This issue(required ".dtd" in system ID) is a known problem regarding Oxygen's validation.
Oxygen uses the ".dtd" suffix to determine which type of validator it should use.
In addition, the following must be true: You'll learn more about namespaces in Validating with XML Schema.
For now, understand that schema validation is a namespace-oriented process.
I have setup a DTD that is accessed through an XML catalog.
Oxygen is resolving though the catalog and validating my document.
Is this expected behavior for the DTD validation in Oxygen?