[dhcp] Request broadcast responses when we already have an IPv4 address

FCoE requires the use of multiple local unicast link-layer addresses.
To avoid the complexity of managing multiple addresses, iPXE operates
in promiscuous mode.  As a consequence, any unicast packets with
non-matching IPv4 addresses are rejected at the IPv4 layer (rather
than at the link layer).

This can cause problems when issuing a second DHCP request: if the
address chosen by the DHCP server does not match the existing address,
then the DHCP response will itself be rejected.

Fix by requesting a broadcast response from the DHCP server if the
network interface already has any IPv4 addresses.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/ip.h

 
 extern struct net_protocol ipv4_protocol __net_protocol;
 
+extern int ipv4_has_any_addr ( struct net_device *netdev );
+
 #endif /* _IPXE_IP_H */

src/net/ipv4.c

  * @v netdev		Network device
  * @ret has_any_addr	Network device has any IPv4 address
  */
-static int ipv4_has_any_addr ( struct net_device *netdev ) {
+int ipv4_has_any_addr ( struct net_device *netdev ) {
 	struct ipv4_miniroute *miniroute;
 
 	list_for_each_entry ( miniroute, &ipv4_miniroutes, list ) {

src/net/udp/dhcp.c

 				      &dhcphdr->flags );
 	memcpy ( dhcphdr->options, options, options_len );
 
+	/* If the network device already has an IPv4 address then
+	 * unicast responses from the DHCP server may be rejected, so
+	 * request broadcast responses.
+	 */
+	if ( ipv4_has_any_addr ( netdev ) )
+		dhcphdr->flags |= htons ( BOOTP_FL_BROADCAST );
+
 	/* Initialise DHCP packet structure */
 	memset ( dhcppkt, 0, sizeof ( *dhcppkt ) );
 	dhcppkt_init ( dhcppkt, data, max_len );