fwd: Don't rewrite inbound multicast destinations

fwd_nat_from_host() (nearly) always rewrites the destination address for
inbound flows to the observed guest address.  Usually, that makes sense:
regardless of the host address to which the new flow arrived, we want to
direct it to the guest.  However, that clearly does not make sense for
multicast - it should still appear as a multicast transmission to the
guest.

In particular this can work very badly for multicast protocols which use
the same source and destination ports by convention (e.g. mDNS).  In this
case, we will attempt to forword multicast packets to our own socket,
causing a forwarding loop (see bug 209 for more details).

While it's certainly not enough to make us handle multicast correctly in
all circumstances, not translating multicast destinations is closer to
correct, and prevents bug 209 at least.

Link: https://bugs.passt.top/show_bug.cgi?id=209
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fwd.c b/fwd.c
index 1369bae..7152169 100644
--- a/fwd.c
+++ b/fwd.c
@@ -1042,7 +1042,8 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
 	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))
+	else if (inany_is_multicast(&ini->oaddr) ||
+		 (c->host_lo_to_ns_lo && inany_is_loopback(&ini->oaddr)))
 		tgt->eaddr = ini->oaddr;
 
 	/* TODO: Allow splicing with specified target address */