First stab at DHCP option handling in a way that will allow us to have

multiple option sources (e.g. multiple DHCP replies, non-volatile
storage etc.)

src/include/gpxe/dhcp.h

@@ -0,0 +1,87 @@
+#ifndef _GPXE_DHCP_H
+#define _GPXE_DHCP_H
+
+/** @file
+ *
+ * Dynamic Host Configuration Protocol
+ *
+ */
+
+#include <stdint.h>
+#include <gpxe/list.h>
+
+/** Construct a tag value for an encapsulated option
+ *
+ * This tag value can be passed to Etherboot functions when searching
+ * for DHCP options in order to search for a tag within an
+ * encapsulated options block.
+ */
+#define DHCP_ENCAP_OPT( encapsulator, encapsulated ) \
+	( ( (encapsulator) << 8 ) | (encapsulated) )
+/** Extract encapsulating option block tag from encapsulated tag value */
+#define DHCP_ENCAPSULATOR( encap_opt ) ( (encap_opt) >> 8 )
+/** Extract encapsulated option tag from encapsulated tag value */
+#define DHCP_ENCAPSULATED( encap_opt ) ( (encap_opt) & 0xff )
+
+/**
+ * @defgroup dhcpopts DHCP option tags
+ * @{
+ */
+
+#define DHCP_PAD 0
+#define DHCP_END 255
+
+#define DHCP_EB_ENCAP 175
+
+#define DHCP_EB_PRIORITY DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 1 )
+
+/** @} */
+
+/**
+ * A DHCP option
+ *
+ * DHCP options consist of a mandatory tag, a length field that is
+ * mandatory for all options except @c DHCP_PAD and @c DHCP_END, and a
+ * payload.  
+ */
+struct dhcp_option {
+	/** Tag
+	 *
+	 * Must be a @c DHCP_XXX value.
+	 */
+	uint8_t tag;
+	/** Length
+	 *
+	 * This is the length of the data field (i.e. excluding the
+	 * tag and length fields).  For the two tags @c DHCP_PAD and
+	 * @c DHCP_END, the length field is implicitly zero and is
+	 * also missing, i.e. these DHCP options are only a single
+	 * byte in length.
+	 */
+	uint8_t len;
+	/** Option data
+	 *
+	 * Interpretation of the content is entirely dependent upon
+	 * the tag.  For fields containing a multi-byte integer, the
+	 * field is defined to be in network-endian order (unless you
+	 * are Intel and feel like violating the spec for fun).
+	 */
+	union {
+		uint8_t byte;
+		uint16_t word;
+		uint32_t dword;
+		uint8_t bytes[0];
+	} data;
+} __attribute__ (( packed ));
+
+/** A DHCP options block */
+struct dhcp_option_block {
+	/** List of option blocks */
+	struct list_head list;
+	/** Option block raw data */
+	void *data;
+	/** Option block length */
+	size_t len;
+};
+
+#endif /* _GPXE_DHCP_H */