git: fd0765933c3c - main - Change lowest address on subnet (host 0) not to broadcast by default.

[Mike Karels]

The branch main has been updated by karels:

URL: https://cgit.FreeBSD.org/src/commit/?id=fd0765933c3ccb059ad7456e657b2e8ed22f58b0

commit fd0765933c3ccb059ad7456e657b2e8ed22f58b0
Author:     Mike Karels <karels at FreeBSD.org>
AuthorDate: 2021-09-05 18:14:04 +0000
Commit:     Mike Karels <karels at FreeBSD.org>
CommitDate: 2021-09-17 00:42:20 +0000

    Change lowest address on subnet (host 0) not to broadcast by default.
    
    The address with a host part of all zeros was used as a broadcast long
    ago, but the default has been all ones since 4.3BSD and RFC1122.  Until
    now, we would broadcast the host zero address as well as the configured
    address.  Change to not broadcasting that address by default, but add a
    sysctl (net.inet.ip.broadcast_lowest) to re-enable it.  Note that the
    correct way to use the zero address for broadcast would be to configure
    it as the broadcast address for the network.
    
    See https:/datatracker.ietf.org/doc/draft-schoen-intarea-lowest-address/
    and the discussion in https://reviews.freebsd.org/D19316.  Note, Linux
    now implements this.
    
    Reviewed by:    rgrimes, tuexen; melifaro (previous version)
    MFC after:      1 month
    Relnotes:       yes
    Differential Revision: https://reviews.freebsd.org/D31861
---
 sys/netinet/in.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/sys/netinet/in.c b/sys/netinet/in.c
index e968a559a13c..b51f1111b88a 100644
--- a/sys/netinet/in.c
+++ b/sys/netinet/in.c
@@ -88,6 +88,12 @@ SYSCTL_INT(_net_inet_ip, OID_AUTO, no_same_prefix, CTLFLAG_VNET | CTLFLAG_RW,
 	&VNET_NAME(nosameprefix), 0,
 	"Refuse to create same prefixes on different interfaces");
 
+VNET_DEFINE_STATIC(bool, broadcast_lowest);
+#define	V_broadcast_lowest		VNET(broadcast_lowest)
+SYSCTL_BOOL(_net_inet_ip, OID_AUTO, broadcast_lowest, CTLFLAG_VNET | CTLFLAG_RW,
+	&VNET_NAME(broadcast_lowest), 0,
+	"Treat lowest address on a subnet (host 0) as broadcast");
+
 VNET_DECLARE(struct inpcbinfo, ripcbinfo);
 #define	V_ripcbinfo			VNET(ripcbinfo)
 
@@ -1170,10 +1176,10 @@ in_ifaddr_broadcast(struct in_addr in, struct in_ifaddr *ia)
 
 	return ((in.s_addr == ia->ia_broadaddr.sin_addr.s_addr ||
 	     /*
-	      * Check for old-style (host 0) broadcast, but
+	      * Optionally check for old-style (host 0) broadcast, but
 	      * taking into account that RFC 3021 obsoletes it.
 	      */
-	    (ia->ia_subnetmask != IN_RFC3021_MASK &&
+	    (V_broadcast_lowest && ia->ia_subnetmask != IN_RFC3021_MASK &&
 	    ntohl(in.s_addr) == ia->ia_subnet)) &&
 	     /*
 	      * Check for an all one subnetmask. These