Package: src:ifupdown2
Version: 1.0~git20170114-1
Severity: important
Dear Maintainer,
it seems that ifupdown2 allow-hotplug keyword works significantly
different from ifupdown allow-hotplug.
In ifupdown, when a new network device appears, a udev trigger fires
where ifquery checks whether there is an "allow-hotplug $INTERFACE"
configuration stanza in /e/n/i. If yes, ifup is called accordingly,
bringing the interface up.
In ifupdown2, when networking.service is started
(/usr/share/ifupdown2/sbin/start-networking start), it checks for all
interfaces marked "allow-hotplug" (function ifup_hotplug). If the
interface is there _and_ has a link (operstate = up) it will be
configured.
This means:
1.) ifupdown2 needs a link, ifupdown just needs the interface to be
there. Since the interface is normally DOWN and is only set UP by
ifupdown(2) it would never configure the link (sort of a dependency
cycle)
2.) allow-hotplug does not get triggered by a udev event when the
interface appears, but expects all interfaces to be available at
startup.
3.) The documentation (man interfaces(5)) is wrong, saying that
"Note that "allow-auto" and "auto" are synonyms."
I think basing the configuration on an operational link (that cannot be
there if ifupdown2 hasn't touched the interface yet, and might not be
there on bootup due to external circumstances or timing issues) will
cause hard to debug issues. Therefor I suggest a targeted fix for
Stretch dropping the operstatus conditional. It still doesn't address
note #2 above, but it would be better than nothing.
Best Regards,
Bernhard
