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Re: [libvirt] IPv6 migration



On Tue, Mar 19, 2013 at 03:21:31PM +0100, Jiri Denemark wrote:
> On Mon, Mar 11, 2013 at 19:40:52 +0100, Ján Tomko wrote:
> > Hello.
> > 
> > We can only tell QEMU on the destination to listen either on IPv6 or on
> > IPv4.
> > 
> > If we're supplied with a numeric v6 address, that's the only thing we
> > need to know to set the listen address to [::].
> > 
> > For hostnames, we can either assume this based on how it resolves by
> > default on the destination (we keep trying all the resolved addresses on
> > the source, but this might break a few cases), which John found
> > disgusting, so that leaves user input:
> > 
> > How about a VIR_DOMAIN_MIGRATE_IPV6 flag, depending on which we set the
> > listen address on the destination and creating a new function
> > virNetSocketNewConnectTCPHints, where we would add IPv4/IPv6 hint
> > based on the presence/absence of this flag?
> 
> Yeah, I think using an explicit flag would be the best approach. As we
> learnt several times, implementing automagic behavior is too fragile and
> leads to ugly code and confusion. IIUC, we would tell QEMU to listen on
> :: iff either migrateuri uses IPv6 address explicitly or
> VIR_DOMAIN_MIGRATE_IPV6 flag is set. In all other cases, 0.0.0.0 address
> will be passed to QEMU. In other words, unless a user takes an explicit
> action, migration will use IPv4 regardless on libvirt version.

That would mean that migration is broken by default in an IPv6 only
environment, so I don't think that is an satisfactory approach. We
should be checking whether.

Listening on '[::]' means that QEMU will accept connections on *both*
IPv4 and IPv6, if configured with dual-stack. So if IPv6 is present
on the target host, it is entirely reasonable to default to '[::]' if
given a hostname.

Daniel
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