fixed ipv6 address with DHCPv6

[Gene Czarcinski <gene czarc net>]

For some time I have been having a problem attempting to have a dnsmasq 
server provide a system with a fixed IPv6 address.  Setting an IPv4 
address and identifying the system with its NIC's MAC address.  But, 
with DHCPv6 there is no relationship defined in the standard for DHCPv6 
to use the MAC.

I tried using the system's name but that has not proven reliable. When 
the system and the dnsmasq server get "out of sync", it takes manual 
intervention to correct things.  When things do work, it works fine.

I looked into using the Client-ID but that "number" is based on the MAC 
plus time and will vary unpredictably.

Suddenly (like yesterday) I found what appears to be the solution and it 
is likely to have been there for some time.  By default, dhclient will 
use LLT (Link-Layer plus Time) to define its DUID (Client-ID).  But, 
there is an command-line override which can change this to LL 
(Link-Layer) which uses the MAC prepended with 0:3:0:1.

The important info is here:
http://tools.ietf.org/html/rfc3315#section-9.4

Also examine the dhclient man age and scroll down to "*-D*/ LL or LLT/"

I then did a quick (two line) patch to NetworkManage 
[src/dhcp-manager/nm-dhcp-dhclient.c] to hardcode the addition of "-D", 
"LL" to the command-line if it is "-6".  It works as advertised.

While this works for me, I do not propose that this be the solution in 
NetworkManager.  Instead, I propose that the default remain the same and 
a new configuration file parameter be added: DUID= which will have only 
two valid values: LL or LLT.  If DUID= is not specified then the default 
is LLT.

Once this is accepted and part of NetworkManager, I will update 
network-manager-applet so the the DUID value can be specified when 
defining an IPv6 interface.  Initially, editing the configuration file 
should be adequate.

Comments?

Gene