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

[ruff]

Ok, here's verbose dump below. Down sequence is executed in the same order as up, although should be in reverse. And to check whether list of remaining interface definitions is empty for any given interface to avoid setting link down. That way dhcp6 will be released first, then dhcp4 release and finally wpa termination and link down. 
Don't know if anyone can produce a patch for that nw source file though %(

Configuring interface wlan0=wlan0 (inet)
run-parts --verbose /etc/network/if-down.d
run-parts: executing /etc/network/if-down.d/resolvconf
run-parts: executing /etc/network/if-down.d/upstart
run-parts: executing /etc/network/if-down.d/wpasupplicant
dhclient -v -r -pf /run/dhclient.wlan0.pid -lf /var/lib/dhcp/dhclient.wlan0.leases wlan0
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/wlan0/xx:xx:xx:xx:xx:xx
Sending on   LPF/wlan0/xx:xx:xx:xx:xx:xx
Sending on   Socket/fallback
DHCPRELEASE on wlan0 to aa.aa.aa.aa port 67
ip link set dev wlan0 down
run-parts --verbose /etc/network/if-post-down.d
run-parts: executing /etc/network/if-post-down.d/avahi-daemon
run-parts: executing /etc/network/if-post-down.d/bridge
run-parts: executing /etc/network/if-post-down.d/wireless-tools
run-parts: executing /etc/network/if-post-down.d/wpasupplicant
wpa_supplicant: terminating wpa_supplicant daemon via pidfile /var/run/wpa_supplicant.wlan0.pid
Stopped /sbin/wpa_supplicant (pid 9677).
wpa_supplicant: removing /run/sendsigs.omit.d/wpasupplicant.wpa_supplicant.wlan0.pid
Configuring interface wlan0=wlan0 (inet6)
run-parts --verbose /etc/network/if-down.d
run-parts: executing /etc/network/if-down.d/resolvconf
run-parts: executing /etc/network/if-down.d/upstart
run-parts: executing /etc/network/if-down.d/wpasupplicant
dhclient -6 -r -pf /run/dhclient6.wlan0.pid -lf /var/lib/dhcp/dhclient6.wlan0.leases wlan0
RTNETLINK answers: Cannot assign requested address
RTNETLINK answers: Cannot assign requested address
RTNETLINK answers: Cannot assign requested address

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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

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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