DHCPv6 – FritzBox 7390 and Windows 7

When connected to a modern ISP, they can deliver native IPv6. That’s good, as the available space in IPv4 is rapidly decreasing.

Although 99% of the home-users is already using PAT – so just one address on the outside exists on which all access is translated – the decrease of available IPv4 space is a serious point of interest. So, from home, a good way to start of with IPv6, also from a professional way of thinking.

In the network at home, I have a big KVM server which runs a couple of VM’s. One of them is the utilities machine, or the one for DNS/DHCP.

The ISP connection is served via a FritxBox 7390 modem, which runs services also. Avm.de was one of the first suppliers enabling the use of IPv6 at home. Their boxes allow for IPv6 tunneling, but also for native IPv6.

Running the DHCP server for IPv4 is common, it’s there for a number of years already. IPv6 works a bit different. One of the major changes is that DHCPv6, when creating a static reservation, does not look at hardware ethernet , however DHCPv6 uses the DUID. This string can be found when using ‘ipconfig /all’ on the windows 7 client. It needs to be put as host-identifier option dhcp6.client-id in the dhcpd6.conf file.

All my VM’s and other Linux based machines do not use DHCP, they are hard coded, also for IPv6. There is one workstation however, running Windows7.

And that was the one where the pain was. Network connection, only internal, not able to resolve DNS queries. As this is the one Windows based workstation which travels also, hardcoding the device was not an option.

In the Fritzbox, under Home Network – Network – Network Settings – ipv6 address, there are a bunch of options to select.
The first bit is enable DHCPv6 or disable DHCPv6. You would expect that when picking the disable DHCPv6 option, the Fritzbox would be silent. No.

The first additional option is to select – there are no other DHCPv6 servers in the home network. But then SLAAC kicks in, automatic configuration of IPv6. No, no, no.

Second choice is to enable the O flag in the DHCP options. So FritzBox delivers the ipv6 address and the other DHCPv6 server delivers the other options (DNS server f.e) to the client. No. I want my machine to get its ipv6 address from my DHCPv6 server. Shut up, Fritzbox.

The third option is to enable M and O flags in the advirtisement messages of the FritzBox. M means ipv6 address and O, discussed above. Yes.

That should work, I thought. No.

One step before, so on Home Network – Network – Network Settings – there is a small part of the screen which mentions Additional IPv6 Routers in the Home Network.


A small line describes that the preference value. The higher, the more important, dictates which DHCPv6 server will give the answer the client should listen to. By default, the Fritzbox is set at 255 – the highest.

That was new to me, as I used the keyword ‘Authoritative’ on DHCPv4 for a long time. This option also exists in DHCPv6, however apparently, no one listens to this option.

Adding the option dhcp6.preference 255; in the dhcpd6.conf file on my isc-dhcp6-server and a reload finally fixed it. Phew..

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