public String getUserDnTemplate() {
        return getUserDn(USERDN_SUBSTITUTION_TOKEN);
    }
Returns the User Distinguished Name (DN) template to use when creating User DNs at runtime - see the
{@link #setUserDnTemplate(String) setUserDnTemplate} JavaDoc for a full explanation.

@return the User Distinguished Name (DN) template to use when creating User DNs at runtime.
core/src/main/java/org/apache/shiro/realm/ldap/DefaultLdapRealm.java