slirp: Fix assertion failure on rejected DHCP requests

The guest network stack might DHCPREQUEST an address that the slirp built
in dhcp server can't let it have - for example if the guest has an old
leases file from another network configuration.  In this case the dhcp
server should and does reject the request and prepares to send a DHCPNAK
to the client.

However, in this case the daddr variable in bootp_reply() is set to
0.0.0.0.  Shortly afterwards, it unconditionally attempts to pre-insert the
new client address into the ARP table.  This causes an assertion failure in
arp_address_add() because of the 0.0.0.0 address.

According to RFC2131, DHCPNAK messages for clients on the same subnet
must be sent to the broadcast address (S3.2, subpoint 2).

Cc: Jan Kiszka <jan.kiszka@siemens.com>

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

slirp/bootp.c

@@ -200,7 +200,8 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)
             daddr.sin_addr = preq_addr;
             memcpy(bc->macaddr, client_ethaddr, ETH_ALEN);
         } else {
-            daddr.sin_addr.s_addr = 0;
+            /* DHCPNAKs should be sent to broadcast */
+            daddr.sin_addr.s_addr = 0xffffffff;
         }
     } else {
         bc = find_addr(slirp, &daddr.sin_addr, bp->bp_hwaddr);