A DHCP session holds a persistent reference to a network device

src/include/gpxe/dhcp.h

 #include <gpxe/udp.h>
 #include <gpxe/async.h>
 #include <gpxe/retry.h>
+#include <gpxe/hotplug.h>
 
 /** BOOTP/DHCP server port */
 #define BOOTPS_PORT 67
 
 	/** Network device being configured */
 	struct net_device *netdev;
+	/** Persistent reference to network device */
+	struct reference netdev_ref;
 
 	/** Options obtained from server */
 	struct dhcp_option_block *options;

src/net/udp/dhcp.c

  * @v rc		Return status code
  */
 static void dhcp_done ( struct dhcp_session *dhcp, int rc ) {
+
 	/* Free up options if we failed */
 	if ( rc != 0 ) {
 		if ( dhcp->options ) {
 		}
 	}
 
+	/* Stop retry timer */
+	stop_timer ( &dhcp->timer );
+
 	/* Close UDP connection */
 	udp_close ( &dhcp->udp );
 
+	/* Release reference on net device */
+	ref_del ( &dhcp->netdev_ref );
+
 	/* Mark async operation as complete */
 	async_done ( &dhcp->aop, rc );
 }
 	.newdata	= dhcp_newdata,
 };
 
+/**
+ * Forget reference to net_device
+ *
+ * @v ref		Persistent reference
+ */
+static void dhcp_forget_netdev ( struct reference *ref ) {
+	struct dhcp_session *dhcp
+		= container_of ( ref, struct dhcp_session, netdev_ref );
+
+	/* Kill DHCP session immediately */
+	dhcp_done ( dhcp, -ENETUNREACH );
+}
+
 /**
  * Initiate DHCP on a network interface
  *
 		goto out;
 	}
 
+	/* Add persistent reference to net device */
+	dhcp->netdev_ref.forget = dhcp_forget_netdev;
+	ref_add ( &dhcp->netdev_ref, &dhcp->netdev->references );
+
 	/* Proof of concept: just send a single DHCPDISCOVER */
 	dhcp_send_request ( dhcp );