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[Launchpad Bug Tracker <1447715@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package ifupdown - 0.8.10ubuntu1.3

---------------
ifupdown (0.8.10ubuntu1.3) xenial; urgency=medium

  [ Wido den Hollander ]
  * Wait properly for Link-Local Address to go through DAD.
    (LP: #1447715)

 -- Dan Streetman <ddstreet@xxxxxxxxxxxxx>  Tue, 17 Apr 2018 17:29:54
-0400

** Changed in: ifupdown (Ubuntu Xenial)
       Status: Fix Committed => Fix Released

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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:
  Fix Released
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]

  Original test case, before dhclient was fixed:
  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.

  New test case, showing failure when using dhclient in 'stateless' mode:
  Configure ifupdown like:

  auto eth0
  iface eth0 inet6 auto
    dhcp 1

  When eth0 is down, run 'sudo ifup eth0', and it will fail as dhclient
  fails to start on the interface.

  [Regression Potential]

  As this modifies the wait-for-dad function, its potential for
  regression is to introduce failures in starting dhclient, possibly by
  failing to wait for DAD to complete or due to some other failure in
  the wait for dad function.

  [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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