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



On 09/15/2012 04:07 PM, Gene Czarcinski wrote:
> On 09/14/2012 06:08 AM, Daniel P. Berrange wrote:
>> FYI, already libvirt aims to work with old dnsmasq, in general there is
>> no problem with using features from new dnsmasq. If the host has an old
>> dnsmasq, they simply won't be able to take advantage of new features
>> libvirt might support. The key is simply that if the host has the old
>> dnsmasq, we should be careful not to invoke it in the wrong way for
>> existing features.
> I am not sure I understand the reasoning for continuing support of old
> versions of dnsmasq ... but so be it.  

Upstream libvirt is targetted at many platforms, past and present, and
must continue to build and function properly on a lot of different
systems that aren't "the latest". For example, because there is a very
large installed base of RHEL5 systems, we still check to make sure that
upstream libvirt builds and runs properly on RHEL5.


> 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
> use.
>
> 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.


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