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Re: [libvirt] libvirt plans: dns support for IPv6?

On 09/15/2012 05:10 PM, Laine Stump wrote:
On 09/15/2012 04:07 PM, Gene Czarcinski wrote:
How is the version of dnsmasq being determined ... at [ugly] runtime
or during configuration/compilation?
You can't rely on any information gathered at compile time, since the
binary may be built on one system and run on another.

Instead, I believe what Dan was talking about is that the default usage
of dnsmasq uses only features that are available on the oldest dnsmasq
we support (which is likely whatever version is currently shipping with
RHEL5 / CentOS5), and any dnsmasq feature used by libvirt that requires
a newer version of dnsmasq will only be attempted when some extra knob
is turned on in the config (for example, if someone wants IPv6 DNS :-),
and will generate an error and fail if dnsmasq on the system isn't new
enough to support whatever feature is required.

Adding IPv6 dns capability to libvirt is turning out to be a little
more involve that I thought it would be.

Right now you can define a IPv6 network manually, define static IPv6
addresses in the guests, and add the definitions to the host's
/etc/hosts file.  That works.

The current dnsmasq 2.63 has a number of options for IPv6 and (to me)
appears to duplicate some/all of the functionality of radvd.  It is
not clear to me that all of these need to be supported.  However,
there is one mode which uses "slaac" IPv6 addresses on a network
running both IPv4 and IPv6 stacks.  It then does a "little dance" and
will add the IPv6 address to cache under the guest system's FQDN.  But
this is not dhcp support.

I am doing some testing but I believe that NetworkManager is not doing
"the right thing" for stateful IPv6 addressing ... it is not sending
the FQDN (not the host name like in IPv4 but the FQDN).  I am trying
not to dig into the NetworkManager code and am doing some testing to
prove that this is the problem.

Assuming that both of these approaches to IPv6 addressing and a dns
service are desirable, this means adding some kind of network
definition parameters so that libvirt knows what dnsmasq parameters to

Any comments appreciated.
A general suggestion when adding things like this: try to make any
required config options related to dhcp/dns terminology rather than the
way they are presented in dnsmasq. Keep in mind that someone may some
day want to provide a backend based on something other than dnsmasq.
OK, how about this approach...

Let's assume we only want the "real dhcp6" provided by dnsmasq. If you do not specify a dhcp range for a family='ipv6' virtual network definition, then everything is as it is today. If you do specify a dhcp-range for an 'ipv6" network, then the extra command line parameter is added and (assuming you are running something like dnsmasq-2.63), it works: you get stateful dhcp6 with updates to the dns.

The only other dnsmasq mode that might be of interest is "ra-names" and yes this is a dnsmasq thing. I need to do more testing to see if I want to bother with it. Again, if it is implemented, the default would be nothing and the command line compatible with old dnsmasq versions. Some special "parameter" would be needed for this to be invoked.

The only interest I have in the 'ra-names" is that I need NetworkManager to work correctly and send fqdn.fqdn information for IPv6 networks and until it does, ra-names works. I have not looked at the code or done testing but, from looking at various documents, I believe that dhcpd-v6 will also need the fqdn.fqdn sent for it to do dynamic dns updating for IPv6 addresses.

As I said earlier, static IPv6 addresses and /etc/hosts entries on the qemu/kvm/libvirt host work today with no changes.

I hope this is a "good enough" solution and I believe it is do-able.


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