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[Dan Streetman <dan.streetman@xxxxxxxxxxxxx>]

*** This bug is a duplicate of bug 1633562 ***
    https://bugs.launchpad.net/bugs/1633562

this is also related to bug 1447715 in which i added the DAD wait to
ifupdown.  however, since bug 1633479 fixed the issue in dhclient
instead of ifupdown, at around the same time, i marked bug 1447715 as
wontfix, since that ifupdown change was no longer required to get
dhclient working.

But, now with this bug, the side-effect of changing dhclient to do its
'PREINIT6' (as mentioned in last comment) seems to be prohibitive and it
probably is much easier to simply re-open bug 1447715 and push that
ifupdown fix into xenial.

That approach does mean that manual running of dhclient -6 -S will
always fail with an interface that's down (or was brought up immediately
before running dhclient -S, which is what ifupdown does).  However,
while manually running dhclient -6 on a currently down interface in
order to both bring it up and get a dhcpv6 address does make sense,
running dhclient -6 -S on a down interface does not really make sense,
since doing that will only get the 'other' network info like DNS server,
and won't actually set up an address on the interface.  So I think I
have to agree with the conclusion in bug 1633562, to mark waiting for
dad when manually running dhclient -6 -S as invalid.

So, I'll mark this bug as a dup of that original bug, and I'll re-open
bug 1447715 to push that ifupdown fix into xenial.

** This bug has been marked a duplicate of bug 1633562
   'dhclient -6 -S' does not bring interface up

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https://bugs.launchpad.net/bugs/1764478

Title:
  dhclient in 'stateless' mode does not wait for ipv6 dad

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 Artful:
  New
Status in isc-dhcp source package in Bionic:
  New

Bug description:
  [impact]

  when configuring an interface in ifupdown to use 'inet6 auto' (SLAAC),
  the option 'dhcp 1' can also be used, which will use isc-dhcp-client
  to perform 'stateless' dhcpv6, which does not get a dhcpv6 address,
  only config info, like DNS domain and server, etc.

  bug 1633479 fixed isc-dhcp-client to properly bring up and wait for
  the interface's ipv6 Duplicate Address Detection (DAD) to complete,
  which is required for isc-dhcp-client to be able to open a socket and
  begin broadcasting DHCPv6 requests.  However, that fix is skipped when
  isc-dhcp-client is used in 'stateless' mode, so it fails in exactly
  the same way, for the same reason; isc-dhcp-client needs to be updated
  to perform the 'PREINIT6' call to the dhclient script, which properly
  sets up the interface for dhclient to use.

  Without that setup, isc-dhcp-client in DHCPv6 'stateless' mode will
  always fail immediately when the interface it's using is down.  If the
  interface is up, it will already have completed DAD and dhclient will
  work.

  [test case]

  configure ifupdown with:

  auto eth0
  iface eth0 inet6 auto
    dhcp 1

  replacing eth0 with the interface to test.  Make sure the interface is
  down, then ifup the interface.  isc-dhcp-client will fail claiming
  that:

  no link-local IPv6 address for eth0

  
  Alternately, ifupdown can be bypassed; for an interface that is currently down, do:

  $ sudo dhclient -6 -S eth0

  which will fail immediately (the -S param uses 'stateless' mode).

  [regression potential]

  changing isc-dhcp-client to perform PREINIT6 adds a new point of
  failure for doing DHCPv6, so there is the potential to break existing
  DHCPv6 clients if the preinit fails.  However, 'normal' DHCPv6 already
  does preinit6 - this only adds it to 'stateless' DHCPv6, so the
  failure potential should be limited to only users of 'stateless'
  DHCPv6, and those users are likely already seeing failures as
  described in this bug.

  [other info]

  This is related to bug 1633479

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