[PATCH RFT] fwd: Only do inbound IPv6 NAT to map_host_loopback / map_guest_addr with matching scope

[PATCH RFT] fwd: Only do inbound IPv6 NAT to map_host_loopback / map_guest_addr with matching scope

[Stefano Brivio]

I'm sharing this mostly for debugging / investigation of:

  https://github.com/containers/container-libs/pull/755#issuecomment-4390420134

even though the change is probably correct and needed regardless of
that.

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 match
the scope of the original packet, otherwise we might unexpectedly
turn global unicast addresses into link-local ones for packets coming
from the host itself.

Link: https://github.com/containers/container-libs/pull/755#issuecomment-4390420134
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 fwd.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

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;
 }
 
+/**
+ * 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
+ */
+bool fwd_scope6_match(const struct in6_addr *a, const struct in6_addr *b)
+{
+	return IN6_IS_ADDR_LINKLOCAL(a) == IN6_IS_ADDR_LINKLOCAL(b);
+}
+
 /**
  * nat_inbound() - Apply address translation for inbound (HOST to TAP)
  * @c:		Execution context
@@ -993,13 +1007,15 @@ bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
 		/* Specifically 127.0.0.1, not 127.0.0.0/8 */
 		*translated = 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)) {
 		translated->a6 = c->ip6.map_host_loopback;
 	} else if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_guest_addr) &&
 		   inany_equals4(addr, &c->ip4.addr)) {
 		*translated = 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)) {
 		translated->a6 = c->ip6.map_guest_addr;
 	} else if (fwd_guest_accessible(c, addr)) {
 		*translated = *addr;
-- 
2.43.0

Re: [PATCH RFT] fwd: Only do inbound IPv6 NAT to map_host_loopback / map_guest_addr with matching scope

[David Gibson]

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On Thu, May 07, 2026 at 06:31:49AM +0200, Stefano Brivio wrote:
> I'm sharing this mostly for debugging / investigation of:
> 
>   https://github.com/containers/container-libs/pull/755#issuecomment-4390420134
> 
> even though the change is probably correct and needed regardless of
> that.
> 
> 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 match
> the scope of the original packet, otherwise we might unexpectedly
> turn global unicast addresses into link-local ones for packets coming
> from the host itself.
> 
> Link: https://github.com/containers/container-libs/pull/755#issuecomment-4390420134
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

There's a real problem here.  However, I don't think this patch really
addresses it.  Details below.

> ---
>  fwd.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> 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;
>  }
>  
> +/**
> + * 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
> + */

Nit: we probably want this helper (or ones like it) in ip.h and/or inany.h.

> +bool fwd_scope6_match(const struct in6_addr *a, const struct in6_addr *b)
> +{
> +	return IN6_IS_ADDR_LINKLOCAL(a) == IN6_IS_ADDR_LINKLOCAL(b);

This considers only linklocal vs. not linklocal.  Officially those are
the only two scopes for unicast IPv6.  But... site-local unicast used
to exist, and while it's deprecated we have once seen it in the wild.
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).

> +}
> +
>  /**
>   * nat_inbound() - Apply address translation for inbound (HOST to TAP)
>   * @c:		Execution context
> @@ -993,13 +1007,15 @@ bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
>  		/* Specifically 127.0.0.1, not 127.0.0.0/8 */
>  		*translated = 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)) {

This test will always be false: we just checked that addr == ::1,
which is not link-local (it's host-local, if we're admitting that
category).

>  		translated->a6 = c->ip6.map_host_loopback;
>  	} else if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_guest_addr) &&
>  		   inany_equals4(addr, &c->ip4.addr)) {
>  		*translated = 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)) {

This may be usually be right in practice, but it's kind of by
accident.

The problem with both these checks is that they compare the scope of a
host side address (addr) with the scope of a guest side address
(c->map_*).  That's not what matters: what matters is that source and
destination on the tap side have the same scope.  In flow table terms,
that is, on a single flowside oaddr and eaddr must have the same
scope (or must they? see later).

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-scope
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 outside
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 the
outside flow is using local-scope addresses which are not meaningful
to the guest).

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 this
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.

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.

If that's not good enough, I can only see two approaches, neither of
which look great.

 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).
    Outgoing connections to the world are only possible for targets
    where we've preconfigured a NAT.

 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.

I suspect that the guest simply won't allow (b) to work for IPv6, but
it might for IPv4, since most things don't actually look for RFC3927
addresses, and NAT is much more expected in general.

>  		translated->a6 = c->ip6.map_guest_addr;
>  	} else if (fwd_guest_accessible(c, addr)) {
>  		*translated = *addr;
> -- 
> 2.43.0
> 

-- 
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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