[pxe] Skip PXE boot server discovery if directed to do so

Option 43.6 can direct us to skip PXE boot server discovery and just
perform a standard DHCP filename boot.

src/include/gpxe/dhcp.h

 /** Vendor encapsulated options */
 #define DHCP_VENDOR_ENCAP 43
 
+/** PXE boot server discovery control */
+#define DHCP_PXE_DISCOVERY_CONTROL DHCP_ENCAP_OPT ( DHCP_VENDOR_ENCAP, 6 )
+
+/** PXE boot server discovery control bits */
+enum dhcp_pxe_discovery_control {
+	/** Inhibit broadcast discovery */
+	PXEBS_NO_BROADCAST = 1,
+	/** Inhibit multicast discovery */
+	PXEBS_NO_MULTICAST = 2,
+	/** Accept only servers in DHCP_PXE_BOOT_SERVERS list */
+	PXEBS_NO_UNKNOWN_SERVERS = 4,
+	/** Skip discovery if filename present */
+	PXEBS_SKIP = 8,
+};
+
 /** PXE boot server multicast address */
 #define DHCP_PXE_BOOT_SERVER_MCAST DHCP_ENCAP_OPT ( DHCP_VENDOR_ENCAP, 7 )
 

src/usr/autoboot.c

  * @ret rc		Return status code
  */
 static int netboot ( struct net_device *netdev ) {
-	struct setting tmp_setting = { .name = NULL };
+	struct setting vendor_class_id_setting
+		= { .tag = DHCP_VENDOR_CLASS_ID };
+	struct setting pxe_discovery_control_setting
+		= { .tag = DHCP_PXE_DISCOVERY_CONTROL };
+	struct setting pxe_boot_menu_setting
+		= { .tag = DHCP_PXE_BOOT_MENU };
 	char buf[256];
 	struct in_addr next_server;
+	unsigned int pxe_discovery_control;
 	int rc;
 
 	/* Open device and display device status */
 	if ( rc != ENOENT )
 		return rc;
 
-	/* Try PXE menu boot, if we have PXE menu options */
-	tmp_setting.tag = DHCP_VENDOR_CLASS_ID;
-	fetch_string_setting ( NULL, &tmp_setting, buf, sizeof ( buf ) );
-	tmp_setting.tag = DHCP_PXE_BOOT_MENU;
+	/* Try PXE menu boot, if applicable */
+	fetch_string_setting ( NULL, &vendor_class_id_setting,
+			       buf, sizeof ( buf ) );
+	pxe_discovery_control =
+		fetch_uintz_setting ( NULL, &pxe_discovery_control_setting );
 	if ( ( strcmp ( buf, "PXEClient" ) == 0 ) &&
-	     setting_exists ( NULL, &tmp_setting ) ) {
+	     setting_exists ( NULL, &pxe_boot_menu_setting ) &&
+	     ( ! ( ( pxe_discovery_control & PXEBS_SKIP ) &&
+		   setting_exists ( NULL, &filename_setting ) ) ) ) {
 		printf ( "Booting from PXE menu\n" );
 		return pxe_menu_boot ( netdev );
 	}