fwd: Rework default address logic for inbound flows

fwd_nat_from_host() needs to determine the guest side destination address
for the new flow.  In some cases that's controlled by the forwarding rule
or -host-lo-to-ns-lo logic, but by default we use the observed guest
address.  We need to pick the right one to match the source address,
though.

Currently this is done with similar, but not quite identical logic in the
spliced and non-spliced paths.  Introduce a new fwd_default_guest_addr()
helper to make explicit:
 * We have the same logic for splice and tap paths
 * This is a fallback path if nothing else determined the address (we
   use this default nearly all the time now, but it might change in future)
 * We're matching IP family and scope with the guest side source address

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 30 insertions, 14 deletions

diff --git a/fwd.c b/fwd.c
index 8440094..90297ef 100644
--- a/fwd.c
+++ b/fwd.c
@@ -1006,6 +1006,27 @@ bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
 }
 
 /**
+ * fwd_default_geuest_addr() - Get appropriate guest address to send to
+ * @c:		Execution context
+ * @guest_addr:	Updated with chosen guest address
+ * @template:	Address to match IP version and scope of
+ *
+ * Sets @guest_addr to have the address of the guest, matching the IP version
+ * and scope of @template where possible.
+ */
+static void fwd_default_guest_addr(const struct ctx *c,
+				   union inany_addr *guest_addr,
+				   const union inany_addr *template)
+{
+	if (inany_v4(template))
+		*guest_addr = inany_from_v4(c->ip4.addr_seen);
+	else if (inany_is_linklocal6(template))
+		guest_addr->a6 = c->ip6.addr_ll_seen;
+	else
+		guest_addr->a6 = c->ip6.addr_seen;
+}
+
+/**
  * fwd_nat_from_host() - Determine to forward a flow from the host interface
  * @c:		Execution context
  * @rule:	Forwarding rule to apply
@@ -1040,13 +1061,9 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 		 */
 		if (c->host_lo_to_ns_lo && inany_is_loopback(&ini->oaddr))
 			tgt->eaddr = ini->oaddr;
-		else if (inany_v4(&ini->eaddr))
-			tgt->eaddr = inany_from_v4(c->ip4.addr_seen);
-		else
-			tgt->eaddr.a6 = c->ip6.addr_seen;
 
 		/* Let the kernel pick source address and port */
-		if (inany_v4(&tgt->eaddr))
+		if (inany_v4(&ini->eaddr))
 			tgt->oaddr = inany_any4;
 		else
 			tgt->oaddr = inany_any6;
@@ -1056,6 +1073,9 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 			/* But for UDP preserve the source port */
 			tgt->oport = ini->eport;
 
+		/* Use guest address as destination, if otherwise unspecified */
+		if (inany_is_unspecified(&tgt->eaddr))
+			fwd_default_guest_addr(c, &tgt->eaddr, &tgt->oaddr);
 		return PIF_SPLICE;
 	}
 
@@ -1074,16 +1094,12 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 	}
 	tgt->oport = ini->eport;
 
-	if (!inany_is_unspecified(&rule->taddr)) {
+	if (!inany_is_unspecified(&rule->taddr))
 		tgt->eaddr = rule->taddr;
-	} else if (inany_v4(&tgt->oaddr)) {
-		tgt->eaddr = inany_from_v4(c->ip4.addr_seen);
-	} else {
-		if (inany_is_linklocal6(&tgt->oaddr))
-			tgt->eaddr.a6 = c->ip6.addr_ll_seen;
-		else
-			tgt->eaddr.a6 = c->ip6.addr_seen;
-	}
+
+	/* Use guest address as destination, if otherwise unspecified */
+	if (inany_is_unspecified(&tgt->eaddr))
+		fwd_default_guest_addr(c, &tgt->eaddr, &tgt->oaddr);
 
 	return PIF_TAP;
 }