A new version of ifupdown has been uploaded to experimental yesterday, which
brings some important changes.
First of all, now it’s possible to specify default values for various
interface configuration options. This eliminates the need of hard coding
of them in C source, as Ubuntu has been doing for some time. End users
are not affected by this change at all, of course.
Second, now ifup behaves differently when it’s called with --all option.
Previously, that was causing all interfaces marked as ‘auto’ to be brought
up. Now, it does exactly the same if --allow option isn’t used. Otherwise,
it brings up the interfaces which are declared to belong to a specified
class using allow-* directive. In other words, ‘auto’ directive indeed
declares interface as belonging to a class ‘auto’, and the default class for
ifupdown is also ‘auto’, so when user runs ifup -a only those interfaces
are brought up.