Re: Thoughts on interface modes / multiple guest addresses

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Dec 18, 2025 at 06:32:49AM +0100, Stefano Brivio wrote:
> On Thu, 18 Dec 2025 14:47:06 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > For multi-address support there are at least four things to consider:
> 
> For the bits related https://bugs.passt.top/show_bug.cgi?id=141, I
> thought Jon was working on a proposal.
> 
> > (a) What goes in our internal list of addresses to give the guest?
> > 
> >     a.1. Everything listed with -a?
> 
> If anything is passed, yes, those, and just those (separately for IP
> version), because the user is clearly overriding addresses (as
> currently implemented and documented).

So far, so good.  But including both explicit addresses and host
addresses seems potentially useful to me (especially for an
intermittently online host).  It's not the first step, but I think we
want to think about how we'd allow this.

> >     a.2. Everything on the host?
> 
> No, because you can't assume you can configure all those addresses on
> a single interface. Adding multiple interfaces is something we could
> consider later.

Hm, depends what you mean by "can".  The only case I can see they
really can't be configured on the same interface is if they're
link-local.  But AFAICT, there's nothing to really stop you putting
any combination of global-scope addresses on a single interface.  It
will less resemble the host's configuration, but again, there are
degrees of transparency not a single standard.

> >     a.3. Everything on the host template interface?
> 
> Everything on the host template interface if available (as currently
> documented).

As a first step, sure.

> >     a.4. A link local address we pick?
> 
> A link-local address if nothing else is available (as currently
> documented). This will need to be permanent for the requirement we
> already discussed months ago with Podman developers.

Right, but if this is permanent it potentially conflicts with
link-local addresses from the host interface

> >     a.5. Some combination of the above.
> > 
> > Unlike routes (that I can see), I'm pretty sure there are use cases
> > where we want both host-copied addresses (for transparency) and
> > explicit addresses (for a stable way of communicating with the host).
> 
> Podman needs some anyway if we start with link-local addresses, to
> keep DNS resolution working.
> 
> The rest just looks beyond the scope of
> https://bugs.passt.top/show_bug.cgi?id=141 to me. I won't stand in the
> way of this discussion, of course, but I won't participate either,
> because I really don't see it as a priority at the moment.
> 
> -- 
> Stefano
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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