[PATCH 21/22] fwd: Distinguish translatable from untranslatable addresses on inbound

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

fwd_nat_from_host() needs to adjust the source address for new flows coming
from an address which is not accessible to the guest.  Currently we always
use our_tap_addr or our_tap_ll.  However in cases where the address is
accessible to the guest via translation (i.e. via --nat-host-loopback) then
it makes more sense to use that translation, rather than the fallback
mapping of our_tap_*.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 fwd.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fwd.c b/fwd.c
index 779278a9..7718f7e2 100644
--- a/fwd.c
+++ b/fwd.c
@@ -386,7 +386,14 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto,
 		return PIF_SPLICE;
 	}
 
-	if (!fwd_guest_accessible(c, &ini->eaddr)) {
+	if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.nat_host_loopback) &&
+	    inany_equals4(&ini->eaddr, &in4addr_loopback)) {
+		/* Specifically 127.0.0.1, not 127.0.0.0/8 */
+		tgt->oaddr = inany_from_v4(c->ip4.nat_host_loopback);
+	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.nat_host_loopback) &&
+		   inany_equals6(&ini->eaddr, &in6addr_loopback)) {
+		tgt->oaddr.a6 = c->ip6.nat_host_loopback;
+	} else if (!fwd_guest_accessible(c, &ini->eaddr)) {
 		if (inany_v4(&ini->eaddr)) {
 			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.our_tap_addr))
 				/* No source address we can use */
-- 
@@ -386,7 +386,14 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto,
 		return PIF_SPLICE;
 	}
 
-	if (!fwd_guest_accessible(c, &ini->eaddr)) {
+	if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.nat_host_loopback) &&
+	    inany_equals4(&ini->eaddr, &in4addr_loopback)) {
+		/* Specifically 127.0.0.1, not 127.0.0.0/8 */
+		tgt->oaddr = inany_from_v4(c->ip4.nat_host_loopback);
+	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.nat_host_loopback) &&
+		   inany_equals6(&ini->eaddr, &in6addr_loopback)) {
+		tgt->oaddr.a6 = c->ip6.nat_host_loopback;
+	} else if (!fwd_guest_accessible(c, &ini->eaddr)) {
 		if (inany_v4(&ini->eaddr)) {
 			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.our_tap_addr))
 				/* No source address we can use */
-- 
2.46.0