From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au Authentication-Results: passt.top; dkim=pass (2048-bit key; secure) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.a=rsa-sha256 header.s=202602 header.b=dhm8trh/; dkim-atps=neutral Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id 650485A0262 for ; Mon, 18 May 2026 05:33:15 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202602; t=1779075193; bh=+HdLrIGs3hBTiI+56mnIgOG9Lp15sgR3Ok9+GyqunQc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=dhm8trh/qtM/S/AUokKf9LKlM7cVLm3PnUH9gldU2AJfoPPqhld/7l8+sfBrya2qL Mm8DVP6VvjjYUkjOtc6DvAUB9u7AVSczKKYDt+bGdjR3CwlAEFw6Q9PxISTkJNrMxA IqtSh9EXeRosJyiaRK+taIGAVbhUPYYYMcDmbm4QX4SSPnfU18xADFQNW/6P6kaIlQ 2LUGumEXVzqPVMtPJxB9CLBsfO+zCGlgQ9pr7buL/dRfif737mEdjaQ9s6H62JDrJH CDAczYCs5QKgluK6DX+mLDX+qByiK0RodVYh9+rBtcjVRCX3XwPggSlDKnZUn+KR3C m6AyhkxiEJbUA== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4gJk0T1C3yz4w9j; Mon, 18 May 2026 13:33:13 +1000 (AEST) Date: Mon, 18 May 2026 13:33:09 +1000 From: David Gibson To: Stefano Brivio Subject: Re: [PATCH RFT] fwd: Only do inbound IPv6 NAT to map_host_loopback / map_guest_addr with matching scope Message-ID: References: <20260507043149.1989693-1-sbrivio@redhat.com> <20260514010816.5ccc02de@elisabeth> <20260515005015.6e23cc47@elisabeth> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="ZuZcPA2v3Nu9qLTv" Content-Disposition: inline In-Reply-To: <20260515005015.6e23cc47@elisabeth> Message-ID-Hash: 2TN7HHUN5FHKAQOOD3QSYDOBLUR4RDCJ X-Message-ID-Hash: 2TN7HHUN5FHKAQOOD3QSYDOBLUR4RDCJ X-MailFrom: dgibson@gandalf.ozlabs.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top, Paul Holzinger X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: --ZuZcPA2v3Nu9qLTv Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, May 15, 2026 at 12:50:23AM +0200, Stefano Brivio wrote: > On Thu, 14 May 2026 14:22:51 +1000 > David Gibson wrote: >=20 > > On Thu, May 14, 2026 at 01:08:16AM +0200, Stefano Brivio wrote: > > > On Wed, 13 May 2026 15:04:35 +1000 > > > David Gibson wrote: > > > =20 > > > > On Thu, May 07, 2026 at 06:31:49AM +0200, Stefano Brivio wrote: =20 > > > > > I'm sharing this mostly for debugging / investigation of: > > > > >=20 > > > > > https://github.com/containers/container-libs/pull/755#issuecomm= ent-4390420134 > > > > >=20 > > > > > even though the change is probably correct and needed regardless = of > > > > > that. > > > > >=20 > > > > > If we have map_guest_addr or map_host_loopback addresses set for = IPv6, > > > > > before using them for inbound NAT from the host, make sure they m= atch > > > > > the scope of the original packet, otherwise we might unexpectedly > > > > > turn global unicast addresses into link-local ones for packets co= ming > > > > > from the host itself. > > > > >=20 > > > > > Link: https://github.com/containers/container-libs/pull/755#issue= comment-4390420134 > > > > > Signed-off-by: Stefano Brivio =20 > > > >=20 > > > > There's a real problem here. However, I don't think this patch rea= lly > > > > addresses it. Details below. > > > > =20 > > > > > --- > > > > > fwd.c | 20 ++++++++++++++++++-- > > > > > 1 file changed, 18 insertions(+), 2 deletions(-) > > > > >=20 > > > > > diff --git a/fwd.c b/fwd.c > > > > > index 0697435..d224c0a 100644 > > > > > --- a/fwd.c > > > > > +++ b/fwd.c > > > > > @@ -974,6 +974,20 @@ uint8_t fwd_nat_from_splice(const struct fwd= _rule *rule, uint8_t proto, > > > > > return PIF_HOST; > > > > > } > > > > > =20 > > > > > +/** > > > > > + * fwd_scope6_match() - Check if the IPv6 scope of two addresses= match > > > > > + * @a: First address > > > > > + * @b: Second address > > > > > + * > > > > > + * Return: true for two IPv6 link-local or both not link-local, = false otherwise > > > > > + * > > > > > + * NOTE: This currently ignores any other difference in scope > > > > > + */ =20 > > > >=20 > > > > Nit: we probably want this helper (or ones like it) in ip.h and/or = inany.h. > > > > =20 > > > > > +bool fwd_scope6_match(const struct in6_addr *a, const struct in6= _addr *b) > > > > > +{ > > > > > + return IN6_IS_ADDR_LINKLOCAL(a) =3D=3D IN6_IS_ADDR_LINKLOCAL(b)= ; =20 > > > >=20 > > > > This considers only linklocal vs. not linklocal. Officially those = are > > > > the only two scopes for unicast IPv6. But... site-local unicast us= ed > > > > to exist, and while it's deprecated we have once seen it in the wil= d. > > > > There's also host-local scope which I'm not sure is a term used by > > > > IPv6, but is used by kernel netlinkg, and kind of exists in practice > > > > (::1 and nothing else is host local). =20 > > >=20 > > > Yes, see the NOTE above. I was trying to find out if this was in any > > > way useful (and it looks like it wasn't, at least from the current > > > progress of https://github.com/containers/container-libs/pull/755). = =20 > >=20 > > Ok. > >=20 > > > > > +} > > > > > + > > > > > /** > > > > > * nat_inbound() - Apply address translation for inbound (HOST t= o TAP) > > > > > * @c: Execution context > > > > > @@ -993,13 +1007,15 @@ bool nat_inbound(const struct ctx *c, cons= t union inany_addr *addr, > > > > > /* Specifically 127.0.0.1, not 127.0.0.0/8 */ > > > > > *translated =3D inany_from_v4(c->ip4.map_host_loopback); > > > > > } else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.map_host_loopback) = && > > > > > - inany_equals6(addr, &in6addr_loopback)) { > > > > > + inany_equals6(addr, &in6addr_loopback) && > > > > > + fwd_scope6_match(&addr->a6, &c->ip6.map_host_loopback)) { = =20 > > > >=20 > > > > This test will always be false: we just checked that addr =3D=3D ::= 1, > > > > which is not link-local (it's host-local, if we're admitting that > > > > category). =20 > > >=20 > > > Oh, right, I didn't actually test this case. > > > =20 > > > > > translated->a6 =3D c->ip6.map_host_loopback; > > > > > } else if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_guest_addr) && > > > > > inany_equals4(addr, &c->ip4.addr)) { > > > > > *translated =3D inany_from_v4(c->ip4.map_guest_addr); > > > > > } else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.map_guest_addr) && > > > > > - inany_equals6(addr, &c->ip6.addr)) { > > > > > + inany_equals6(addr, &c->ip6.addr) && > > > > > + fwd_scope6_match(&addr->a6, &c->ip6.map_guest_addr)) { = =20 > > > >=20 > > > > This may be usually be right in practice, but it's kind of by > > > > accident. > > > >=20 > > > > The problem with both these checks is that they compare the scope o= f a > > > > host side address (addr) with the scope of a guest side address > > > > (c->map_*). =20 > > >=20 > > > Note that inany_equals6(addr, &c->ip6.addr) is pre-existing. =20 > >=20 > > Right - that's the confusing but necessary semantics of > > --map-guest-addr. It's a translation for the thing on the outside > > that has the same address as the guest does on the inside. > >=20 > > What I didn't spot before is that makes the scope check equivalent to: > > fwd_scope6_match(&c->ip6.addr, &c->ip6.map_guest_addr) > > which we should be able to implement at conf() time (someday at > > address update time). >=20 > There might be cases where one wants to have different scopes for > those, though, and just apply the map_guest_addr inbound remapping for > packets matching its scope. I'm not sure if it's useful. I don't really understand what you're saying here. By definition the map_guest_addr translation applies with the outside address matches the (assigned) inside guest address. We know the scope of the assigned guest address in advance, we don't have to wait until we get a connection. > > > Further, this "mismatch" is actually intended (see commit message and > > > Podman's pull request I mentioned above), as I was trying to (quickly) > > > make sure that we don't turn a global unicast request into a link-loc= al > > > one. =20 > >=20 > > Ok, I had misunderstood the problem somewhat. Looking at that github > > comment, I'm still pretty confused TBH :/. >=20 > We didn't investigate further because it's really not that critical at > this point, as the follow-ups to that report should indicate. As I commented on github, I've now understood (I'm pretty sure) what's going on there. It's not actually related to map_guest_addr or nat_inbound() at all, but is instead the "translation of last resort" in fwd_nat_from_host(). > > > > That's not what matters: what matters is that source and > > > > destination on the tap side have the same scope. =20 > > >=20 > > > Not for this particular issue: again, I was just trying to make sure > > > that a global unicast request doesn't get translated to a link-local > > > one. =20 > >=20 > > Hrm. It's not really clear to me why that's bad. >=20 > Because it's surprising that a request to a valid global unicast > address that's assigned to a container, from another global > unicast address, gets translated to anything else. There's no need for > that. Isn't there? If the outside source is using the same address as the guest uses inside we have to apply some sort of translation (or drop it entirely). So, if we want to preserve scope as well, we have to pluck a global scope address from somewhere, and it's not clear how we can do that. > > > This can probably checked in an indirect and more correct way "at the > > > source". > > > =20 > > > > In flow table terms, > > > > that is, on a single flowside oaddr and eaddr must have the same > > > > scope (or must they? see later). > > > >=20 > > > > The scopes on one side of the flow don't need to match the scopes on > > > > the other side of the flow. In fact we need to allow them to be > > > > different: --map-host-loopback is *always* transforming a host-scope > > > > flow on the outside to something else on the inside (either link-sc= ope > > > > or global-scope will work, as long as it's the same for both > > > > addresses). We don't do it yet, but I can imagine cases where it > > > > would be useful to translate a flow that's global-scope on the outs= ide > > > > to local-scope on the inside (because for some reason we want to or > > > > have to hide the external peer's address from the guest). Or from > > > > local-scope on the outside to global-scope on the inside (because t= he > > > > outside flow is using local-scope addresses which are not meaningful > > > > to the guest). =20 > > >=20 > > > Yes, definitely, that might actually be a feature, I just think we > > > don't want to do that by default / mistake. =20 > >=20 > > I mean, --map-host-loopback is kind of already this. > >=20 > > > In this case we had an inbound request to a global unicast address th= at > > > was translated for some reason (we didn't really investigate) to a > > > link-local destination address. =20 > >=20 > > Hrm, ok. I really want to understand why that happened. >=20 > In the short term it's probably easier if you try out yourself something > like Paul described, because there are other more critical issues we > discovered later that we're tackling at the moment. >=20 > > What was the source address? >=20 > I *think* another global unicast address. But maybe not and that would > then explain the non-issue. >=20 > > This seems like it would be handled by the selection > > of the guest side eaddr in fwd_nat_from_host(), which explicitly tries > > to match scope with the (translated) source address. >=20 > Maybe, yes. >=20 > > > But if it's explicit it should be allowed, by all means. > > > =20 > > > > This has some tricky implications for what we do about assigning > > > > addresses for "local mode" or any future variant where we need to > > > > assign a guest address, but can't take one from the host. If we > > > > assign a link-local address, as was our plan, that implies under th= is > > > > assumption that the guest can only talk to link-local machines. In > > > > practice that would mean only the host (via -map-*) or in future > > > > things we added explicit NATs, where the guest side address is > > > > link-local. The guest would have no ability to contact the internet > > > > at large. =20 > > >=20 > > > I don't think that's desirable. =20 > >=20 > > Neither do I, but how to avoid it is not obvious to me. >=20 > By making it a matter of preference: if there's another, more fitting > (in terms of scope) address, we use that. Otherwise fall back to a > link-local. Sure, but where would we get a global scope address from? > > > > At least once we have the netlink monitor, maybe that's ok. While = the > > > > host has no address, the guest has only link local, so it can only > > > > talk to the host (or explicitly configured forwards/NATs). But the > > > > host has no connectivity anyway, so there's nothing else to talk to > > > > anyway. When the host gets connectivity we add a global-scope guest > > > > address, so it gets connectivity too. > > > >=20 > > > > If that's not good enough, I can only see two approaches, neither of > > > > which look great. > > > >=20 > > > > a) For incoming connections from the world, to a guest with only an > > > > LL address, we NAT *both* source and destination address (ugh, = the > > > > bookkeeping). =20 > > >=20 > > > The bookkeeping is already in place though. =20 > >=20 > > Well.. we can DNAT easily enough, but to match scope we also need to > > SNAT. That means picking a link-local source address (guest oaddr) > > for each incoming flow. Maybe we can use our_tap_ll for that? I'd forgotten when I wrote this, but we're already doing this. That's what's causing the odd behaviour seen here. > Actually, we don't really need to match the scope, though. I just think > it's preferable when doable. So a) could be optional, and b) could be > the default. At that point: But as noted, I'm not sure (b) works *at all* for IPv6. It also wouldn't help for this case: we *cannot* use the original source address if it's the same as the guest's. > > But that means all incoming connections will appear to come from there > > regardless of whether they are actually the same peer or not. If the > > guest is talking to enough peers we risk running out of source ports. >=20 > ...this would be a marginal risk. The user enabled that explicitly. At present, they didn't, it's always on. But if we don't do that, that implies the guest cannot be reached from peers that have the same address as it. > > Or for UDP, where we preserve source port, we risk collision between > > flows that are separate on the outside. Theoretically, we could avoid > > this by assigning a distinct, link-local, guest side address for each > > peer. Doing _that_ is a lot of new bookkeeping which is what I was > > thinking of. > >=20 > > > > Outgoing connections to the world are only possible for targets > > > > where we've preconfigured a NAT. > > > >=20 > > > > b) We _do_ allow different scopes on the two guest-side addresses. > > > > This implies that the guest *expects and requires* their gateway > > > > (us) to SNAT them. > > > >=20 > > > > I suspect that the guest simply won't allow (b) to work for IPv6, b= ut > > > > it might for IPv4, since most things don't actually look for RFC3927 > > > > addresses, and NAT is much more expected in general. =20 > > >=20 > > > I think it's rather complicated to define this before having played > > > with the netlink monitor implementation itself, but again, this is we= ll > > > beyond the scope of this patch. =20 > >=20 > > Maybe. > >=20 > > >=20 > > > The idea here is that *if* we have a global unicast address in a > > > container, the mere fact that we have a link-local address in > > > --map-guest-addr (not actually the case, it seems, but we haven't > > > investigated further), shouldn't cause inbound traffic to be mapped to > > > that link-local address. =20 > >=20 > > Indeed it should not, but I don't yet see why that's happening. >=20 > Me neither, but when Paul raised that, it looked like the most visible > issue we could have solved and "got everything to work". Things turned > out to be rather different in reality, so we didn't look for an > explanation, at least not yet. >=20 > > > But note that I'm not sure if it's an actual problem or if it's even > > > happening at all, at this stage. =20 >=20 > --=20 > Stefano >=20 --=20 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 --ZuZcPA2v3Nu9qLTv Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmoKiHQACgkQzQJF27ox 2GfbRQ/+NFUVpeqxCv/b+hGVKs7h81lPs/JRlwgGKln41nKsfeRugjTSvgXaoSSJ p9ufRmCZpyW8hm2VvjXGs/TObLl6y1gSMPeuMK5+C4tJtDHzvaROyuNb188TDRCb WCfHdy0Xbdp9PXbCOqWo8r6DaP7t9vIZVXLua4HGieWAfarESHT+DS91lULmudbF 9oER/byyax4brhY7oVo6vnYr1nX80a/GXgdpwP81uh+mbAjSnN23iTiGL+ofgXUS ZnuEhQhSLaYrla7Vf6DufI1ug4VbMH7T4xEZHLRBF6E3ryKkrBZAfecceTBdwBfR /m0Q7WtBPf81XtckUj6yJir3LP9kIrhWGe3B87b2Gx1dNi93+g8qrq68sdW9xnzX mQrNu2LTUceFUUKf8MnuCbBb1+amJeCWlPbTEwCIKhCNJxKtqiP+gLkFLkvWC9a2 1RGRLWf3LqR9xlV21Pltf2qlwwXE1kGzwhSZchvMO+HQzAvtpfEAc4oN/9rNP14c THhxRaETBB8tTUE8MJhwDJX8Kot5IWu+SVgPtmBeqaeK6T/L5x+sHSz9gieNI7Pq YfxarZRkCY2xyR72wk+T8DUQFFyCtUIc5Kro0TMqv++A8rcYvIMDNkLo9XVdF7+T JU4SAMHbkmpoFof8aK8Ra9mnwh9kesl/e3JUCe+se7zTH6nH0Mw= =5tEq -----END PGP SIGNATURE----- --ZuZcPA2v3Nu9qLTv--