diff --git a/lib/dns/include/dns/rdataset.h b/lib/dns/include/dns/rdataset.h
index 581c52937d..a7b2ed45f2 100644
--- a/lib/dns/include/dns/rdataset.h
+++ b/lib/dns/include/dns/rdataset.h
@@ -207,7 +207,8 @@ dns_result_t
 dns_rdataset_towire(dns_rdataset_t *rdataset,
 		    dns_name_t *owner_name,
 		    dns_compress_t *cctx,
-		    isc_buffer_t *target);
+		    isc_buffer_t *target,
+		    unsigned int *countp);
 /*
  * Convert 'rdataset' to wire format, compressing names as specified
  * in cctx, and storing the result in 'target'.
@@ -215,16 +216,23 @@ dns_rdataset_towire(dns_rdataset_t *rdataset,
  * Notes:
  *	The rdata cursor position will be changed.
  *
+ *	The number of RRs added to target will be added to *countp.
+ *
  * Requires:
  *	'rdataset' is a valid rdataset.
  *
  *	'rdataset' is not empty.
  *
+ *	'countp' is a valid pointer.
+ *
  * Ensures:
  *	On a return of DNS_R_SUCCESS, 'target' contains a wire format
  *	for the data contained in 'rdataset'.  Any error return leaves
  *	the buffer in an undefined state.
  *
+ *	*countp has been incremented by the number of RRs added to
+ *	target.
+ *
  * Returns:
  *	DNS_R_SUCCESS		- all ok
  *	DNS_R_NOSPACE		- 'target' doesn't have enough room
diff --git a/lib/dns/rdataset.c b/lib/dns/rdataset.c
index a1791d8a1a..ecf056b4ba 100644
--- a/lib/dns/rdataset.c
+++ b/lib/dns/rdataset.c
@@ -265,11 +265,13 @@ dns_result_t
 dns_rdataset_towire(dns_rdataset_t *rdataset,
 		    dns_name_t *owner_name,
 		    dns_compress_t *cctx,
-		    isc_buffer_t *target)
+		    isc_buffer_t *target,
+		    unsigned int *countp)
 {
 	dns_rdata_t rdata;
 	isc_region_t r;
 	dns_result_t result;
+	unsigned int count;
 
 	/*
 	 * Convert 'rdataset' to wire format, compressing names as specified
@@ -279,7 +281,9 @@ dns_rdataset_towire(dns_rdataset_t *rdataset,
 	REQUIRE(DNS_RDATASET_VALID(rdataset));
 	result = dns_rdataset_first(rdataset);
 	REQUIRE(result == DNS_R_SUCCESS);
+	REQUIRE(countp != NULL);
 
+	count = 0;
 	do {
 		/*
 		 * copy out the name, type, class, ttl.
@@ -310,11 +314,15 @@ dns_rdataset_towire(dns_rdataset_t *rdataset,
 		if (result != DNS_R_SUCCESS)
 			return (result);
 
+		count++;
+
 		result = dns_rdataset_next(rdataset);
 	} while (result == DNS_R_SUCCESS);
 
 	if (result != DNS_R_NOMORE)
 		return (result);
 
+	*countp += count;
+
 	return (DNS_R_SUCCESS);
 }