dhclient6, IA_PD hints, Comcast, and a "pd hint"

[Troy Telford]

I'm using ISC dhclient 4.2.4

Comcast has rolled out native IPv6 in my area.

By default, Comcast assigns a /64 prefix to users. I'm not too happy 
about this, as I have a few subnets I'd like to keep separated, and you 
can't do that with a /64 from what I understand.

According to the thread at:
http://forums.comcast.com/t5/Home-Networking-Router-WiFi/IPv6-prefix-size-and-home-routing/td-p/1495933 


Comcast will provide a /60 if the DHCP client requests it in IA_PD. 
(I'm less familiar with the details of what that actually means, so 
bear with me).

The instructions given by Comcast are for Cisco iOS, and are something 
along the lines of:
interface GigabitEthernet0/0
 description Comcast Public
 ipv6 address dhcp
 ipv6 enable
 ipv6 dhcp client pd hint ::/60
 ipv6 dhcp client pd prefix-from-Comcast
 ipv6 traffic-filter ipv6_inbound in

The same thread has working configurations for wide-dhcpv6-client (the 
bottom of the thread), where the prefix size is set by:

id-assoc pd 1 {
    prefix ::/60 infinity

I've been unable to find any documentation that describes how to 
configure ISC dhclient to request a prefix size of /60.  (I can only 
find references to 'dhclient -6 -P ethxyz' - which results in a /64 
being assigned.

Does anybody know how to configure dhclient to request a /60 instead of 
letting the server use its default of /64?

It'd be nice to know and have documented (and easy to find), as Comcast 
is in the process of deploying IPv6 across their network. I'm sure I'm 
not the only or last person who will ask to be able to subnet my IPv6 
network.

Thanks.
-- 
Troy Telford