[Bug 1633479] Re: dhclient does not wait for ipv6 dad (duplicate address detection)

[Stéphane Graber]

So I'm not convinced that doing this in dhclient is necessarily right.
Unlike IPv4 where DHCP operates over a raw socket, in IPv6 mode,
dhclient is just a regular network client and as such requires a source
address to be set so it can contact the network (the link-local address
in this case).

As dhclient is supposed to only be triggered after a RA has been
received and parsed, it should be the job of whatever handles the RA to
wait until a viable link-local has been established before triggering
dhclient.

That being said, since we don't seem to have anything that handles that
first configuration step properly, this workaround certainly won't hurt
and will fix the current race condition.

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https://bugs.launchpad.net/bugs/1633479

Title:
  dhclient does not wait for ipv6 dad (duplicate address detection)

Status in isc-dhcp package in Ubuntu:
  Confirmed

Bug description:
  dhclient -6 does not wait for an interface go through duplicate address detection.
  As a result the following will almost always fail:

   $ dev=eth0
   $ ip link set down dev $dev
   $ dhclient -6 -1 -v eth0
   Internet Systems Consortium DHCP Client 4.3.3
   Copyright 2004-2015 Internet Systems Consortium.
   All rights reserved.
   For info, please visit https://www.isc.org/software/dhcp/

   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..
   $ echo $?
   1

  Related bugs:
   * bug 1447715: dhclient -6: Can't bind to dhcp address: Cannot assign requested address
   * bug 1633562: 'dhclient -6 -S' does not bring interface up 

  ProblemType: Bug
  DistroRelease: Ubuntu 16.10
  Package: isc-dhcp-client 4.3.3-5ubuntu15
  ProcVersionSignature: Ubuntu 4.8.0-22.24-generic 4.8.0
  Uname: Linux 4.8.0-22-generic x86_64
  NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
  ApportVersion: 2.20.3-0ubuntu8
  Architecture: amd64
  CurrentDesktop: Unity
  Date: Fri Oct 14 09:18:52 2016
  EcryptfsInUse: Yes
  InstallationDate: Installed on 2015-07-23 (449 days ago)
  InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150722.1)
  SourcePackage: isc-dhcp
  UpgradeStatus: No upgrade log present (probably fresh install)

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