Re: [PATCH v4] Send an initial ARP and NDP request to resolve the guest IP address

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 11 Sep 2025 21:47:56 +0200
Volker Diels-Grabsch <v@njh.eu> wrote:

> Stefano Brivio wrote:
> > >  		memcpy(c->guest_mac, eh->h_source, ETH_ALEN);
> > > +		info("Guest MAC address: %s", eth_ntop(c->guest_mac, bufmac, sizeof(bufmac)));  
> > 
> > This is one line you can break easily instead. But I don't think it
> > should be info(), otherwise the guest can happily spam system logs on
> > the host.
> > 
> > I'm undecided between suggesting debug() or trace(), because on one
> > hand it's a rather relevant information (maybe say "New guest MAC
> > address"?), on the other hand nothing prevents a guest from using a
> > different MAC address every other frame, and make debugging with --debug
> > impossible if this happens.  
> 
> For debugging this issue that message was extremely helpful, so I
> strongly recommend to not hide it in trace().  I personally would
> even like to keep it info(), but I'll demote it to debug() as you
> request.

I think you're right, it should ideally be info(), but as long as we
don't have a mechanism to rate limit messages, we need to be mindful of
potential denial of service vectors. System loggers will, in general,
not limit us much.

> > We don't have (yet) a generic rate limiting mechanism for prints.
> > Maybe we could go with trace() for the moment, and the day we have it,
> > switch this to a rate-limited debug() (or even info(), at that point).  
> 
> In the unlikely case a client really does spam us with an every
> changing MAC address, we can still easily grep those messages out.

...unless we're using a log file (quite common if we debug) and we
exceed the maximum size. There's a mandatory limit on it, again for
security reasons, and once we exceed it we start overwriting entries.

Of course, if we're debugging a specific issue in this sense, we can
just increase the size limit, so debug() should be kind of fine as well.

> I'll soon provide v5 of my patch.

Thanks! I'll review it in a bit.

-- 
Stefano