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 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,

Can we do an RTM_GETNEIGH, with no address specified?  It's something
like that we do to get all our links and addresses in other places.

> but even if it were it wouldn't help us much
> if the entry isn't here, which is also a problematic case. See below.
> 
> > 
> > > 2: The first UDP is attempted sent from the guest. An ARP request is
> > >     sent to PASTA, and responded to with the 9a:9a: address.
> > 
> > Maybe we still need to explicitly ask for an ARP resolution when the
> > guest ARPs.
> 
> I think so. If we limit this to ARP and NDP, this should be unproblematic.

I just realised this is harder than I thought, though.  At least if we
want to get the right answer for the first guest ARP.  It's not just a
netlink request, we'd need to wait for the host to ARP, which means
timeouts, and state we need to track, and ...

-- 
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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