When building Openfire (using the Ant buildfiles), internet connectivity is required. The build will fail when no connectivity is availble.
As Openfire ships with all dependencies, this is undersirable. Openfire should simply build, without accessing internet resources.
It appears that the cause of the issue is validation of XSDs and/or DTDs. Further investigation is needed.
This describes how Apache Ant works with XML Catalogs: https://ant.apache.org/manual/Types/xmlcatalog.html