Constructing User DN using principal name

[diptivs@gmail.com]

Our application needs to construct User DN after successful authentication
for authorization.Currently what application has is user principal name.

User principal name is of format: Userame at RealmName And user DN is of the
form
cn=<Username>,cn=Users,cn=<DomainName>,cn=com

As Realm need not be always same as Domain name: How to get this
<DomainName> using the Realm in the user name?
Any suggestions to solve this problem would be of help.

Currently we are using MIT libraries v1.5.3. Soon we will upgrade to latest.

Thanks,
Dipti