On old installs it looks like this:
------------
# The first network card - this entry was created during the Debian installation
auto eth0
iface eth0 inet dhcp
---------
While on new installs it looks like this:
----------
# This is a list of hotpluggable network interfaces.
# They will be activated automatically by the hotplug subsystem.
mapping eth0
script grep
map eth0
We haven't used the "script grep" hack in quite some time, this
entire stanza is not present in new installs, though it might be present
on some oldish ones.
# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp
Yes, it writes "allow-hotplug" instead of "auto" if the interface seems
to be hotpluggable. (Most interfaces appear that way with udev.)
An old system that has "auto eth0" here will work fine though, unless
eth0 is in fact a hotpluggable interface, in which case hotplugging it
will not work. Is there a case where hotplugging would have worked in
sarge and won't in etch, with the old config? That would be one for the
release notes.