[Bug 1764478] [NEW] dhclient in 'stateless' mode does not wait for ipv6 dad

[Dan Streetman]

Public bug reported:

[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

** Affects: isc-dhcp (Ubuntu)
     Importance: Undecided
         Status: New

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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

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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