Adding a User Principal Name, or UPN, to Active Directory lets you match user logon names to email addresses, for example, among other things. This page describes how to add a UPN to Active Directory, so you can assign the alternate name to a user in Active Directory Users and Computers.
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This provides the search syntax for using the dsquery command on a Domain Controller to find the correct Active Directory DN for a user for use in LDAP authentication scenarios. For example:
dsquery user dc=example,dc=com -name user1*
which outputs:
"CN=user1,CN=Users,DC=example,DC=com"