Comment 7 for bug 1693819

This is specifically important for any configuration that wants to have both a IPv4 address and DHCPv6 IPv6 address on a single interface, and uses ifupdown for interface management, and wants independent control of the 4 and 6 address. Meaning, to be able to add or remove an interface's IPv6 DHCPv6 address, while leaving the interface up and still configured with its IPv4 address. The ifupdown program does not have any mechanism for controlling IPv4 and IPv6 addresses on a single interface separately, so interface aliases are the most convenient way to keep using ifupdown and be able to control the addresses separately. An example ifupdown configuration might be:

auto eth0
iface eth0 inet manual
  pre-down ifdown eth0:1
  pre-down ifdown eth0:2

auto eth0:1
iface eth0:1 inet dhcp

auto eth0:2
iface eth0:2 inet6 dhcp

That configuration will start dhcp for both IPv4 and IPv6 addresses on eth0 at boot, and will allow running 'sudo ifdown eth0:1' to remove the IPv4 address only, while leaving the IPv6 address, or 'sudo ifdown eth0:2' to remove the IPv6 address only, while leaving the IPv4 address. Or, 'sudo ifdown eth0' will remove both addresses and take down the physical interface. Since this configuration uses an interface alias for DHCPv6, the current isc-dhcp-client fails, and requires the patch from the attached debdiffs to work.