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[Dan Streetman <dan.streetman@xxxxxxxxxxxxx>]

I'm re-opening this for Xenial, moving from Wontfix back to in progress.
This is because while 'normal' dhcpv6 works without this fix because of
the patch to dhclient (see my explanation in comment 23), there is a
special case of using dhclient in 'stateless' mode, i.e. dhclient -6 -S.
In that mode, dhclient does not call its 'PREINIT6' which is where the
wait-for-dad patch was added.  When configuring ifupdown in this way,
dhclient still fails due to this bug; e.g.:

auto eth0
iface eth0 inet6 auto
  dhcp 1

if eth0 is down, then running ifup eth0 with the above config will
result in dhclient always failing due to this bug.

** Changed in: ifupdown (Ubuntu Xenial)
       Status: Won't Fix => In Progress

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https://bugs.launchpad.net/bugs/1447715

Title:
  dhclient -6: Can't bind to dhcp address: Cannot assign requested
  address

Status in ifupdown package in Ubuntu:
  Fix Released
Status in ifupdown source package in Xenial:
  In Progress
Status in ifupdown package in Debian:
  Fix Released

Bug description:
  [Impact]

  When using dhcpv6 configured via ifupdown, the interface's dhcp client
  fails to start at boot with the error:

  Can't bind to dhcp address: Cannot assign requested address

  This is because ifupdown doesn't wait, after bringing the interface
  up, for it to complete its link-local Duplicate Address Detection
  (DAD), and the dhcp client can't bind to the link-local address
  because it's still in "tentative" state (until DAD completes).

  This is fixed upstream in debian ifupdown in version 0.8.11 by adding
  '-tentative' to the /lib/ifupdown/wait-for-ll6.sh script; before the
  script was only waiting for the link-local address to appear, now it
  waits until the link-local address appears and is not in 'tentative'
  state.

  [Test Case]

  Configure an interface, using ifupdown, with dhcpv6 (e.g. iface eth0
  inet6 dhcp) and reboot. It will fail to get a dhcp address during
  boot.

  [Regression Potential]

  None

  [Other Info]

  After upgrading to Ubuntu 15.04 my computer is unable to obtain an
  IPv6 address via DHCPv6. The root cause is that dhclient is exiting
  with the following message:

  -----------------------------------------------------
  Can't bind to dhcp address: Cannot assign requested address
  Please make sure there is no other dhcp server
  running and that there's no entry for dhcp or
  bootp in /etc/inetd.conf.   Also make sure you
  are not running HP JetAdmin software, which
  includes a bootp server.

  If you think you have received this message due to a bug rather
  than a configuration issue please read the section on submitting
  bugs on either our web page at www.isc.org or in the README file
  before submitting a bug.  These pages explain the proper
  process and the information we find helpful for debugging..

  exiting.
  -----------------------------------------------------

  The problem seems to exist in isc-dhcp-client 4.3.1-5ubuntu2 (15.04
  version) because downgrading to isc-dhcp-client 4.2.4-7ubuntu14 (14.10
  version) allows me to obtain an address.

  NetworkManager is the one launching dhclient. The two invocations (one
  for IPv4 and one for IPv6) are:

  dhclient_start(): running: /sbin/dhclient -d -q -sf
  /usr/lib/NetworkManager/nm-dhcp-helper -pf /run/sendsigs.omit.d
  /network-manager.dhclient-eth0.pid -lf /var/lib/NetworkManager
  /dhclient-c0a3dfde-5c0b-4cef-9c3b-563cfb1fb9d2-eth0.lease -cf
  /var/lib/NetworkManager/dhclient-eth0.conf eth0

  dhclient_start(): running: /sbin/dhclient -d -q -6 -N -sf
  /usr/lib/NetworkManager/nm-dhcp-helper -pf /run/sendsigs.omit.d
  /network-manager.dhclient6-eth0.pid -lf
  /var/lib/NetworkManager/dhclient6-c0a3dfde-5c0b-4cef-9c3b-
  563cfb1fb9d2-eth0.lease -cf
  /var/lib/NetworkManager/dhclient6-eth0.conf eth0

  I do not see anything suspicious with the way both dhclients are
  invoked.

  Given that a downgrade allows me to obtain an IPv6 address I think
  this might be a bug in the ISC DHCP client rather than in
  NetworkManager.

  Related Bugs:
   * bug 1633479: dhclient does not wait for ipv6 dad (duplicate address detection)

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