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

On Wed, Jul 15, 2026 at 09:11:30AM +0200, Stefano Brivio wrote:

On Mon, 13 Jul 2026 15:38:52 +1000 David Gibson 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

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