Description of problem:

Our ISP has allocated us an IPv6 global prefix which is a /56. We are using it directly (for now at least). I have verified using tcpdump that it is being advertised correctly by our router as a /56.

However while NM sets the interface with the correct prefix numerically, it sets it as a /64, which therefore doesn't work. On our network, we use both Router Advertisement (RA) for normal IPv6 allocation, and DHCPv6 in order to use well known addresses, DNS mapping of clients, and set things like DNS resolver to use, etc. Therefore NM is set with method "Automatic", and not the "Addresses only" nor "DHCP only" alternatives.

I've done a bit of investigation, and I think I see the problem but I'm not certain. DHCPv6 can allocate addresses, but it does not deal with distributing prefixes. See RFC3315. That's not in its remit. That's the job for RA.

The problem seems to be that dhclient (from ISC DHCP) will return a "new_ip6_prefixlen" of 64 no matter what. From dhclient's dhc6.c in dhc_marshall_values():
                        /* Current practice is that all subnets are /64's, but
                         * some suspect this may not be permanent.
                         */
                        client_envadd(client, prefix, "ip6_prefixlen",
                                      "%d", 64);

But as far as I can tell, subnets must not be smaller than a /64, but they can be larger. "Current practice" isn't the same as what may exist!

But I think the problem here is not from dhclient. It's because NetworkManager uses the prefix length, despite DHCPv6 inherently being unable to provide it. 

I may be wrong, but I think in NetworkManager's nm-device.c:merge_dhcp_config_to_master(), the address (including prefix) should not be wholly merged into the config, but the prefix length should be treated specially and the value from DHCP should not override the value provided by RA.


Version-Release number of selected component (if applicable):

NetworkManager-0.8.1-9.el6_1.3.x86_64

It is also known to be reproducible on F14:
NetworkManager-0.8.5.92-1.git20110927.fc14.x86_64


How reproducible:

100%

Steps to Reproduce:

Configure a global prefix of a /56 on a network's IPv6 router and advertise it.
  
Actual results:

NM will assign the prefix but as a /64.