Dec 052013

There already exist many tutorials how to setup a basic IPv6 network environment on Linux. Most of the time they are limited to an example of a radvd.conf enabling Router Advertisement and Stateless Address Auto Configuration (SLAAC), an example how to configure BIND to serve DNS requests for AAAA records and reply to IPv6 reverse lookups. Sometimes even DHCPv6 is mentioned as an alternative way to assign IPv6 client addresses, but mostly with a very basic example configuration without fixed address assignments. Building on this basic knowledge, I want to summarize some further settings which might be interesting, when playing around with DHCPv6 in a Linux environment.

radvd and DHCPv6

As I’ve already mentioned earlier there is no way around radvd, at least if your router is based on Linux. Unlike with IPv4, in IPv6 the router can announce its presence with ICMPv6 Router Advertisement messages, triggered by radvd. By default these messages include the ‘Autonomous’ flag, which enables SLAAC and therefore ask a receiver to autonomously configure its address based on the given IPv6 prefix. In a DHCPv6 subnet you may want to disable this behaviour in the radvd.conf with the following prefix option:

AdvAutonomous off;

There are two more radvd.conf configuration directives which are important in a DHCPv6 setup:

AdvManagedFlag on;
AdvOtherConfigFlag on;

The ManagedFlag option (M flag) will hint the receiver to obtain a stateful address via DHCPv6. The OtherConfigFlag option (O flag) is used to inform the receiver that various other configuration information such as DNS, SIP or NTP server address lists can be requested via DHCPv6. The latter is often also used together with SLAAC, in case a client doesn’t understand RDNSS and DNSSL announcements.

Important: Not all IPv6 clients can handle address assignment if SLAAC is disabled! According to the RFC4294 (IPv6 Node Requirement), a IPv6 client must only support address auto-configuration via SLAAC. DHCPv6 may be supported optionally. Especially Windows XP but also Google Android (see Issue #32621) won’t be able to auto-configure a routable IPv6 address without SLAAC.

Another interesting radvd option not directly related to DHCPv6, but helpful if you like to analyse your network traffic without being distracted by all the Router Advertisement noise is the following interface option:

UnicastOnly on;

This will prevent radvd from broadcasting Router Advertisements and only reply with a unicast message if it receives a Router Solicitation message from a IPv6 host refreshing its routing table. Together with the possibility to only respond to a predefined list of host IPs in the radvd.conf, its even possible to run your router in complete stealth mode towards unknown IPv6 clients:

clients {

radvd and ip6tables

To protect the router you may want to enable ip6tables. Additionally to the default ICMPv6 messages, such as Neighbor Solicitation/Advertisement and Destination Unreachable, which should be allowed on every IPv6 host anyway, the following rules must be configured to whitelist the radvd communication channels:

Allow incoming Router Solicitation (ICMPv6 Type 133) messages:

ip6tables -A INPUT -i <netdevice> -p ipv6-icmp -m icmp6 \
    --icmpv6-type router-solicitation -j ACCEPT

Allow outgoing Router Advertisement (ICMPv6 Type 134) messages:

ip6tables -A OUTPUT -o <netdevice> -p ipv6-icmp -m icmp6 \
    --icmpv6-type router-advertisement -j ACCEPT

That’s it for the moment. In part 2 of my DHCPv6 series, I’ll show you some interesting Linux DHCP server configuration directives. Stay tuned and don’t hesitate to leave a comment in case this article was helpful for you or if I got it all wrong… 😉