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Re: [libvirt] RFC: Supporting IPv6 on libvirt virtual networks



On 11/03/2010 06:36 AM, Daniel P. Berrange wrote:
On Tue, Nov 02, 2010 at 02:22:08PM -0400, Laine Stump wrote:
    is going to need radvd config information. The list of options is
    rather long, and I don't see how we could assume a hardcoded
    default for any of them, so if we're going to have an<radvd>
    subelement, it's going to need a *lot* of attributes/subelements of
    its own:

      http://linux.die.net/man/5/radvd.conf

    For this reason, I think we can at least initially *not* include all
    this config, and not attempt to run radvd ourselves.
The list of radvd.conf options is long, because it is a tool that has
to serve a million&  one different deployment scenarios. The libvirt
virtual networking is just 1/2 scenarios and thus we don't need to
deal with the entire of radvd configuration.

Okay, I'll take your word for that, and use your example in the other email as the model config.

Can I assume it's okay to do this:

1) run a separate radvd instance for each interface
2) store the pid in /var/run/libvirt/network/${netname}-radvd.pid
3) create the conf file to be used by radvd in /var/run/libvirt/network/${netname}-radvd.conf

??

Also, similar to dnsmasq, I will leave it running if libvirtd exits, and check for it the next time libvirtd starts up.


    However, we can't merely ignore radvd, since our bridges don't
    exist at the time radvd is started by the system, and even if
    you've put the proper config in the static radvd.conf file, it
    can't be recognized until the interface exists, and the method of
    informing radvd that it needs to re-scan the conf file to find new
    interfaces is to send it a SIGHUP.
That isn't going to work. radvd is something that you only ever run
on machines that are network gateways. Virtalization hosts are thus
very unlikely to be running radvd at all. We are taking a non-gateway
host, and making it serve as a gateway solely for the virutal network
interface. As such, we need to be able to run radvd ourselves, solely
for the virbr0 device (and any other configured networks we have).
Running libvirt on a host that is already a network router, and thus
does have radvd running, is something that is really out of scope
for this.


But once I get this all running, that's exactly what I will be doing (using the same machine as a virt host and an IPv6 router). :-P

Sure, it's not what I'd do if I had lots of equipment and space, but in my limited hardware setup at home, I only have a single machine that I can depend on to be always on, and always connected to the network.


Anyway, as long as radvd can have multiple instances running (as long as none of them are attempting to use the same interface), this shouldn't be a problem.


  Therefore we should entirely manage radvd as we do for
dnsmasq.

    So, I'm thinking we can add an<radvd/>  subelement to IP that, for
    now, will have no attributes and no subelements. If the ip/radvd
    subelement exists, libvirt will send radvd a SIGHUP when the
    network is brought up, and again if it is brought down.
I don't think we need a radvd element really. If the forwarding
method is 'routed', then we always need radvd to make that
functionality work. If it is 'isolated', then we don't want it.
To configure radvd, all we need to know is the interface name,
and bridge IPv6 address and the bridge prefix. We have all that
info already in the XML.

So I guess the idea is that in isolated mode, the machines could talk amongst themselves using the link-local IPv6 address (fe80:...), right?

As far as in routed mode, I have two questions:


1) Can we be 100% sure that users will always want want (or at least be able to tolerate) route discovery on a routed IPv6 network? (I don't have enough experience using IPv6 in the real world to know the answer)

2) What about the people who do want to adjust the radvd config in some manner, should we require them to setup their own bridge from scratch?

3) (kind of related to (2)) If no IPv6 addresses are given in the XML, should we continue to disable IPv6 capabilities on the bridge (thus preventing someone from rolling their own radvd/whatever without needing to create their own bridge).



Configuring the IP addresses of the bridge
------------------------------------------

Unfortunately, libnl isn't available on all Linux platforms (libvirt's
virtual networks are only available on Linux), for example RHEL5 has
libnl, but it's an older version that is API-incompatible with the
version used by libvirt.
How badly API incompatible ?  Stupid little differences that can be
trivially #ifdef'd, or major differences that make it look like an
entirely different API ?


A good question. I'm not sure. My statement here is based totally on an email where the NetworkManager maintainer (Dan Williams) told me that when they started using libnl-1.1 in the version of NetworkManager that's in RHEL-5, they had to build their own private copy of libnl, since the -1.0 in RHEL5 was ABI incompatible.

A quick look through the header files of 1.0 vs 1.1 shows that the name of at least one header file has changed, and there are some new functions, and some others eliminated. Much of it is the same, though. My worry would be about functions whose signature remains the same, but have different semantics.


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