This is a special directory like /etc/net/ifaces/default is. If /etc/net
receives a request to configure an interface, which does not have its own
configuration directory, and ALLOW_UNKNOWN is enabled, /etc/net/ifaces/unknown
will be used as configuration source. 'default' and 'unknown' are reserved
names, so don't assign them to real interfaces. Please pay attention that
you can't use all options for 'unknown' interfaces. Default settings should
result in attempt to perform DHCP/IPv4LL configuration after enabling
ALLOW_UNKNOWN variable in global options. The most natural way to do it is to
create a distribution/site specific options file in /etc/net/options.d to
override default value in 00-default file.