Out of the box Ubuntu 14.04 LTS supports stateless IPv6. This means you can run IPv6 without DHCPv6 server. If you want to run an DHCPv6 server to automatically register IPv6 addresses in your local DNS, or you want to port forward certain ports for a service (webserver / bittorrent) to a specific IPv6 address (DHCPv6 reservation) you end up having to set up a DHCPv6 statefull client. This is quiet easy; assuming you've already got an DHCPv6 server running in your network. In this example we're using "wide dhcpv6 client":

sudo apt-get install wide-dhcpv6-client

sudo vim /etc/wide-dhcpv6/dhcp6c.conf

modify the contents of this file so it looks like this:

# Default dhpc6c configuration: it assumes the address is autoconfigured using
# router advertisements.

profile default {

request domain-name-servers;
request domain-name;

script "/etc/wide-dhcpv6/dhcp6c-script";

interface eth0 {
send rapid-commit;
send ia-na 0;
request domain-name-servers;

id-assoc na 0 {};

Just restart the services and you're done.

sudo /etc/init.d/wide-dhcpv6-client stop
sudo /etc/init.d/wide-dhcpv6-client start