[Bug 1013597] Re: No default route for stateful DHCPv6

[Andrew Collins]

I'd like to request a backport of this for 12.04 LTS as well, this issue
has forced me to move to NetworkManager on my server instead of
ifupdown, as DHCPv6 stateful support is seriously broken unless I
manually add default routes.

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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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