diff --git a/lib/dns/db.c b/lib/dns/db.c
index 0f123d3671..deeeb41ac5 100644
--- a/lib/dns/db.c
+++ b/lib/dns/db.c
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: db.c,v 1.63 2000/11/17 01:06:37 bwelling Exp $ */
+/* $Id: db.c,v 1.64 2000/11/30 13:19:04 marka Exp $ */
 
 /***
  *** Imports
@@ -615,6 +615,8 @@ dns_db_addrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
 	REQUIRE(((db->attributes & DNS_DBATTR_CACHE) == 0 && version != NULL)||
 		((db->attributes & DNS_DBATTR_CACHE) != 0 &&
 		 version == NULL && (options & DNS_DBADD_MERGE) == 0));
+	REQUIRE((options & DNS_DBADD_EXACT) == 0 ||
+		(options & DNS_DBADD_MERGE) != 0);
 	REQUIRE(DNS_RDATASET_VALID(rdataset));
 	REQUIRE(dns_rdataset_isassociated(rdataset));
 	REQUIRE(rdataset->rdclass == db->rdclass);
diff --git a/lib/dns/include/dns/db.h b/lib/dns/include/dns/db.h
index 6dbe403fe0..7ee53679d5 100644
--- a/lib/dns/include/dns/db.h
+++ b/lib/dns/include/dns/db.h
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: db.h,v 1.60 2000/11/16 22:33:50 bwelling Exp $ */
+/* $Id: db.h,v 1.61 2000/11/30 13:19:08 marka Exp $ */
 
 #ifndef DNS_DB_H
 #define DNS_DB_H 1
@@ -192,6 +192,7 @@ struct dns_db {
  */
 #define DNS_DBADD_MERGE			0x01
 #define DNS_DBADD_FORCE			0x02
+#define DNS_DBADD_EXACT			0x04
 
 /*****
  ***** Methods
@@ -1014,6 +1015,8 @@ dns_db_addrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
  *	without regard to trust levels.  If not forcing the data, then the
  *	rdataset will only be added if its trust level is >= the trust level of
  *	any existing rdataset.  Forcing is only meaningful for cache databases.
+ *	If DNS_DBADD_EXACT is set then there must be no rdata in common between
+ *	the old and new rdata sets.
  *
  *	The 'now' field is ignored if 'db' is a zone database.  If 'db' is
  *	a cache database, then the added rdataset will expire no later than
diff --git a/lib/dns/include/dns/rdataslab.h b/lib/dns/include/dns/rdataslab.h
index 2a5683d54e..77d7c077bd 100644
--- a/lib/dns/include/dns/rdataslab.h
+++ b/lib/dns/include/dns/rdataslab.h
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: rdataslab.h,v 1.16 2000/10/18 23:53:30 marka Exp $ */
+/* $Id: rdataslab.h,v 1.17 2000/11/30 13:19:09 marka Exp $ */
 
 #ifndef DNS_RDATASLAB_H
 #define DNS_RDATASLAB_H 1
@@ -55,6 +55,9 @@
 
 ISC_LANG_BEGINDECLS
 
+#define DNS_RDATASLAB_FORCE 0x1
+#define DNS_RDATASLAB_EXACT 0x2
+
 /***
  *** Functions
  ***/
@@ -96,11 +99,12 @@ isc_result_t
 dns_rdataslab_merge(unsigned char *oslab, unsigned char *nslab,
 		    unsigned int reservelen, isc_mem_t *mctx,
 		    dns_rdataclass_t rdclass, dns_rdatatype_t type,
-		    isc_boolean_t force, unsigned char **tslabp);
+		    unsigned int flags, unsigned char **tslabp);
 /*
  * Merge 'oslab' and 'nslab'.
  *
  * XXX
+ * DNS_RDATASLAB_FORCE and DNS_RDATASLAB_EXACT are mutually exclusive.
  */
 
 isc_result_t
diff --git a/lib/dns/journal.c b/lib/dns/journal.c
index 63ae5ecac4..b2a1937d61 100644
--- a/lib/dns/journal.c
+++ b/lib/dns/journal.c
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: journal.c,v 1.63 2000/11/14 23:29:50 bwelling Exp $ */
+/* $Id: journal.c,v 1.64 2000/11/30 13:19:05 marka Exp $ */
 
 #include <config.h>
 
@@ -397,7 +397,8 @@ dns_diff_apply(dns_diff_t *diff, dns_db_t *db, dns_dbversion_t *ver)
 			if (op == DNS_DIFFOP_ADD) {
 				result = dns_db_addrdataset(db, node, ver,
 							    0, &rds,
-							    DNS_DBADD_MERGE,
+							    DNS_DBADD_MERGE|
+							    DNS_DBADD_EXACT,
 							    NULL);
 			} else if (op == DNS_DIFFOP_DEL) {
 				result = dns_db_subtractrdataset(db, node, ver,
diff --git a/lib/dns/rbtdb.c b/lib/dns/rbtdb.c
index c23afa00e6..a94371913e 100644
--- a/lib/dns/rbtdb.c
+++ b/lib/dns/rbtdb.c
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: rbtdb.c,v 1.134 2000/11/22 01:56:02 halley Exp $ */
+/* $Id: rbtdb.c,v 1.135 2000/11/30 13:19:06 marka Exp $ */
 
 /*
  * Principal Author: Bob Halley
@@ -3108,7 +3108,6 @@ add(dns_rbtdb_t *rbtdb, dns_rbtnode_t *rbtnode, rbtdb_version_t *rbtversion,
 	rdatasetheader_t *topheader, *topheader_prev, *header;
 	unsigned char *merged;
 	isc_result_t result;
-	isc_boolean_t force = ISC_FALSE;
 	isc_boolean_t header_nx;
 	isc_boolean_t newheader_nx;
 	isc_boolean_t merge;
@@ -3274,10 +3273,13 @@ add(dns_rbtdb_t *rbtdb, dns_rbtnode_t *rbtnode, rbtdb_version_t *rbtversion,
 		 * that is the union of 'newheader' and 'header'.
 		 */
 		if (merge) {
+			unsigned int flags = 0;
 			INSIST(rbtversion->serial >= header->serial);
 			merged = NULL;
+			if ((options & DNS_DBADD_EXACT) != 0)
+				flags |= DNS_RDATASLAB_EXACT;
 			if (newheader->ttl != header->ttl)
-				force = ISC_TRUE;
+				flags |= DNS_RDATASLAB_FORCE;
 			result = dns_rdataslab_merge(
 					     (unsigned char *)header,
 					     (unsigned char *)newheader,
@@ -3285,8 +3287,7 @@ add(dns_rbtdb_t *rbtdb, dns_rbtnode_t *rbtnode, rbtdb_version_t *rbtversion,
 					     rbtdb->common.mctx,
 					     rbtdb->common.rdclass,
 					     (dns_rdatatype_t)header->type,
-					     force,
-					     &merged);
+					     flags, &merged);
 			if (result == ISC_R_SUCCESS) {
 				/*
 				 * If 'header' has the same serial number as