Is there a Maximum Length for userPrincipalName in Active Directory?

Question

I am writing an application that is linked to Active Directory, and I need to store the userPrincipalName in a database table, but I do not know how big the field would need to be.

On MSDN, no Length is given, and neither in RFC 822. Now, before I revert to the DOMAIN\Username that has a defined Length (sAMAccountName is less than 20 chars, NETBIOS Domain Name is max. 15 chars), I wonder if anyone knows what the limit is either by standard, or by the implementation within both Windows 2003 and Windows 2008 domains.

Answer 1

On Win2k3 SP2 the longest userPrincipleName it allows me to create is 1013 characters long.

Answer 2

While trying to answer this question for myself today I stumbled across a documented answer.

2.381 Attribute userPrincipalName defines the userPrincipalName in the following way:

 cn: User-Principal-Name
 ldapDisplayName: userPrincipalName
 attributeId: 1.2.840.113556.1.4.656
 attributeSyntax: 2.5.5.12
 omSyntax: 64
 isSingleValued: TRUE
 schemaIdGuid: 28630ebb-41d5-11d1-a9c1-0000f80367c1
 systemOnly: FALSE
 searchFlags: fATTINDEX
 rangeUpper: 1024
 attributeSecurityGuid: e48d0154-bcf8-11d1-8702-00c04fb96050
 isMemberOfPartialAttributeSet: TRUE
 systemFlags: FLAG_SCHEMA_BASE_OBJECT | FLAG_ATTR_REQ_PARTIAL_SET_MEMBER

This specifies the maximum length as 1024 according to the rangeUpper attribute definition.

In this context 1024 means octets (bytes), as opposed to Unicode code points, as omSyntax: 64 is defined as String(Unicode) in LDAP Representations and references RFC 2252 LDAPv3 Attributes 6.10 Directory String which describes it as the following:

6.10. Directory String

( 1.3.6.1.4.1.1466.115.121.1.15 DESC 'Directory String' )

A string in this syntax is encoded in the UTF-8 form of ISO 10646 (a superset of Unicode). Servers and clients MUST be prepared to receive encodings of arbitrary Unicode characters, including characters not presently assigned to any character set.

With UTF-8 being a variable length encoding this means that the maximum string length is however many code points you can UTF-8 encode into 1024 octets (bytes). i.e.: For purely ASCII strings that's 1024 code points, for anything with non-ASCII characters it means something less than 1024 code points.