I simplified the problem a bit by switching to a simple test document.
The problem still appears, however. Using the same catalog file before,
and the following input file, it still hits the Oasis server but doesn't
download as many files:
testbook.xml
----------------------------------------
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">
<book>
<bookinfo>
<title>Test book</title>
<author>
<firstname>Some</firstname>
<surname>Person</surname>
</author>
</bookinfo>
<chapter>
<title>Chain of command</title>
<para>Moo.</para>
<para>Meep.</para>
</chapter>
</book>
--------------------------------
If, however, I change the Doctype declaration to read:
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
"file:///usr/share/xml/docbook/schema/dtd/4.3/docbookx.dtd">
then there is no network traffic. The verbose log file, however, is
identical in both cases (level 4, excerpt but still long):