fwd: Reorder DNAPT and SNAT steps in fwd_nat_from_host()

The order in which we translate source and destination addresses is a bit
unclear in fwd_nat_from_host().  Reorder things to make it clearer:
   1. Pick guest-side destination address, where options require it
   2. Pick guest-side source address (needs to be different for SPLICE and
      TAP)
   3. If (1) didn't determine destination, pick a fallback to match family
      and scope of source address from (2).

As a small bonus this lets us make step (1) common between SPLICE and TAP
paths.  The value of this is a bit dubious right now, but it will make some
future changes clearer.

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

1 files changed, 7 insertions, 7 deletions

diff --git a/fwd.c b/fwd.c
index de444d9..1369bae 100644
--- a/fwd.c
+++ b/fwd.c
@@ -1038,8 +1038,12 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 			  const struct fwd_rule *rule, uint8_t proto,
 			  const struct flowside *ini, struct flowside *tgt)
 {
-	/* Common for spliced and non-spliced cases */
+	/* DNAPT: Common for splice and non-spliced where possible */
 	tgt->eport = rule->to + (ini->oport - rule->first);
+	if (!inany_is_unspecified(&rule->taddr))
+		tgt->eaddr = rule->taddr;
+	else if (c->host_lo_to_ns_lo && inany_is_loopback(&ini->oaddr))
+		tgt->eaddr = ini->oaddr;
 
 	/* TODO: Allow splicing with specified target address */
 	if (!c->no_splice && inany_is_unspecified(&rule->taddr) &&
@@ -1056,10 +1060,8 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 		 * In either case, let the kernel pick the source address to
 		 * match.
 		 */
-		if (c->host_lo_to_ns_lo && inany_is_loopback(&ini->oaddr))
-			tgt->eaddr = ini->oaddr;
 
-		/* Let the kernel pick source address and port */
+		/* SNAT: (implicit) let the kernel pick source addr/port */
 		if (inany_v4(&ini->eaddr))
 			tgt->oaddr = inany_any4;
 		else
@@ -1079,6 +1081,7 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 	if (c->splice_only)
 		return PIF_NONE;
 
+	/* SNAT: translate source address if necessary */
 	if (!nat_inbound(c, &ini->eaddr, &tgt->oaddr)) {
 		if (inany_v4(&ini->eaddr)) {
 			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.our_tap_addr))
@@ -1091,9 +1094,6 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 	}
 	tgt->oport = ini->eport;
 
-	if (!inany_is_unspecified(&rule->taddr))
-		tgt->eaddr = rule->taddr;
-
 	/* Use guest address as destination, if otherwise unspecified */
 	if (inany_is_unspecified(&tgt->eaddr))
 		tgt->eaddr = fwd_default_guest_addr(c, &tgt->oaddr);