Add ProxyDHCP support.

src/include/gpxe/dhcp.h

 #include <gpxe/in.h>
 #include <gpxe/refcnt.h>
 #include <gpxe/tables.h>
+#include <latch.h>
 
 struct net_device;
 struct job_interface;
 /**
  * Drop reference to DHCP options block
  *
- * @v options		DHCP options block
+ * @v options		DHCP options block, or NULL
  */
 static inline __attribute__ (( always_inline )) void
 dhcpopt_put ( struct dhcp_option_block *options ) {
 	ref_put ( &options->refcnt );
 }
 
+/** Maximum time that we will wait for ProxyDHCP offers */
+#define PROXYDHCP_WAIT_TIME ( TICKS_PER_SEC * 2 )
+
 extern struct list_head dhcp_option_blocks;
 
 extern unsigned long dhcp_num_option ( struct dhcp_option *option );

src/net/udp/dhcp.c

 	 * (e.g. @c DHCPDISCOVER).
 	 */
 	int state;
-	/** Options obtained from server */
+	/** Options obtained from DHCP server */
 	struct dhcp_option_block *options;
+	/** Options obtained from ProxyDHCP server */
+	struct dhcp_option_block *proxy_options;
 	/** Retransmission timer */
 	struct retry_timer timer;
+	/** Session start time (in ticks) */
+	unsigned long start;
 };
 
 /**
 
 	netdev_put ( dhcp->netdev );
 	dhcpopt_put ( dhcp->options );
+	dhcpopt_put ( dhcp->proxy_options );
 	free ( dhcp );
 }
 
 		container_of ( xfer, struct dhcp_session, xfer );
 	const struct dhcphdr *dhcphdr = data;
 	struct dhcp_option_block *options;
+	struct dhcp_option_block **store_options;
+	int is_proxy;
 	unsigned int msgtype;
+	unsigned long elapsed;
 
 	/* Check for matching transaction ID */
 	if ( dhcphdr->xid != dhcp_xid ( dhcp->netdev ) ) {
 		return -EINVAL;
 	}
 
-	/* Determine message type */
+	/* Determine and verify message type */
+	is_proxy = ( dhcphdr->yiaddr.s_addr == 0 );
 	msgtype = find_dhcp_num_option ( options, DHCP_MESSAGE_TYPE );
-	DBGC ( dhcp, "DHCP %p received %s\n",
-	       dhcp, dhcp_msgtype_name ( msgtype ) );
+	DBGC ( dhcp, "DHCP %p received %s%s\n", dhcp,
+	       ( is_proxy ? "Proxy" : "" ), dhcp_msgtype_name ( msgtype ) );
+	if ( ( ( dhcp->state != DHCPDISCOVER ) || ( msgtype != DHCPOFFER ) ) &&
+	     ( ( dhcp->state != DHCPREQUEST ) || ( msgtype != DHCPACK ) ) ) {
+		DBGC ( dhcp, "DHCP %p discarding %s while in %s state\n",
+		       dhcp, dhcp_msgtype_name ( msgtype ),
+		       dhcp_msgtype_name ( dhcp->state ) );
+		goto out_discard;
+	}
 
-	/* Handle DHCP reply */
+	/* Update stored standard/ProxyDHCP options, if the new
+	 * options have equal or higher priority than the
+	 * currently-stored options.
+	 */
+	store_options = ( is_proxy ? &dhcp->proxy_options : &dhcp->options );
+	if ( ( ! *store_options ) || 
+	     ( find_dhcp_num_option ( options, DHCP_EB_PRIORITY ) >=
+	       find_dhcp_num_option ( *store_options, DHCP_EB_PRIORITY ) ) ) {
+		dhcpopt_put ( *store_options );
+		*store_options = options;
+	} else {
+		dhcpopt_put ( options );
+	}
+
+	/* Handle DHCP response */
 	switch ( dhcp->state ) {
 	case DHCPDISCOVER:
-		if ( msgtype != DHCPOFFER )
-			goto out_discard;
-		dhcp->state = DHCPREQUEST;
+		/* If we have received a valid standard DHCP response
+		 * (i.e. one with an IP address), and we have allowed
+		 * sufficient time for ProxyDHCP reponses, then
+		 * transition to making the DHCPREQUEST.
+		 */
+		elapsed = ( currticks() - dhcp->start );
+		if ( dhcp->options &&
+		     ( elapsed > PROXYDHCP_WAIT_TIME ) ) {
+			stop_timer ( &dhcp->timer );
+			dhcp->state = DHCPREQUEST;
+			dhcp_send_request ( dhcp );
+		}
 		break;
 	case DHCPREQUEST:
-		if ( msgtype != DHCPACK )
-			goto out_discard;
-		dhcp->state = DHCPACK;
+		/* DHCP finished; register options and exit */
+		if ( dhcp->proxy_options )
+			dhcp->register_options ( dhcp->netdev,
+						 dhcp->proxy_options );
+		dhcp->register_options ( dhcp->netdev, dhcp->options );
+		dhcp_finished ( dhcp, 0 );
 		break;
 	default:
 		assert ( 0 );
-		goto out_discard;
 	}
 
-	/* Stop timer and update stored options */
-	stop_timer ( &dhcp->timer );
-	if ( dhcp->options )
-		dhcpopt_put ( dhcp->options );
-	dhcp->options = options;
-
-	/* Transmit next packet, or terminate session */
-	if ( dhcp->state < DHCPACK ) {
-		dhcp_send_request ( dhcp );
-	} else {
-		dhcp->register_options ( dhcp->netdev, dhcp->options );
-		dhcp_finished ( dhcp, 0 );
-	}
 	return 0;
 
  out_discard:
 	dhcp->register_options = register_options;
 	dhcp->timer.expired = dhcp_timer_expired;
 	dhcp->state = DHCPDISCOVER;
+	dhcp->start = currticks();
 
 	/* Instantiate child objects and attach to our interfaces */
 	if ( ( rc = xfer_open_socket ( &dhcp->xfer, SOCK_DGRAM,