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[Eric Desrochers <eric.desrochers@xxxxxxxxxxxxx>]

** Also affects: isc-dhcp (Ubuntu Artful)
   Importance: Undecided
       Status: New

** Also affects: isc-dhcp (Ubuntu Trusty)
   Importance: Undecided
       Status: New

** Also affects: isc-dhcp (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Also affects: isc-dhcp (Ubuntu Zesty)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1718568

Title:
  dhclient-script fails to wait for link-local address

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 Zesty:
  New
Status in isc-dhcp source package in Artful:
  New
Status in isc-dhcp package in Debian:
  New

Bug description:
  Summary:
  ========

  If a interface does not yet have a link-local address (as it may have
  just been brought up), dhclient -6 <ifname> will fail. The built-in
  "wait for link-local address" loop does not function properly, causing
  DHCP failure.

  Discussion:
  ===========

  In trying to configure isc-dhcp-client 4.3.5-3ubuntu1 for IPv6 on
  Ubuntu 17.04, I was finding that on boot I was getting failures with
  the logged message "no link-local IPv6 address for <ifname>"

  I found that it took several seconds for the link-local address to be
  assigned when the interface came up (in this case, the ISP/modem-
  facing interface), and worked around it with a script that looks at

    /sbin/ifconfig $IFACE | /bin/fgrep -q 'scopeid 0x20'

  and loops for a fixed number of times for that to be successful.

  On looking at /sbin/dhclient-script it appears that it *tries* to do
  the same thing in

    # set the link up and wait for ipv6 link local dad to finish
    ipv6_link_up_and_dad()

  this code sets

    out=$(ip -6 -o address show dev "$dev" scope link)

  then checks it with a case statement inside of a loop for

          case " $out " in
              *\ dadfailed\ *)
                  error "$dev: ipv6 dad failed."
                  return 1;;
              *\ tentative\ *) :;;
              *) return 0;;
          esac

  If there is no link-local address, $out will be empty. The default
  case is taken, and the loop exits immediately:

  $ echo "'$out'" ; case " $out " in
  >     *\ dadfailed\ *)
  >         echo "dadfailed"
  >         ;;
  >     *\ tentative\ *)
  >         echo "tentative"
  >         ;;
  >     *)
  >         echo "default"
  > esac
  ''
  default

  As a result, there is no "wait for link-local address" and when there
  is no link-local address, dhclient fails later on.

  Possible Fix:
  =============

  Adding "the missing case" for "no address" case that continues the
  loop is one possible solution.

  .        case " $out " in
  .            *\ dadfailed\ *)
  .                error "$dev: ipv6 dad failed."
  .                return 1;;
  .            *\ tentative\ *) :;;
  +            "  ")
  +                :
  +                ;;
  .            *) return 0;;
  .        esac

  At least in my situation, this prevents the failure of dhclient due to
  the link-local address not being "ready" yet.

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