When dumping a RRD file, the generated XML contains the following header:
The Upgrade Tools uses the class RrdParseUtils from opennms-rrd-model to generate the object representation of a RRD Dump XML. This in turn uses JAXB (through JaxbUtils) to parse the XML.
For some reason, JAXB is trying to retrieve the DTD because the DOCTYPE entry exists, and this is causing problems on those situations on which the OpenNMS machine cannot reach the Internet.
The idea is to pre-process the XML to remove those headers, or use a different parser on which we can turn off validations.