Configure IPv6 on server running Ubuntu Xenial

When you request IPv6 range for your server at, support will give you DUID. Record it and keep it safe: it's your key to get IPv6 connectivity.

To get connectivity actually working, you need to request IPv6 prefix delegation via DHCPv6. Xenial Xerus has almost all parts in place, but one tweak is still necessary. Xenial's version of ifupdown has no capability to run dhclient with -P option that is necessary to request prefix delegation from the router.

Upgrade ifupdown package

Luckily, yakkety's version of ifupdown has this capability, and can be installed cleanly on xenial. Get the package from here (or any mirror):

At the time of this writing, 2016-09-01, the latest version is:


Install it, after installation you should see this:

# ifup --version
ifup version 0.8.13ubuntu1

You can reboot to check that everyting still works honky dory.

Configure your system to request prefix delegation

Now, the configuration part. You need to modify two files: /etc/network/interfaces and /etc/dhcp/dhclient.conf. Let us assume that the name of the Ethernet interface is em1.

Edit /etc/dhcp/dhclient.conf

In the file /etc/dhcp/dhclient.conf, add the following statement:

send dhcp6.client-id <DUID-that-support-has-given-you>;

or if you have more that one interface (e.g. you run some virtual networks),

interface em1 {
  send dhcp6.client-id <DUID-that-support-has-given-you>;

Edit /etc/network/interfaces

Now, your /etc/network/interfaces file initially looks like this:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto em1
iface em1 inet dhcp

You need to add another, IPv6 specific, entry for em1:

iface em1 inet6 dhcp
  autoconf 0
  request_prefix 1

(autoconf 0 is optional).

Test that it works

Now restart the networking unit:

systemctl restart networking

Now if you look at the process list, you will see two instances of dhclient running:

# ps ax|grep dhclient
 2404 ?        Ss     0:00 /sbin/dhclient -1 -v -pf /run/ -lf /var/lib/dhcp/dhclient.em1.leases -I -df /var/lib/dhcp/dhclient6.em1.leases em1
 2489 ?        Ss     0:00 /sbin/dhclient -1 -6 -pf /run/ -lf /var/lib/dhcp/dhclient6.em1.leases -I -P -N -df /var/lib/dhcp/dhclient.em1.leases em1
 3131 pts/0    S+     0:00 grep --color=auto dhclient

The important part here is that the -6 instance is launced with -P and -N options. Shortly, you should get a public address on the em1 interface, and be able to ping6 Reboot once again to verify that everyting gets configured properly on boot.

That is all.

Eugene Crosser <crosser at average dot org>