protected String getUserDnSuffix() {
        return userDnSuffix;
    }
Returns the User DN suffix to use when building a runtime User DN value.  or {@code null} if no
{@link #getUserDnTemplate() userDnTemplate} has been configured.  If configured, this value is the text that
occurs after the {@link #USERDN_SUBSTITUTION_TOKEN} in the {@link #getUserDnTemplate() userDnTemplate} value.

@return the User DN suffix to use when building a runtime User DN value or {@code null} if no
{@link #getUserDnTemplate() userDnTemplate} has been configured.
core/src/main/java/org/apache/shiro/realm/ldap/DefaultLdapRealm.java