Split DHCP packet creation into two parts: creating the basic packet

structure, and populating it with options.  This should allow us to
use the same basic options list for both DHCPDISCOVER and DHCPREQUEST,
plus making it much easier to set the non-constant parameters
(e.g. requested IP address) in request packets.

src/include/gpxe/dhcp.h

@@ -11,88 +11,6 @@
 #include <gpxe/list.h>
 #include <gpxe/in.h>
 
-/**
- * A DHCP packet
- *
- */
-struct dhcp_packet {
-	/** Operation
-	 *
-	 * This must be either @c BOOTP_REQUEST or @c BOOTP_REPLY.
-	 */
-	uint8_t op;
-	/** Hardware address type
-	 *
-	 * This is an ARPHRD_XXX constant.  Note that ARPHRD_XXX
-	 * constants are nominally 16 bits wide; this could be
-	 * considered to be a bug in the BOOTP/DHCP specification.
-	 */
-	uint8_t htype;
-	/** Hardware address length */
-	uint8_t hlen;
-	/** Number of hops from server */
-	uint8_t hops;
-	/** Transaction ID */
-	uint32_t xid;
-	/** Seconds since start of acquisition */
-	uint16_t secs;
-	/** Flags */
-	uint16_t flags;
-	/** "Client" IP address
-	 *
-	 * This is filled in if the client already has an IP address
-	 * assigned and can respond to ARP requests.
-	 */
-	struct in_addr ciaddr;
-	/** "Your" IP address
-	 *
-	 * This is the IP address assigned by the server to the client.
-	 */
-	struct in_addr yiaddr;
-	/** "Server" IP address
-	 *
-	 * This is the IP address of the next server to be used in the
-	 * boot process.
-	 */
-	struct in_addr siaddr;
-	/** "Gateway" IP address
-	 *
-	 * This is the IP address of the DHCP relay agent, if any.
-	 */
-	struct in_addr giaddr;
-	/** Client hardware address */
-	uint8_t chaddr[16];
-	/** Server host name (null terminated)
-	 *
-	 * This field may be overridden and contain DHCP options
-	 */
-	uint8_t sname[64];
-	/** Boot file name (null terminated)
-	 *
-	 * This field may be overridden and contain DHCP options
-	 */
-	uint8_t file[128];
-	/** DHCP magic cookie
-	 *
-	 * Must have the value @c DHCP_MAGIC_COOKIE.
-	 */
-	uint32_t magic;
-	/** DHCP options
-	 *
-	 * Variable length; extends to the end of the packet.
-	 */
-	uint8_t options[0];
-};
-
-/** Opcode for a request from client to server */
-#define BOOTP_REQUEST 1
-
-/** Opcode for a reply from server to client */
-#define BOOTP_REPLY 2
-
-/** DHCP magic cookie */
-#define DHCP_MAGIC_COOKIE 0x63825363UL
-
 /** Construct a tag value for an encapsulated option
  *
  * This tag value can be passed to Etherboot functions when searching
@@ -276,6 +194,123 @@ struct dhcp_option_block {
 	signed int priority;
 };
 
+/**
+ * A DHCP header
+ *
+ */
+struct dhcphdr {
+	/** Operation
+	 *
+	 * This must be either @c BOOTP_REQUEST or @c BOOTP_REPLY.
+	 */
+	uint8_t op;
+	/** Hardware address type
+	 *
+	 * This is an ARPHRD_XXX constant.  Note that ARPHRD_XXX
+	 * constants are nominally 16 bits wide; this could be
+	 * considered to be a bug in the BOOTP/DHCP specification.
+	 */
+	uint8_t htype;
+	/** Hardware address length */
+	uint8_t hlen;
+	/** Number of hops from server */
+	uint8_t hops;
+	/** Transaction ID */
+	uint32_t xid;
+	/** Seconds since start of acquisition */
+	uint16_t secs;
+	/** Flags */
+	uint16_t flags;
+	/** "Client" IP address
+	 *
+	 * This is filled in if the client already has an IP address
+	 * assigned and can respond to ARP requests.
+	 */
+	struct in_addr ciaddr;
+	/** "Your" IP address
+	 *
+	 * This is the IP address assigned by the server to the client.
+	 */
+	struct in_addr yiaddr;
+	/** "Server" IP address
+	 *
+	 * This is the IP address of the next server to be used in the
+	 * boot process.
+	 */
+	struct in_addr siaddr;
+	/** "Gateway" IP address
+	 *
+	 * This is the IP address of the DHCP relay agent, if any.
+	 */
+	struct in_addr giaddr;
+	/** Client hardware address */
+	uint8_t chaddr[16];
+	/** Server host name (null terminated)
+	 *
+	 * This field may be overridden and contain DHCP options
+	 */
+	uint8_t sname[64];
+	/** Boot file name (null terminated)
+	 *
+	 * This field may be overridden and contain DHCP options
+	 */
+	uint8_t file[128];
+	/** DHCP magic cookie
+	 *
+	 * Must have the value @c DHCP_MAGIC_COOKIE.
+	 */
+	uint32_t magic;
+	/** DHCP options
+	 *
+	 * Variable length; extends to the end of the packet.
+	 */
+	uint8_t options[0];
+};
+
+/** Opcode for a request from client to server */
+#define BOOTP_REQUEST 1
+
+/** Opcode for a reply from server to client */
+#define BOOTP_REPLY 2
+
+/** DHCP magic cookie */
+#define DHCP_MAGIC_COOKIE 0x63825363UL
+
+/** DHCP packet option block fill order
+ *
+ * This is the order in which option blocks are filled when
+ * reassembling a DHCP packet.  We fill the smallest field ("sname")
+ * first, to maximise the chances of being able to fit large options
+ * within fields which are large enough to contain them.
+ */
+enum dhcp_packet_option_block_fill_order {
+	OPTS_SNAME = 0,
+	OPTS_FILE,
+	OPTS_MAIN,
+	NUM_OPT_BLOCKS
+};
+
+/**
+ * A DHCP packet
+ *
+ */
+struct dhcp_packet {
+	/** The DHCP packet contents */
+	struct dhcphdr *dhcphdr;
+	/** Maximum length of the DHCP packet buffer */
+	size_t max_len;
+	/** Used length of the DHCP packet buffer */
+	size_t len;
+	/** DHCP option blocks within a DHCP packet
+	 *
+	 * A DHCP packet contains three fields which can be used to
+	 * contain options: the actual "options" field plus the "file"
+	 * and "sname" fields (which can be overloaded to contain
+	 * options).
+	 */
+	struct dhcp_option_block options[NUM_OPT_BLOCKS];
+};
+
 /** A DHCP session */
 struct dhcp_session {
 	/** Network device being configured */