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[Corin Langosch <info@xxxxxxxxxxxxxxxxx>]

This is still broken on latest ubuntu server 14.04.  No wonder that ipv6
gets no adoption when one needs hours (or dig through bug reports) to
get a basic ipv4/ipv6 dual stack setup working.

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https://bugs.launchpad.net/bugs/1013597

Title:
  No default route for stateful DHCPv6

Status in ifupdown package in Ubuntu:
  Fix Released
Status in ifupdown source package in Precise:
  Triaged

Bug description:
  The default route cannot be provided via DHCPv6, it must be obtained
  from router advertisements. However, when using the dhcp method for
  inet6 in /etc/network/interfaces (e.g. "iface eth0 inet6 dhcp"), ifup
  will set net.ipv6.conf.<IFACE>.accept_ra=0, resulting in no default
  route for IPv6. Instead, it should explicitly set accept_ra=1.

  A workaround is to set it in a post-up script. However, Linux
  apparently only sends router solicitations when the interface is
  brought up, so if unsolicited RA:s are infrequent the host will be
  without a default route until the next RA.

  Another problem is that when bringing the interface down on dual-stack
  hosts, there will be a long timeout. This is because bringing down
  IPv4 also brings the interface link down, and afterwards dhclient
  cannot release the DHCPv6 lease due to the link being down ("RTNETLINK
  answers: Cannot assign requested address"). The workaround is to bring
  the link back up before bringing IPv6 down.

  So the workarounds (which obviously should not be needed) for a dual-
  stack dhcp client look like this:

  iface eth0 inet dhcp
  iface eth0 inet6 dhcp
      up sysctl net.ipv6.conf.$IFACE.accept_ra=1
      pre-down ip link set dev $IFACE up

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