dhcpv6 returning bad subnet - linux

Question

I have IPv6 on a linux machine and my network and it works. Now I want to set up DHCP for it. I set up the isc-dhcp-server and configured the subnet.
Another linux machine (both debian 7) acts as test-client and gets the IP, but not in the range configured, and, much worse, gets a /64 subnet and not a /80.
Since the IP pool available on the router is already a subset of the /64 assigned to another upstream-machine, I need a smaller subnet. I cannot allow it to be /64.
Config of dhcp server:
subnet6 2a01:4f8:202:6106:acda::/80 {
range6 2a01:4f8:202:6106:acda:f000::/84;
option dhcp6.name-servers 2a01:4f8:202:6106::2;
prefix6 2a01:4f8:202:6106:acda:c000:: 2a01:4f8:202:6106:acda:f000:: /84;
}
ifconfig output on client:
debian#arm:~$ sudo ifconfig
[sudo] password for debian:
eth0 Link encap:Ethernet HWaddr c8:a0:30:ae:48:24
inet addr:192.168.0.104 Bcast:192.168.0.255 Mask:255.255.255.0
inet6 addr: fe80::caa0:30ff:feae:4824/64 Scope:Link
inet6 addr: 2a01:4f8:202:6106:acda:ff2f:452c:b7b5/64 Scope:Global
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:185 errors:0 dropped:0 overruns:0 frame:0
TX packets:201 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:18222 (17.7 KiB) TX bytes:23159 (22.6 KiB)
Interrupt:56
A windows-7 machine also connected does not get an IPv6 address at all.
config of the radvd on the server (in case it matters)
interface eth0 {
AdvSendAdvert on;
prefix 2a01:4f8:202:6106:acda::/80 {
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr on;
};
route 2000::/3
{
};
};
What's wrong? Why is this not working? Is the bad subnet-size a bug in the server? Or the client?
Result of the bad subnet-size is that for example the nameserver, 2a01:4f8:202:6106::2 , in the 64-bit range, is not reachable. The client thinks it should be on the lan segment and tries to get the link-local IPv6 and the ethernet MAC of it and fails. It needs to go via the router. When I set the subnet manually to /80 everything works fine.

Answer 1

First of course the general warning: using non-/64 subnet sizes will break things. Your ISP should really give you a decent amount of address space to work with, like a /48 or a /56. You can then route /64s out of that wherever you want.
Then you have to look at how your ISP gives you your current /64. If they route it to a LAN that is connected to your server's eth0 interface then there is nothing much you can do except bridging or proxy-ND because you have to make it look like everything is directly connected to that LAN. Both methods have their own complications.
If you are bridging to a LAN managed by your ISP then you shouldn't run anything like radvd or dhcpd because you'll interfere with your ISP.
If you are using proxy-ND or your ISP routes the /64 to your server (so you have a different IPv6 address on your server's interface to the ISP and the ISP routes the /64 to that interface) then you should indeed run radvd and dhcpd but only on the internal interface, not towards your ISP.
Back to your RA+DHCPv6 setup because that part of your question is easy to answer. There are three things wrong with your radvd setup (so, yes, it matters a lot :)
First you cannot do SLAAC (StateLess Address Auto Configuration) on anything except a /64 so you'll have to turn AdvAutonomous off. Then you have to tell the clients that a managed (stateful) DHCPv6 server is available so turn the AdvManagedFlag on. And the route 2000::/3 is also unnecessary. You are advertising that you are the default gateway and this more-specific doesn't add anything useful.
interface eth1 {
AdvSendAdvert on;
AdvManagedFlag on;
prefix 2a01:4f8:202:6106:acda::/80 {
AdvOnLink on;
AdvAutonomous off;
AdvRouterAddr on;
};
};