platform: generate IFA_BROADCAST address based on the peer IFA_ADDRESSth/ifa-broadcast

This is also what iproute2 does ([1]) when creating a default broadcast address
with `ip addr add 192.168.1.5/24 brd + dev eth0`.

Also, kernel does in fib_add_ifaddr() ([2]):
```
        __be32 addr = ifa->ifa_local;
        __be32 prefix = ifa->ifa_address & mask;

        ...

        /* Add broadcast address, if it is explicitly assigned. */
        if (ifa->ifa_broadcast && ifa->ifa_broadcast != htonl(0xFFFFFFFF))
                fib_magic(RTM_NEWROUTE, RTN_BROADCAST, ifa->ifa_broadcast, 32,
                          prim, 0);

        if (!ipv4_is_zeronet(prefix) && !(ifa->ifa_flags & IFA_F_SECONDARY) &&
            (prefix != addr || ifa->ifa_prefixlen < 32)) {
                if (!(ifa->ifa_flags & IFA_F_NOPREFIXROUTE))
                        fib_magic(RTM_NEWROUTE,
                                  dev->flags & IFF_LOOPBACK ? RTN_LOCAL : RTN_UNICAST,
                                  prefix, ifa->ifa_prefixlen, prim,
                                  ifa->ifa_rt_priority);

                /* Add network specific broadcasts, when it takes a sense */
                if (ifa->ifa_prefixlen < 31) {
                        fib_magic(RTM_NEWROUTE, RTN_BROADCAST, prefix, 32,
                                  prim, 0);
                        fib_magic(RTM_NEWROUTE, RTN_BROADCAST, prefix | ~mask,
                                  32, prim, 0);
                }
        }
```

Which means by default kernel already adds those special broadcast routes which
are identical to what we configure with IFA_BROADCAST. However, kernel too bases
them on the peer (IFA_ADDRESS).

[1] https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/tree/ip/ipaddress.c?id=d5391e186f04214315a5a80797c78e50ad9f5271#n2380
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/fib_frontend.c?id=bef1d88263ff769f15aa0e1515cdcede84e61d15#n1109

1 files changed, 4 insertions, 3 deletions

diff --git a/src/platform/nm-platform.h b/src/platform/nm-platform.h
index f1acd1df1e..bd99052b3e 100644
--- a/src/platform/nm-platform.h
+++ b/src/platform/nm-platform.h
@@ -1171,10 +1171,11 @@ nm_platform_ip4_broadcast_address_from_addr (const NMPlatformIP4Address *addr)
 	if (addr->use_ip4_broadcast_address)
 		return addr->broadcast_address;
 
-	/* the set broadcast-address gets ignored, and we determine a default brd. */
-	if (   addr->address != 0u
+	/* the set broadcast-address gets ignored, and we determine a default brd base
+	 * on the peer IFA_ADDRESS. */
+	if (   addr->peer_address != 0u
 	    && addr->plen < 31 /* RFC3021 */)
-		return nm_platform_ip4_broadcast_address_create (addr->address, addr->plen);
+		return nm_platform_ip4_broadcast_address_create (addr->peer_address, addr->plen);
 	return 0u;
 }