Comment 30 for bug 1447715

Xenial verification:

ubuntu@lp1447715:/etc/network/interfaces.d$ dpkg -l | grep ifupdown
ii ifupdown 0.8.10ubuntu1.2 amd64 high level tools to configure network interfaces

ubuntu@lp1447715:/etc/network/interfaces.d$ grep -A 2 ens7 50-cloud-init.cfg
auto ens7
iface ens7 inet6 auto
 dhcp 1

ubuntu@lp1447715:/etc/network/interfaces.d$ sudo ip l show ens7
3: ens7: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:b0:86:a5 brd ff:ff:ff:ff:ff:ff

ubuntu@lp1447715:/etc/network/interfaces.d$ sudo ifup -v ens7
Parsing file /etc/network/interfaces.d/50-cloud-init.cfg
Configuring interface ens7=ens7 (inet6)
/bin/run-parts --exit-on-error --verbose /etc/network/if-pre-up.d
run-parts: executing /etc/network/if-pre-up.d/ethtool
run-parts: executing /etc/network/if-pre-up.d/ifenslave
+ [ inet6 = meta ]
+ IF_BOND_SLAVES=
+ [ ]
+ [ ]
+ [ -z ]
+ exit
run-parts: executing /etc/network/if-pre-up.d/vlan
/sbin/modprobe -q net-pf-10 > /dev/null 2>&1 || true # ignore failure.

/sbin/sysctl -q -e -w net.ipv6.conf.ens7.accept_ra=2
/sbin/sysctl -q -e -w net.ipv6.conf.ens7.autoconf=1
/bin/ip link set dev ens7 up
/lib/ifupdown/wait-for-ll6.sh
/sbin/dhclient -1 -6 -S -pf /run/dhclient6.ens7.pid -lf /var/lib/dhcp/dhclient6.ens7.leases -I -df /var/lib/dhcp/dhclient.ens7.leases ens7
Failed to bring up ens7.

ubuntu@lp1447715:/etc/network/interfaces.d$ dpkg -l | grep ifupdown
ii ifupdown 0.8.10ubuntu1.3 amd64 high level tools to configure network interfaces

ubuntu@lp1447715:/etc/network/interfaces.d$ grep -A 2 ens7 50-cloud-init.cfg
auto ens7
iface ens7 inet6 auto
 dhcp 1

ubuntu@lp1447715:/etc/network/interfaces.d$ sudo ip l show ens7
3: ens7: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:b0:86:a5 brd ff:ff:ff:ff:ff:ff

ubuntu@lp1447715:/etc/network/interfaces.d$ sudo ifup -v ens7
Parsing file /etc/network/interfaces.d/50-cloud-init.cfg
Configuring interface ens7=ens7 (inet6)
/bin/run-parts --exit-on-error --verbose /etc/network/if-pre-up.d
run-parts: executing /etc/network/if-pre-up.d/ethtool
run-parts: executing /etc/network/if-pre-up.d/ifenslave
+ [ inet6 = meta ]
+ IF_BOND_SLAVES=
+ [ ]
+ [ ]
+ [ -z ]
+ exit
run-parts: executing /etc/network/if-pre-up.d/vlan
/sbin/modprobe -q net-pf-10 > /dev/null 2>&1 || true # ignore failure.

/sbin/sysctl -q -e -w net.ipv6.conf.ens7.accept_ra=2
/sbin/sysctl -q -e -w net.ipv6.conf.ens7.autoconf=1
/bin/ip link set dev ens7 up
/lib/ifupdown/wait-for-ll6.sh
/sbin/dhclient -1 -6 -S -pf /run/dhclient6.ens7.pid -lf /var/lib/dhcp/dhclient6.ens7.leases -I -df /var/lib/dhcp/dhclient.ens7.leases ens7
/bin/run-parts --exit-on-error --verbose /etc/network/if-up.d
run-parts: executing /etc/network/if-up.d/000resolvconf
run-parts: executing /etc/network/if-up.d/ethtool
run-parts: executing /etc/network/if-up.d/ifenslave
+ [ inet6 = meta ]
+ [ ]
run-parts: executing /etc/network/if-up.d/ip
run-parts: executing /etc/network/if-up.d/openssh-server
run-parts: executing /etc/network/if-up.d/upstart

verified.