Re: [PATCH v2] Send an initial ARP request to resolve the guest IP address

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

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On Tue, Sep 09, 2025 at 05:55:30PM +0200, Stefano Brivio wrote:
> On Tue, 9 Sep 2025 12:10:45 +0200
> Volker Diels-Grabsch <v@njh.eu> wrote:
> 
> > David Gibson wrote:
> > > Looks good to me.  It would be nice to also send an Neighbour
> > > Discovery request to accomplish the same thing for IPv6 only guests,
> > > but that could be a separate patch.  
> > 
> > Good point!  I'd prefer to do it in the same patch, as I'd also like
> > fix another minor detail in this one.
> > 
> > Just for the sake of clarity: I had a look at RFC4861 and there are
> > many types of Neighbour Discovery requests.  For our purpose, I believe
> > that we want to send a "Neighbor Solicitation Message" described in
> > section 4.3:
> > 
> > https://datatracker.ietf.org/doc/html/rfc4861#section-4.3
> > 
> > Do you agree, or did you have something else in mind?
> 
> I was about to comment on this, but from the new patch you sent, I see
> you already figured out it's a Neighbour Solicitation and that we
> already have some bits of code for that. :)
> 
> > > Note that even with this patch, active TCP connections (and in some
> > > cases UDP flows) will be broken by a passt restart.  
> > 
> > Indeed, that is unavoidable for a user-space tool opening TCP and UDP
> > connections, I guess, unless "passt" itself is wrapped into another
> > process or system tool that keeps those connections open.  But let's
> > not go into that.
> 
> Actually, it's not really unavoidable, in the sense that we recently
> added (see migrate.c, repair.c, passt-repair.c, and passt-repair(1))
> support for migration of live TCP connections triggered by vhost-user
> commands:

Right, but doing this requires knowing in advance, support from qemu
and a bunch of infrastructure.  It's not going to work if you just
kill and restart passt.

-- 
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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