diff --git a/src/include/gpxe/dhcp.h b/src/include/gpxe/dhcp.h
index cc9b94eb..e34d5de5 100644
--- a/src/include/gpxe/dhcp.h
+++ b/src/include/gpxe/dhcp.h
@@ -23,6 +23,8 @@
 /** Extract encapsulated option tag from encapsulated tag value */
 #define DHCP_ENCAPSULATED( encap_opt ) ( (encap_opt) & 0xff )
 
+#define DHCP_IS_ENCAP_OPT( opt ) DHCP_ENCAPSULATOR( opt )
+
 /**
  * @defgroup dhcpopts DHCP option tags
  * @{
@@ -74,6 +76,17 @@ struct dhcp_option {
 	} data;
 } __attribute__ (( packed ));
 
+/**
+ * Length of a DHCP option header
+ *
+ * The header is the portion excluding the data, i.e. the tag and the
+ * length.
+ */
+#define DHCP_OPTION_HEADER_LEN ( offsetof ( struct dhcp_option, data ) )
+
+/** Maximum length for a single DHCP option */
+#define DHCP_MAX_LEN 0xff
+
 /** A DHCP options block */
 struct dhcp_option_block {
 	/** List of option blocks */
@@ -82,17 +95,35 @@ struct dhcp_option_block {
 	void *data;
 	/** Option block length */
 	size_t len;
+	/** Option block maximum length */
+	size_t max_len;
+	/** Block priority
+	 *
+	 * This is determined at the time of the call to
+	 * register_options() by searching for the @c DHCP_EB_PRIORITY
+	 * option.
+	 */
+	signed int priority;
 };
 
 extern unsigned long dhcp_num_option ( struct dhcp_option *option );
-extern struct dhcp_option * find_dhcp_option ( unsigned int tag,
-					   struct dhcp_option_block *options );
+extern struct dhcp_option *
+find_dhcp_option ( struct dhcp_option_block *options, unsigned int tag );
+extern struct dhcp_option * find_global_dhcp_option ( unsigned int tag );
+extern void register_dhcp_options ( struct dhcp_option_block *options );
+extern void unregister_dhcp_options ( struct dhcp_option_block *options );
+extern struct dhcp_option_block * alloc_dhcp_options ( size_t max_len );
+extern void free_dhcp_options ( struct dhcp_option_block *options );
+extern struct dhcp_option *
+set_dhcp_option ( struct dhcp_option_block *options, unsigned int tag,
+		  const void *data, size_t len );
+
 
 /**
  * Find DHCP numerical option, and return its value
  *
- * @v tag		DHCP option tag to search for
  * @v options		DHCP options block
+ * @v tag		DHCP option tag to search for
  * @ret value		Numerical value of the option, or 0 if not found
  *
  * This function exists merely as a notational shorthand for a call to
@@ -103,8 +134,37 @@ extern struct dhcp_option * find_dhcp_option ( unsigned int tag,
  * check that find_dhcp_option() returns a non-NULL value.
  */
 static inline unsigned long
-find_dhcp_num_option ( unsigned int tag, struct dhcp_option_block *options ) {
-	return dhcp_num_option ( find_dhcp_option ( tag, options ) );
+find_dhcp_num_option ( struct dhcp_option_block *options, unsigned int tag ) {
+	return dhcp_num_option ( find_dhcp_option ( options, tag ) );
+}
+
+/**
+ * Find DHCP numerical option, and return its value
+ *
+ * @v tag		DHCP option tag to search for
+ * @ret value		Numerical value of the option, or 0 if not found
+ *
+ * This function exists merely as a notational shorthand for a call to
+ * find_global_dhcp_option() followed by a call to dhcp_num_option().
+ * It is not possible to distinguish between the cases "option not
+ * found" and "option has a value of zero" using this function; if
+ * this matters to you then issue the two constituent calls directly
+ * and check that find_global_dhcp_option() returns a non-NULL value.
+ */
+static inline unsigned long
+find_global_dhcp_num_option ( unsigned int tag ) {
+	return dhcp_num_option ( find_global_dhcp_option ( tag ) );
+}
+
+/**
+ * Delete DHCP option
+ *
+ * @v options		DHCP options block
+ * @v tag		DHCP option tag
+ */
+static inline void delete_dhcp_option ( struct dhcp_option_block *options,
+					unsigned int tag ) {
+	set_dhcp_option ( options, tag, NULL, 0 );
 }
 
 #endif /* _GPXE_DHCP_H */
diff --git a/src/net/dhcpopts.c b/src/net/dhcpopts.c
index faa3a50a..6b06b9a2 100644
--- a/src/net/dhcpopts.c
+++ b/src/net/dhcpopts.c
@@ -19,8 +19,10 @@
 #include <stdint.h>
 #include <byteswap.h>
 #include <errno.h>
+#include <string.h>
 #include <malloc.h>
 #include <assert.h>
+#include <vsprintf.h>
 #include <gpxe/list.h>
 #include <gpxe/dhcp.h>
 
@@ -33,6 +35,26 @@
 /** List of registered DHCP option blocks */
 static LIST_HEAD ( option_blocks );
 
+/**
+ * Obtain printable version of a DHCP option tag
+ *
+ * @v tag		DHCP option tag
+ * @ret name		String representation of the tag
+ *
+ */
+static inline char * dhcp_tag_name ( unsigned int tag ) {
+	static char name[8];
+
+	if ( DHCP_IS_ENCAP_OPT ( tag ) ) {
+		snprintf ( name, sizeof ( name ), "%d.%d",
+			   DHCP_ENCAPSULATOR ( tag ),
+			   DHCP_ENCAPSULATED ( tag ) );
+	} else {
+		snprintf ( name, sizeof ( name ), "%d", tag );
+	}
+	return name;
+}
+
 /**
  * Obtain value of a numerical DHCP option
  *
@@ -64,44 +86,52 @@ unsigned long dhcp_num_option ( struct dhcp_option *option ) {
 }
 
 /**
- * Calculate length of a DHCP option
+ * Calculate length of a normal DHCP option
+ *
+ * @v option		DHCP option
+ * @ret len		Length (including tag and length field)
+ *
+ * @c option may not be a @c DHCP_PAD or @c DHCP_END option.
+ */
+static inline unsigned int dhcp_option_len ( struct dhcp_option *option ) {
+	assert ( option->tag != DHCP_PAD );
+	assert ( option->tag != DHCP_END );
+	return ( option->len + DHCP_OPTION_HEADER_LEN );
+}
+
+/**
+ * Calculate length of any DHCP option
  *
  * @v option		DHCP option
  * @ret len		Length (including tag and length field)
  */
-static inline unsigned int dhcp_option_len ( struct dhcp_option *option ) {
+static inline unsigned int dhcp_any_option_len ( struct dhcp_option *option ) {
 	if ( ( option->tag == DHCP_END ) || ( option->tag == DHCP_PAD ) ) {
 		return 1;
 	} else {
-		return ( option->len + 2 );
+		return dhcp_option_len ( option );
 	}
 }
 
 /**
  * Find DHCP option within DHCP options block, and its encapsulator (if any)
  *
- * @v tag		DHCP option tag to search for
  * @v options		DHCP options block
- * @ret encapsulator	Encapsulating option (if applicable, may be NULL)
+ * @v tag		DHCP option tag to search for
  * @ret option		DHCP option, or NULL if not found
  *
  * Searches for the DHCP option matching the specified tag within the
  * block of data.  Encapsulated options may be searched for by using
- * DHCP_ENCAP_OPT() to construct the tag value.  If the option is an
- * encapsulated option, and @c encapsulator is non-NULL, it will be
- * filled in with a pointer to the encapsulating option, if present.
- * Note that the encapsulating option may be present even if the
- * encapsulated option is absent, in which case @c encapsulator will
- * be set but the function will return NULL.
+ * DHCP_ENCAP_OPT() to construct the tag value.
  *
  * This routine is designed to be paranoid.  It does not assume that
  * the option data is well-formatted, and so must guard against flaws
  * such as options missing a @c DHCP_END terminator, or options whose
  * length would take them beyond the end of the data block.
  */
-static struct dhcp_option *
-find_dhcp_option_encap ( unsigned int tag, struct dhcp_option_block *options,
-			 struct dhcp_option **encapsulator ) {
+struct dhcp_option * find_dhcp_option ( struct dhcp_option_block *options,
+					unsigned int tag ) {
+	unsigned int original_tag __attribute__ (( unused )) = tag;
 	struct dhcp_option *option = options->data;
 	ssize_t remaining = options->len;
 	unsigned int option_len;
@@ -111,22 +141,24 @@ find_dhcp_option_encap ( unsigned int tag, struct dhcp_option_block *options,
 		 * if the length is malformed (i.e. takes us beyond
 		 * the end of the data block).
 		 */
-		option_len = dhcp_option_len ( option );
+		option_len = dhcp_any_option_len ( option );
 		remaining -= option_len;
 		if ( remaining < 0 )
 			break;
 		/* Check for matching tag */
-		if ( option->tag == tag )
+		if ( option->tag == tag ) {
+			DBG ( "Found DHCP option %s (length %d)\n",
+			      dhcp_tag_name ( original_tag ), option->len );
 			return option;
+		}
 		/* Check for explicit end marker */
 		if ( option->tag == DHCP_END )
 			break;
 		/* Check for start of matching encapsulation block */
-		if ( DHCP_ENCAPSULATOR ( tag ) &&
+		if ( DHCP_IS_ENCAP_OPT ( tag ) &&
 		     ( option->tag == DHCP_ENCAPSULATOR ( tag ) ) ) {
 			/* Continue search within encapsulated option block */
-			if ( encapsulator )
-				*encapsulator = option;
+			tag = DHCP_ENCAPSULATED ( tag );
 			remaining = option->len;
 			option = ( void * ) &option->data;
 			continue;
@@ -136,46 +168,6 @@ find_dhcp_option_encap ( unsigned int tag, struct dhcp_option_block *options,
 	return NULL;
 }
 
-/**
- * Find DHCP option within DHCP options block
- *
- * @v tag		DHCP option tag to search for
- * @v options		DHCP options block
- * @ret option		DHCP option, or NULL if not found
- *
- * Searches for the DHCP option matching the specified tag within the
- * block of data.  Encapsulated options may be searched for by using
- * DHCP_ENCAP_OPT() to construct the tag value.
- */
-struct dhcp_option * find_dhcp_option ( unsigned int tag,
-					struct dhcp_option_block *options ) {
-	return find_dhcp_option_encap ( tag, options, NULL );
-}
-
-/**
- * Find length of used portion of DHCP options block
- *
- * @v options		DHCP options block
- * @ret len		Length of used portion of data block
- *
- * This searches for the @c DHCP_END marker within the options block.
- * If found, the length of the used portion of the block (i.e. the
- * portion containing everything @b before the @c DHCP_END marker, but
- * excluding the @c DHCP_END marker itself) is returned.
- *
- * If no @c DHCP_END marker is present, the length of the whole
- * options block is returned.
- */
-size_t dhcp_option_block_len ( struct dhcp_option_block *options ) {
-	void *dhcpend;
-
-	if ( ( dhcpend = find_dhcp_option ( DHCP_END, options ) ) ) {
-		return ( dhcpend - options->data );
-	} else {
-		return options->len;
-	}
-}
-
 /**
  * Find DHCP option within all registered DHCP options blocks
  *
@@ -197,7 +189,7 @@ struct dhcp_option * find_global_dhcp_option ( unsigned int tag ) {
 	struct dhcp_option *option;
 
 	list_for_each_entry ( options, &option_blocks, list ) {
-		if ( ( option = find_dhcp_option ( tag, options ) ) )
+		if ( ( option = find_dhcp_option ( options, tag ) ) )
 			return option;
 	}
 	return NULL;
@@ -211,21 +203,19 @@ struct dhcp_option * find_global_dhcp_option ( unsigned int tag ) {
  * Register a block of DHCP options.
  */
 void register_dhcp_options ( struct dhcp_option_block *options ) {
-	struct dhcp_option_block *existing_options;
-	signed int existing_priority;
-	signed int priority;
+	struct dhcp_option_block *existing;
 
 	/* Determine priority of new block */
-	priority = find_dhcp_num_option ( DHCP_EB_PRIORITY, options );
+	options->priority = find_dhcp_num_option ( options, DHCP_EB_PRIORITY );
+	DBG ( "Registering DHCP options block with priority %d\n",
+	      options->priority );
 
 	/* Insert after any existing blocks which have a higher priority */
-	list_for_each_entry ( existing_options, &option_blocks, list ) {
-		existing_priority = find_dhcp_num_option ( DHCP_EB_PRIORITY,
-							   existing_options );
-		if ( priority > existing_priority )
+	list_for_each_entry ( existing, &option_blocks, list ) {
+		if ( options->priority > existing->priority )
 			break;
 	}
-	list_add_tail ( &options->list, &existing_options->list );
+	list_add_tail ( &options->list, &existing->list );
 }
 
 /**
@@ -240,23 +230,24 @@ void unregister_dhcp_options ( struct dhcp_option_block *options ) {
 /**
  * Allocate space for a block of DHCP options
  *
- * @v len		Maximum length of option block
- * @ret options		Option block, or NULL
+ * @v max_len		Maximum length of option block
+ * @ret options		DHCP option block, or NULL
  *
  * Creates a new DHCP option block and populates it with an empty
  * options list.  This call does not register the options block.
  */
-struct dhcp_option_block * alloc_dhcp_options ( size_t len ) {
+struct dhcp_option_block * alloc_dhcp_options ( size_t max_len ) {
 	struct dhcp_option_block *options;
 	struct dhcp_option *option;
 
-	options = malloc ( sizeof ( *options ) + len );
+	options = malloc ( sizeof ( *options ) + max_len );
 	if ( options ) {
 		options->data = ( ( void * ) options + sizeof ( *options ) );
-		options->len = len;
-		if ( len ) {
+		options->max_len = max_len;
+		if ( max_len ) {
 			option = options->data;
 			option->tag = DHCP_END;
+			options->len = 1;
 		}
 	}
 	return options;
@@ -265,8 +256,124 @@ struct dhcp_option_block * alloc_dhcp_options ( size_t len ) {
 /**
  * Free DHCP options block
  *
- * @v options		Option block
+ * @v options		DHCP option block
  */
 void free_dhcp_options ( struct dhcp_option_block *options ) {
 	free ( options );
 }
+
+/**
+ * Resize a DHCP option
+ *
+ * @v options		DHCP option block
+ * @v option		DHCP option to resize
+ * @v encapsulator	Encapsulating option (or NULL)
+ * @v old_len		Old length (including header)
+ * @v new_len		New length (including header)
+ * @ret rc		Return status code
+ */
+static int resize_dhcp_option ( struct dhcp_option_block *options,
+				struct dhcp_option *option,
+				struct dhcp_option *encapsulator,
+				size_t old_len, size_t new_len ) {
+	void *source = ( ( ( void * ) option ) + old_len );
+	void *dest = ( ( ( void * ) option ) + new_len );
+	void *end = ( options->data + options->max_len );
+	ssize_t delta = ( new_len - old_len );
+	size_t new_options_len;
+	size_t new_encapsulator_len;
+
+	/* Check for sufficient space, and update length fields */
+	if ( new_len > DHCP_MAX_LEN )
+		return -ENOMEM;
+	new_options_len = ( options->len + delta );
+	if ( new_options_len > options->max_len )
+		return -ENOMEM;
+	if ( encapsulator ) {
+		new_encapsulator_len = ( encapsulator->len + delta );
+		if ( new_encapsulator_len > DHCP_MAX_LEN )
+			return -ENOMEM;
+		encapsulator->len = new_encapsulator_len;
+	}
+	options->len = new_options_len;
+
+	/* Move remainder of option data */
+	memmove ( dest, source, ( end - dest ) );
+
+	return 0;
+}
+
+/**
+ * Set value of DHCP option
+ *
+ * @v options		DHCP option block
+ * @v tag		DHCP option tag
+ * @v data		New value for DHCP option
+ * @v len		Length of value, in bytes
+ * @ret option		DHCP option, or NULL
+ *
+ * Sets the value of a DHCP option within the options block.  The
+ * option may or may not already exist.  Encapsulators will be created
+ * (and deleted) as necessary.
+ *
+ * This call may fail due to insufficient space in the options block.
+ * If it does fail, and the option existed previously, the option will
+ * be left with its original value.
+ */
+struct dhcp_option * set_dhcp_option ( struct dhcp_option_block *options,
+				       unsigned int tag,
+				       const void *data, size_t len ) {
+	static const uint8_t empty_encapsulator[] = { DHCP_END };
+	struct dhcp_option *option;
+	void *insertion_point = options->data;
+	struct dhcp_option *encapsulator = NULL;
+	unsigned int encap_tag = DHCP_ENCAPSULATOR ( tag );
+	size_t old_len = 0;
+	size_t new_len = ( len ? ( len + DHCP_OPTION_HEADER_LEN ) : 0 );
+
+	/* Find old instance of this option, if any */
+	option = find_dhcp_option ( options, tag );
+	if ( option ) {
+		old_len = dhcp_option_len ( option );
+		DBG ( "Resizing DHCP option %s from length %d to %d\n",
+		      dhcp_tag_name ( tag ), option->len, len );
+	} else {
+		old_len = 0;
+		DBG ( "Creating DHCP option %s (length %d)\n",
+		      dhcp_tag_name ( tag ), new_len );
+	}
+	
+	/* Ensure that encapsulator exists, if required */
+	if ( DHCP_IS_ENCAP_OPT ( tag ) ) {
+		encapsulator = find_dhcp_option ( options, encap_tag );
+		if ( ! encapsulator )
+			encapsulator = set_dhcp_option ( options, encap_tag,
+							 empty_encapsulator,
+						sizeof ( empty_encapsulator) );
+		if ( ! encapsulator )
+			return NULL;
+		insertion_point = &encapsulator->data;
+	}
+
+	/* Create new option if necessary */
+	if ( ! option )
+		option = insertion_point;
+	
+	/* Resize option to fit new data */
+	if ( resize_dhcp_option ( options, option, encapsulator,
+				  old_len, new_len ) != 0 )
+		return NULL;
+
+	/* Copy new data into option, if applicable */
+	if ( len ) {
+		option->tag = tag;
+		option->len = len;
+		memcpy ( &option->data, data, len );
+	}
+
+	/* Delete encapsulator if there's nothing else left in it */
+	if ( encapsulator && ( encapsulator->len <= 1 ) )
+		set_dhcp_option ( options, encap_tag, NULL, 0 );
+
+	return option;
+}