From: YASUOKA Masahiko <yasuoka@openbsd.org>
Subject: diff: setting M_BCAST flag
To: tech@openbsd.org
Date: Sat, 08 Mar 2025 12:54:35 +0900

Hello,

Our ip_input() doesn't set M_BCAST flag for 0.0.0.0 or 255.255.255.255
if it's not set by the lower layer.  The flag must be set since it is
used by the upper layers.  (eg. TCP use the flag to drop the packet.)

The modification on in_ouraddr() is to set M_BCAST flag consistently
if the packet is for directed broadcast.  Previous the flag isn't set
if the packet is diverted by pf(4).

(eg. on the host that runs a transparent proxy for 80/tcp, when it
receives a packet for  directed broadcast but its destination mac
address is not ff:ff:ff:ff:ff:ff (eg. wrong netmask), TCP wil not drop
it and the connection will  be established.)

ok? comments?

Index: sys/netinet/ip_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_input.c,v
diff -u -p -r1.404 ip_input.c
--- sys/netinet/ip_input.c	2 Mar 2025 21:28:32 -0000	1.404
+++ sys/netinet/ip_input.c	8 Mar 2025 03:51:45 -0000
@@ -508,6 +508,7 @@ ip_input_if(struct mbuf **mp, int *offp,
 
 	if (ip->ip_dst.s_addr == INADDR_BROADCAST ||
 	    ip->ip_dst.s_addr == INADDR_ANY) {
+		m->m_flags |= M_BCAST; /* Make sure M_BCAST is set */
 		nxt = ip_ours(mp, offp, nxt, af, ns);
 		goto out;
 	}
@@ -518,7 +519,7 @@ ip_input_if(struct mbuf **mp, int *offp,
 	} else {
 		ro = &ns->ns_route;
 	}
-	switch(in_ouraddr(m, ifp, ro, flags)) {
+	switch (in_ouraddr(m, ifp, ro, flags)) {
 	case 2:
 		goto bad;
 	case 1:
@@ -851,18 +852,6 @@ in_ouraddr(struct mbuf *m, struct ifnet 
 	struct ip		*ip;
 	int			 match = 0;
 
-#if NPF > 0
-	switch (pf_ouraddr(m)) {
-	case 0:
-		return (0);
-	case 1:
-		return (1);
-	default:
-		/* pf does not know it */
-		break;
-	}
-#endif
-
 	ip = mtod(m, struct ip *);
 
 	rt = route_mpath(ro, &ip->ip_dst, &ip->ip_src, m->m_pkthdr.ph_rtableid);
@@ -883,6 +872,18 @@ in_ouraddr(struct mbuf *m, struct ifnet 
 			m->m_flags |= M_BCAST;
 		}
 	}
+
+#if NPF > 0
+	switch (pf_ouraddr(m)) {
+	case 0:
+		return (0);
+	case 1:
+		return (1);
+	default:
+		/* pf does not know it */
+		break;
+	}
+#endif
 
 	if (!match) {
 		struct ifaddr *ifa;