From 502aa5d8026099d86da780e109fa91f191881e0f Mon Sep 17 00:00:00 2001
From: Roy Marples <roy@marples.name>
Date: Wed, 18 Dec 2019 16:30:12 +0000
Subject: [PATCH] inet: Add support for IPv4 /31 prefixes, as described in RFC
 3021.

To run a /31 network, participating hosts MUST drop support
for directed broadcasts, and treat the first and last addresses
on subnet as unicast. The broadcast address for the prefix
should be the link local broadcast address, INADDR_BROADCAST.

Taken from FreeBSD, r226402.
---
 sys/netinet/in.c | 19 +++++++++++++------
 sys/netinet/in.h |  2 ++
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/sys/netinet/in.c b/sys/netinet/in.c
index 457ec864cc..57b92b8de4 100644
--- a/sys/netinet/in.c
+++ b/sys/netinet/in.c
@@ -1037,10 +1037,15 @@ in_ifinit(struct ifnet *ifp, struct in_ifaddr *ia,
 	 */
 	ia->ia_ifa.ifa_metric = ifp->if_metric;
 	if (ifp->if_flags & IFF_BROADCAST) {
-		ia->ia_broadaddr.sin_addr.s_addr =
-			htonl(ia->ia_subnet | ~ia->ia_subnetmask);
-		ia->ia_netbroadcast.s_addr =
-			htonl(ia->ia_net | ~ ia->ia_netmask);
+		if (ia->ia_subnetmask == IN_RFC3021_MASK) {
+			ia->ia_broadaddr.sin_addr.s_addr = INADDR_BROADCAST;
+			ia->ia_netbroadcast.s_addr = INADDR_BROADCAST;
+		} else {
+			ia->ia_broadaddr.sin_addr.s_addr =
+				htonl(ia->ia_subnet | ~ia->ia_subnetmask);
+			ia->ia_netbroadcast.s_addr =
+				htonl(ia->ia_net | ~ ia->ia_netmask);
+		}
 	} else if (ifp->if_flags & IFF_LOOPBACK) {
 		ia->ia_dstaddr = ia->ia_addr;
 		flags |= RTF_HOST;
@@ -1298,9 +1303,11 @@ in_broadcast(struct in_addr in, struct ifnet *ifp)
 		    (in.s_addr == ia->ia_broadaddr.sin_addr.s_addr ||
 		     in.s_addr == ia->ia_netbroadcast.s_addr ||
 		     /*
-		      * Check for old-style (host 0) broadcast.
+		      * Check for old-style (host 0) broadcast, but
+		      * taking into account that RFC 3021 obsoletes it.
 		      */
-		     t == ia->ia_subnet || t == ia->ia_net) &&
+		     (ia->ia_subnetmask != IN_RFC3021_MASK &&
+		     (t == ia->ia_subnet || t == ia->ia_net))) &&
 		     /*
 		      * Check for an all one subnetmask. These
 		      * only exist when an interface gets a secondary
diff --git a/sys/netinet/in.h b/sys/netinet/in.h
index 24c77dfc87..2f6fac974f 100644
--- a/sys/netinet/in.h
+++ b/sys/netinet/in.h
@@ -309,6 +309,8 @@ struct in_addr {
 
 #define	IN_LOOPBACKNET		127			/* official! */
 
+#define	IN_RFC3021_MASK		(uint32_t)0xfffffffe
+
 /*
  * Socket address, internet style.
  */
-- 
2.11.4.GIT