Re: [PATCH v2 1/5] fwd: Clarify semantics of --host-lo-to-ns-lo

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

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On Wed, Jul 15, 2026 at 09:11:30AM +0200, Stefano Brivio wrote:
> On Mon, 13 Jul 2026 15:38:52 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > The semantics of --host-lo-to-ns-lo as described in the man page don't
> > quite make sense: It says without the option forwarded packets will appear
> > to come _from_ the guest's public address, which is not usually true.
> > Instead the packets will arrive *to* the guest's public address.  The exact
> > semantics are also a bit confusing in general.
> > 
> > Rewrite both the man page and code to clarify this.  The new rule is that
> > it redirects connections addressed to a host loopback address to the same
> > loopback address in the guest.  This is notionally different from what we
> > had in two ways:
> >   * We can now deliver to nonstandard loopback addresses within the guest,
> >     not just the default one.  This is technically a behavioural change,
> >     but I think will be less surprising behaviour.
> >   * The decision is now made on the original _destination_ address, rather
> >     than source address.  That's different theoretically, but not in
> >     practice, since loopback packets must have loopback addresses for both
> >     source and destination.
> > 
> > We make it explicitly incompatible with --no-splice - previously it
> > was allowed, but would have no effect in that case.
> > 
> > As well as being more precise right now, these semantics will intersect
> > better with other upcoming changes to the forwarding logic.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> 
> I added:
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=212

Thanks.  Having fixed the bug before it was reported, I chose not to
strain my powers of precognition by also predicting the bug number :).

-- 
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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