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

[Stefano Brivio <sbrivio@redhat.com>]

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

-- 
Stefano