BSD: Fix netmask sockaddr family and length for incoming route msgs

Netmask family and length are ignored by traditional
userland tools such as route and netstat and are assumed
to match the destination sockaddr.
On the wire, sa_family is 0 (AF_UNSPEC)
for default routes or 255 for subnet/prefix routes.
Default routes also have zero length netmask.

In both cases we can just sub in the values from
the destination address.
This is important, because if we feed the same values
back we receive back into RTM_DELETE or RTM_ADD then
the kernel doesn't react very well.

This is currently true for all BSD kernels.

Author: rsmarples

src/if-bsd.c

@@ -887,10 +887,27 @@ if_copyrt(struct dhcpcd_ctx *ctx, struct rt *rt, const struct rt_msghdr *rtm)
 
 	rt->rt_flags = (unsigned int)rtm->rtm_flags;
 	if_copysa(&rt->rt_dest, rti_info[RTAX_DST]);
+
 	if (rtm->rtm_addrs & RTA_NETMASK) {
 		if_copysa(&rt->rt_netmask, rti_info[RTAX_NETMASK]);
-		if (rt->rt_netmask.sa_family == 255) /* Why? */
-			rt->rt_netmask.sa_family = rt->rt_dest.sa_family;
+		/*
+		 * Netmask family and length are ignored by traditional
+		 * userland tools such as route and netstat and are assumed
+		 * to match the destination sockaddr.
+		 * On the wire, sa_family is 0 (AF_UNSPEC)
+		 * for default routes or 255 for subnet/prefix routes.
+		 * Default routes also have zero length netmask.
+		 *
+		 * In both cases we can just sub in the values from
+		 * the destination address.
+		 * This is important, because if we feed the same values
+		 * back we receive back into RTM_DELETE or RTM_ADD then
+		 * the kernel doesn't react very well.
+		 *
+		 * This is currently true for all BSD kernels.
+		 */
+		rt->rt_netmask.sa_family = rt->rt_dest.sa_family;
+		rt->rt_netmask.sa_len = rt->rt_dest.sa_len;
 	}
 
 	/* dhcpcd likes an unspecified gateway to indicate via the link.

src/sa.c

@@ -419,11 +419,6 @@ sa_cmp(const struct sockaddr *sa1, const struct sockaddr *sa2)
 	assert(sa1 != NULL);
 	assert(sa2 != NULL);
 
-	/* Treat AF_UNSPEC as the unspecified address. */
-	if ((sa1->sa_family == AF_UNSPEC || sa2->sa_family == AF_UNSPEC) &&
-	    sa_is_unspecified(sa1) && sa_is_unspecified(sa2))
-		return 0;
-
 	if (sa1->sa_family != sa2->sa_family)
 		return sa1->sa_family - sa2->sa_family;