocklist-clean-expired-entries.log lists the expired entries that UDM deleted.

6.9.5. LDAP ACLs for block lists#

By default every UCS node in the domain and every member of the Domain Admins group can write block list entries. And everybody can read. You can configure the permissions on the Primary Directory Node and the Backup Directory Nodes with the following Univention Configuration Registry Variables:

For example, if you want to give a user the permission to delete block list entries who isn’t member of the Domain Admins group, you need to create a group with that user as member and add the LDAP DN of this group to ldap/database/internal/acl/blocklists/groups/write.