It was big surprise for that in Debian
/etc/network/interface you cannot us
iface ethX inet6 dhcp
option to use DHCPv6 client to set IP address. If you need IPv6 address from DHCP server, you should can either define interface manually:
iface eth1 inet6 manual
up /sbin/ip link set eth1 up
post-up /etc/init.d/wide-dhcpv6-client start
pre-down /etc/init.d/wide-dhcpv6-client stop
down /sbin/ip link set eth1 down
or plug it into IPv4 definition (i.e.
inet dhcp).
inet dhcp
iface eth1 inet dhcp
post-up /etc/init.d/wide-dhcpv6-client start
pre-down /etc/init.d/wide-dhcpv6-client stop
