The big downside of IPv6 addresses is their length. While manually configured addresses can be chosen to allow for a short textual representation by using the :: abbreviation, using auto-configuration tends to produce addresses which are quite the opposite - constructing a 64bit host part by expansion of the 48bit MAC address renders even the shortest prefix nearly unrecognizable.

Using DHCPv6 might cause relief to this misery due to it's possibility to have a static lease for identified hosts. This is how to get things going.

Sadly, it is not possible for now to use the same instance of dhcpd for both IPv4 and IPv6 address configuration. Therefore one has to maintain a separate dhcpd6.conf, as well.

/etc/dhcpd6.conf

option domain-name "ip6.local";
default-lease-time 600;
max-lease-time 7200;
authoritative;

subnet6 dead:beef::/64 {
  range6 dead:beef::1000 dead:beef::ffff;

}

host workstation {
  host-identifier option dhcp6.client-id 00:01:00:01:1c:42:94:23:00:ff:17:0b:ac:49;
  fixed-address6 dead:beef::3;
}

The above config will dynamically assign addresses in the prefix dead:beef::/64, with host parts from 1000 till ffff. The host named workstation (only by dhcpd, not anywhere else) gets dead:beef::3 iff it identifies correctly (see below).

Start the dedicated dhcpd for IPv6 like so:

dhcpd -6 -cf /etc/dhcpd6.conf <iface>

The scope of this article covers using dhcpcd as DHCP client only, not least because it is able to simultaneously configure an interface with IPv4 and IPv6 address.

/etc/dhcpcd.duid

00:01:00:01:1c:42:94:23:00:ff:17:0b:ac:49

The above file should exist in every system where dhcpcd is installed. If not, one might just create it - it's contents are supposed to be rather random, with focus on hopefully having a unique DUID for every host in the same network.

Firing up dhcpcd is then as easy as this:

dhcpcd <iface>

Since one can't assume every host joining the network to implement DHCPv6 (yet), it is desirable to offer stateless auto-configuration as well. This is surprisingly easy, only two additional lines have to be added to radvd.conf to make radvd and dhcpd get along with each other:

/etc/radvd.conf

interface <iface> {
	AdvSendAdvert on;

	# here we go
	AdvManagedFlag on;
	AdvOtherConfigFlag on;

	prefix dead:beef::/64 {
		AdvOnLink on;
		AdvAutonomous on;
		AdvRouterAddr off;
	};
	RDNSS dead:beef::1 {
		AdvRDNSSLifetime 86400;
	};
};

First things first, standard stateless auto-config options above: radvd is told to advertise the prefix dead:beef::/64 for autonomous configuration on interface <iface>. Besides, dead:beef::1 is advertised as nameserver to use.

Now for the interesting part: AdvManagedFlag signals hosts to use stateful auto-configuration in addition to the stateless configured ones. AdvOtherConfigFlag enables the same for non-address information.

As the attentive reader might have guessed already, just adding the above two lines to radvd.conf leads to DHCPv6-aware hosts assigning a stateful as well as a stateless address to the configured interface, at least when using dhcpcd. This might even be called a feature, as it makes dhcpcd establish a working interface configuration no matter what style of auto-configuration is necessary. On the other hand, I have not found a way to tell dhcpcd to prefer stateful over stateless and fall back to the later only if no DHCPv6 lease was received in time. From this point of view the behaviour is rather that of a bug than anything else. At least it is possible though to turn stateless auto-configuration off in dhcpcd by adding the following statement to dhcpcd.conf:

/etc/dhcpcd.conf

ipv6ra_noautoconf

If I wanted to configure the interface statelessly, I would not run dhcpcd in the first place, would I?

Remarkably, none of the autoconf or accept_ra sysctl settings have to be switched off in order for this to work.