Re: [PATCH v9 9/9] arp/ndp: send gratuitous ARP / unsolicitated NA when MAC cache entry added

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

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On Fri, Sep 26, 2025 at 07:05:56PM -0400, Jon Maloy wrote:
> 
> 
> On 2025-09-25 20:55, David Gibson wrote:
> > On Thu, Sep 25, 2025 at 09:14:42AM -0400, Jon Maloy wrote:
> > > 
> > > 
> > > On 2025-09-25 02:36, David Gibson wrote:
> > > > On Wed, Sep 24, 2025 at 06:18:52PM -0400, Jon Maloy wrote:
> > > > > 
> > > [...]
> > > > > 
> > > > > I experimented a bit with this. My test program is a simple UDP
> > > > > client-server pair, exchanging first 3 UDP messages client->server, followed
> > > > > by
> > > > > 3 messages server->client.
> > > > 
> > > > With the client on the guest, and server outside?  How is the outside
> > > > machine arranged - is it a physically separate host? A bridged VM or
> > > > container on the same host?  Something else?
> > > 
> > > It is a physically separate host.
> > > 
> > > > 
> > > > > First, I changed the main() loop a bit, so that netlink events are
> > > > > handled before all other events, if any. (Basically, I added
> > > > > an extra loop before the main loop, only handling netlink events, before
> > > > > moving on to the main loop (where netlink events had been excluded.)
> > > > > This should secure absolute priority of netlink events before any other
> > > > > events. As you will see below, this made no difference to the scenarios
> > > > > I describe.
> > > > 
> > > > Drat.
> > > > > 1: When starting the container, I notice that there is no subscription
> > > > >      event in PASTA, even though I can see the entry for the remote host
> > > > >      is present in the host's ARP table. There is never any event coming
> > > > >      up even if I wait for 10+ minutes.
> > > > 
> > > > Huh.... do we need to do something to ensure we get events for
> > > > existing entries in the host ARP table, not just ones that are added
> > > > or updated after we're running?
> > > 
> > > It doesn't seem to be possible,
> 
> It actually *is* possible, and I just implemented it. It doesn't solve all
> problems, but makes a huge difference. I will add it in v10.

Hooray!  I thought there ought to be a way.

-- 
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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