Understanding unique attributes in Active Directory

In this blog, I would like to help the reader understand the rules that govern unique attributes in Active Directory (AD) along with the Open Protocols documentation set. I provide examples for user naming attributes. I also clarify common misunderstandings about attribute uniqueness and attribute indexing.

This blog post does not intend to provide a tutorial on LDAP. I assume the reader is familiar with   directory services concepts as detailed in IETF LDAP specifications (RFC2251 through RFC2256, RFC3377, or the latest RFC4510 through 4519, etc). The reader is also assumed to have hands-on experience with Active Directory.

The common issue                                      

You may have encountered some of the following LDAP errors as results of attempts to add or modify entries in an Active Directory server:

  • constraintViolation (19) [RFC4511]
  • attributeOrValueExists (20) [RFC4511]
  • entryAlreadyExists (68) [RFC4511]

Now you wonder the intricacies of the processing behind the operation and why it didn't succeed, even though you have attempted to add the entry in a newly created sub-tree. You decide to probe the schema, and pull out the relevant classSchema objects and attributeSchema objects. But still, there is no conclusive information. I will try to discuss this in the next paragraphs.

Attribute uniqueness in AD

Attribute uniqueness is enforced by constraints or pre-operation triggers that apply associated checks. The directory server performs checks on all update operations before it makes changes to the directory. Examples of update operations are Add, Modify, and Rename.

Active Directory does not provide any means for identifying unique attributes. Some attributes are unique as a result of their specific processing rules defined by the protocols. In a broader AD view, this translates into associated constraints and triggers applied to objects during replica update operations. These rules depend on objects and may apply to a naming context (NC), a container, a distinguished name (DN), a domain, or an entire forest, etc.

Clarifying attribute indexing and uniqueness

Attribute indexing and attribute value uniqueness are not interrelated concepts in AD. A single-valued or multi-valued attribute can be indexed. And the values may require uniqueness or not, depending on their rules as we previously introduced. For each attribute, the scope of uniqueness is defined by the processing rules and constraints specified by the protocol.

Key references in protocol specifications

Updates operations and their constraints are generally defined in [MS-ADTS] In addition, SAM-specific objects creation and modification apply additional constraints defined in [MS-SAMR] Section

In the following examples, we introduce some unique attributes and link them to the documentation.

Example1: Distinguished names

Distinguished names (DNs) are unique and they unambiguously identify objects in the directory. The directory server does not allow two objects with the same Relative DN (RDN) under the same parent or container. A DN is composed of a RDN, and its container's path. Therefore the uniqueness of RDNs guarantees the uniqueness of DNs. For more details see [MS-ADTS]   Multivalued and Multiple-Attribute RDNs.

A simple example of DN is: CN=username1,OU=Staff,DC=contoso,DC=com

Example 2: sAMAccountName

[MS-SAMR] "3.1.1   Abstract Data Model" explains SAM-related constraints relationships between attributes and triggers defined respectively in Sections and Section of [MS-SAMR] describes the uniqueness requirements for sAMAccountName ([MS-ADA3] 2.221).  

SAMAccountName is unique and also indexed. The sAMAccountName must be unique among all security principal objects within the domain. For instance, attempting to add the following objects will result in an LDAP "Already exists" error.

dn: CN=username1,OU=Staff,DC=contoso,DC=com

 sAMAccountName: username1

 objectClass: user

 dn: CN=username1,OU=Managers,DC=contoso,DC=com

 sAMAccountName: username1

 objectClass: user

Example 3: Uniqueness scope of user naming attributes

A user's RDN is the value of its canonical name or cn attribute. Like other directory objects, a user object has names in the form of cn, name, distinguishedName, and objectGUID. A user object is a security principal object, and it has additional naming attributes. We summarize the scope of uniqueness of these attributes in the following table.



Uniqueness scope


The userPrincipalName  is a single-valued and indexed attribute that is a string that specifies the user principal name (UPN). The UPN is an Internet-style login name for the user based on the Internet standard [RFC822]. It should map to the user e-mail name in the form of <name>@<tree name> where <name> is the user name such as the sAMAccountName and the <tree name> is the domain.

[MS-ADA3] 2.348 Attribute userPrincipalName

[MS-ADTS]   Add Operation Contraints

The UPN must be unique among all security principal objects within the directory forest.


The objectGUID attribute is a single-valued attribute that is the unique identifier for the object. This attribute is a Globally Unique Identifier (GUID).

[MS-ADA3] 2.43 Attribute objectGUID

[MS-ADTS]   Basics, objectGUID, and Special Attribute Behavior

[MS-ADTS]   Add Operation Contraints

The GUID is unique across the enterprise and anywhere else.


The sAMAccountName attribute is a single-valued attribute that is the logon name used to support clients and servers from a previous version of Windows (such as Windows NT 4.0 and earlier, Windows 95, Windows 98, and LAN Manager). The sAMAccountName should be less than 20 characters to support these clients and servers.

[MS-ADA3] 2.221 Attribute sAMAccountName

 [MS-SAMR] Uniqueness requirements for sAMAccountName

The sAMAccountName must be unique among all security principal objects within the domain.


The objectSid attribute is a single-valued attribute that specifies the security identifier (SID) of the user. The SID is a unique value used to identify the user as a security principal. It is a binary value that is set by the system when the user is created.

[MS-ADA3] 2.44 Attribute objectSid

[MS-ADTS]   Basics, objectGUID, and Special Attribute Behavior

[MS-ADTS]   Add Operation Contraints

The objectSid is unique across the domain.



The sIDHistory attribute is a multi-valued attribute that contains previous SIDs used for the user object if the user was moved from another domain.

[MS-ADA3] 2.266 Attribute sIDHistory

ObjectSid is unique per domain, and the sIDHistory attribute contains SIDs from the user's previous domain moves.

Note: Naming attributes can be viewed and managed by the Active Directory User and Computers snap-in.


[MS-ADTS]: Active Directory Technical Specification

[MS-ADSC]: Active Directory Schema Classes

[MS-ADA1]: Active Directory Schema Attributes A-L 

[MS-ADA2]: Active Directory Schema Attributes M

[MS-ADA3]: Active Directory Schema Attributes N-Z

[MS-SAMR]: Security Account Manager (SAM) Remote Protocol Specification (Client-to-Server)

User Naming Attributes

Indexed attributes