[Bug 1693819] Re: dhclient DHCPv6 does not work with interface alias

[Dan Streetman]

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.

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https://bugs.launchpad.net/bugs/1693819

Title:
  dhclient DHCPv6 does not work with interface alias

Status in isc-dhcp package in Ubuntu:
  New
Status in isc-dhcp source package in Trusty:
  New
Status in isc-dhcp source package in Xenial:
  New
Status in isc-dhcp source package in Yakkety:
  New
Status in isc-dhcp source package in Zesty:
  New
Status in isc-dhcp source package in Artful:
  New

Bug description:
  [Impact]

  The isc-dhcp-client dhclient program does not work with interface
  alias names, e.g. eth0:1.  It fails immediately.

  [Test Case]

  On trusty through artful, find an interface that is connected to a
  network that has a DHCPv6 server on it.  First verify it can get a
  DHCPv6 address, e.g.:

  $ sudo dhclient -6 -v ens7

  that should successfully set up a DHCPv6 address on the interface.
  Then release the interface:

  $ sudo dhclient -6 -v -r ens7

  Now, try again but use the interface alias, e.g.:

  $ sudo dhclient -6 -v ens7:1

  It should succeed, but it will fail with:

  no link-local IPv6 address for ens7:1

  [Regression Potential]

  Changing how isc-dhcp handles (enumerates/lists/compares) interface
  names may lead to isc-dhcp using the wrong interface or failing to
  find the correct interface to use.  However, if the only change is to
  strip the alias part from the interface during comparison, the
  possibility of error should be small, since all interface aliases
  point back to the same physical interface.

  [Other Info]

  This bug exists in the upstream isc-dhcp; the ISC has no public bug
  tracker, and only accepts bug reports via private email, which has
  been sent.

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