The use case I found was in initramfs-tools, which recently SRU'd a change that 
uses dhclient to support 'ip=' command lines and dhcpv6.

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

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:

  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'

  [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

  [Regression Potential]


  [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..


  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
  563cfb1fb9d2-eth0.lease -cf
  /var/lib/NetworkManager/dhclient6-eth0.conf eth0

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

  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

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