diff --git a/bin/named/config.c b/bin/named/config.c
index fbf29cf7a8..85bad35716 100644
--- a/bin/named/config.c
+++ b/bin/named/config.c
@@ -79,6 +79,7 @@ options {\n\
 	listen-on-v6 {any;};\n\
 #	lock-file \"" NAMED_LOCALSTATEDIR "/run/named/named.lock\";\n\
 	match-mapped-addresses no;\n\
+	max-ixfr-ratio 100%;\n\
 	max-rsa-exponent-size 0; /* no limit */\n\
 	max-udp-size 4096;\n\
 	memstatistics-file \"named.memstats\";\n\
diff --git a/bin/named/zoneconf.c b/bin/named/zoneconf.c
index 30ec0c430f..7d2c1fb208 100644
--- a/bin/named/zoneconf.c
+++ b/bin/named/zoneconf.c
@@ -1400,6 +1400,15 @@ named_zone_configure(const cfg_obj_t *config, const cfg_obj_t *vconfig,
 					   ixfrdiff);
 		}
 
+		obj = NULL;
+		result = named_config_get(maps, "max-ixfr-ratio", &obj);
+		INSIST(result == ISC_R_SUCCESS && obj != NULL);
+		if (cfg_obj_isstring(obj)) {
+			dns_zone_setixfrratio(zone, 0);
+		} else {
+			dns_zone_setixfrratio(zone, cfg_obj_aspercentage(obj));
+		}
+
 		obj = NULL;
 		result = named_config_get(maps, "request-expire", &obj);
 		INSIST(result == ISC_R_SUCCESS);
diff --git a/bin/tests/system/checkconf/bad-maxratio1.conf b/bin/tests/system/checkconf/bad-maxratio1.conf
new file mode 100644
index 0000000000..f0d06be669
--- /dev/null
+++ b/bin/tests/system/checkconf/bad-maxratio1.conf
@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+zone example {
+	type master;
+	masterfile-format map;
+	file "example.db";
+	max-ixfr-ratio 0.9;
+};
diff --git a/bin/tests/system/checkconf/bad-maxratio2.conf b/bin/tests/system/checkconf/bad-maxratio2.conf
new file mode 100644
index 0000000000..902c334423
--- /dev/null
+++ b/bin/tests/system/checkconf/bad-maxratio2.conf
@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+zone example {
+	type master;
+	masterfile-format map;
+	file "example.db";
+	max-ixfr-ratio 0%;
+};
diff --git a/bin/tests/system/checkconf/good-maxratio1.conf b/bin/tests/system/checkconf/good-maxratio1.conf
new file mode 100644
index 0000000000..80ff113f21
--- /dev/null
+++ b/bin/tests/system/checkconf/good-maxratio1.conf
@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+zone example {
+	type master;
+	masterfile-format map;
+	file "example.db";
+	max-ixfr-ratio 50%;
+};
diff --git a/bin/tests/system/checkconf/good-maxratio2.conf b/bin/tests/system/checkconf/good-maxratio2.conf
new file mode 100644
index 0000000000..e057dfe6e1
--- /dev/null
+++ b/bin/tests/system/checkconf/good-maxratio2.conf
@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+zone example {
+	type master;
+	masterfile-format map;
+	file "example.db";
+	max-ixfr-ratio unlimited;
+};
diff --git a/bin/tests/system/checkconf/good.conf b/bin/tests/system/checkconf/good.conf
index 2eb2850f10..f2dae97362 100644
--- a/bin/tests/system/checkconf/good.conf
+++ b/bin/tests/system/checkconf/good.conf
@@ -79,6 +79,7 @@ options {
 		"corp";
 	};
 	dnssec-policy "test";
+	max-ixfr-ratio 90%;
 	transfer-source 0.0.0.0 dscp 63;
 	zone-statistics none;
 };
@@ -90,11 +91,13 @@ view "first" {
 		type master;
 		file "xxx";
 		update-policy local;
+		max-ixfr-ratio 20%;
 		notify-source 10.10.10.10 port 53 dscp 55;
 	};
 	zone "clone" {
 		type master;
 		file "yyy";
+		max-ixfr-ratio unlimited;
 	};
 	dnssec-validation auto;
 	zone-statistics terse;
diff --git a/bin/tests/system/checkconf/tests.sh b/bin/tests/system/checkconf/tests.sh
index 07fc06cd1f..ba8b314cb6 100644
--- a/bin/tests/system/checkconf/tests.sh
+++ b/bin/tests/system/checkconf/tests.sh
@@ -511,9 +511,16 @@ awk 'BEGIN { ok = 0; } /cut here/ { ok = 1; getline } ok == 1 { print }' good-ka
 [ -s good-kasp.conf.in ] || ret=1
 $CHECKCONF -p good-kasp.conf.in | grep -v '^good-kasp.conf.in:' > good-kasp.conf.out 2>&1 || ret=1
 cmp good-kasp.conf.in good-kasp.conf.out || ret=1
-
 if [ $ret != 0 ]; then echo_i "failed"; fi
 status=`expr $status + $ret`
 
+n=`expr $n + 1`
+echo_i "check that max-ixfr-ratio 100% generates a warning ($n)"
+ret=0
+$CHECKCONF warn-maxratio1.conf > checkconf.out$n 2>/dev/null || ret=1
+grep "exceeds 100%" < checkconf.out$n > /dev/null || ret=1
+if [ $ret != 0 ]; then echo_i "failed"; ret=1; fi
+status=`expr $status + $ret`
+
 echo_i "exit status: $status"
 [ $status -eq 0 ] || exit 1
diff --git a/bin/tests/system/checkconf/warn-maxratio1.conf b/bin/tests/system/checkconf/warn-maxratio1.conf
new file mode 100644
index 0000000000..519dcc484c
--- /dev/null
+++ b/bin/tests/system/checkconf/warn-maxratio1.conf
@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+zone example {
+	type master;
+	masterfile-format map;
+	file "example.db";
+	max-ixfr-ratio 101%;
+};
diff --git a/bin/tests/system/ixfr/ns4/named.conf.in b/bin/tests/system/ixfr/ns4/named.conf.in
index d3490b18e4..086211684a 100644
--- a/bin/tests/system/ixfr/ns4/named.conf.in
+++ b/bin/tests/system/ixfr/ns4/named.conf.in
@@ -37,6 +37,7 @@ view "primary" {
 		type slave;
 		file "mytest.db";
 		masters { 10.53.0.3; };
+		max-ixfr-ratio unlimited;
 	};
 	zone "sub.test" IN {
 		type slave;
diff --git a/lib/bind9/check.c b/lib/bind9/check.c
index dbae875c87..037c9e218f 100644
--- a/lib/bind9/check.c
+++ b/lib/bind9/check.c
@@ -1565,6 +1565,24 @@ check_options(const cfg_obj_t *options, isc_log_t *logctx, isc_mem_t *mctx,
 		}
 	}
 
+	obj = NULL;
+	(void)cfg_map_get(options, "max-ixfr-ratio", &obj);
+	if (obj != NULL && cfg_obj_ispercentage(obj)) {
+		uint32_t percent = cfg_obj_aspercentage(obj);
+		if (percent == 0) {
+			cfg_obj_log(obj, logctx, ISC_LOG_ERROR,
+				    "'ixfr-max-ratio' must be a nonzero "
+				    "percentage or 'unlimited')");
+			if (result == ISC_R_SUCCESS) {
+				result = ISC_R_RANGE;
+			}
+		} else if (percent > 100) {
+			cfg_obj_log(obj, logctx, ISC_LOG_WARNING,
+				    "'ixfr-max-ratio %d%%' exceeds 100%%",
+				    percent);
+		}
+	}
+
 	return (result);
 }
 
diff --git a/lib/dns/include/dns/zone.h b/lib/dns/include/dns/zone.h
index 4ca41d65e1..c5c363ab0e 100644
--- a/lib/dns/include/dns/zone.h
+++ b/lib/dns/include/dns/zone.h
@@ -2345,6 +2345,25 @@ dns_zone_setrequestixfr(dns_zone_t *zone, bool flag);
  * \li	'zone' to be valid.
  */
 
+uint32_t
+dns_zone_getixfrratio(dns_zone_t *zone);
+/*%
+ * Returns the zone's current IXFR ratio.
+ *
+ * Requires:
+ * \li	'zone' to be valid.
+ */
+
+void
+dns_zone_setixfrratio(dns_zone_t *zone, uint32_t ratio);
+/*%
+ * Sets the ratio of IXFR size to zone size above which we use an AXFR
+ * response, expressed as a percentage. Cannot exceed 100.
+ *
+ * Requires:
+ * \li	'zone' to be valid.
+ */
+
 void
 dns_zone_setserialupdatemethod(dns_zone_t *zone, dns_updatemethod_t method);
 /*%
diff --git a/lib/dns/win32/libdns.def.in b/lib/dns/win32/libdns.def.in
index b5f2e57c65..3c25fec239 100644
--- a/lib/dns/win32/libdns.def.in
+++ b/lib/dns/win32/libdns.def.in
@@ -1203,6 +1203,7 @@ dns_zone_getgluecachestats
 dns_zone_getidlein
 dns_zone_getidleout
 dns_zone_getincludes
+dns_zone_getixfrratio
 dns_zone_getjournal
 dns_zone_getjournalsize
 dns_zone_getkasp
@@ -1305,6 +1306,7 @@ dns_zone_setforwardacl
 dns_zone_setidlein
 dns_zone_setidleout
 dns_zone_setisself
+dns_zone_setixfrratio
 dns_zone_setjournal
 dns_zone_setjournalsize
 dns_zone_setkasp
diff --git a/lib/dns/zone.c b/lib/dns/zone.c
index d1459c1919..e408e7ff4d 100644
--- a/lib/dns/zone.c
+++ b/lib/dns/zone.c
@@ -408,6 +408,7 @@ struct dns_zone {
 	 * whether ixfr is requested
 	 */
 	bool requestixfr;
+	uint32_t ixfr_ratio;
 
 	/*%
 	 * whether EDNS EXPIRE is requested
@@ -1114,6 +1115,7 @@ dns_zone_create(dns_zone_t **zonep, isc_mem_t *mctx) {
 	zone->sourceserial = 0;
 	zone->sourceserialset = false;
 	zone->requestixfr = true;
+	zone->ixfr_ratio = 100;
 	zone->requestexpire = true;
 	ISC_LIST_INIT(zone->rss_events);
 	ISC_LIST_INIT(zone->rss_post);
@@ -20083,6 +20085,18 @@ dns_zone_getrequestixfr(dns_zone_t *zone) {
 	return (zone->requestixfr);
 }
 
+void
+dns_zone_setixfrratio(dns_zone_t *zone, uint32_t ratio) {
+	REQUIRE(DNS_ZONE_VALID(zone));
+	zone->ixfr_ratio = ratio;
+}
+
+uint32_t
+dns_zone_getixfrratio(dns_zone_t *zone) {
+	REQUIRE(DNS_ZONE_VALID(zone));
+	return (zone->ixfr_ratio);
+}
+
 void
 dns_zone_setrequestexpire(dns_zone_t *zone, bool flag) {
 	REQUIRE(DNS_ZONE_VALID(zone));
diff --git a/lib/isccfg/namedconf.c b/lib/isccfg/namedconf.c
index 516be5cc2b..d3b0d6a41f 100644
--- a/lib/isccfg/namedconf.c
+++ b/lib/isccfg/namedconf.c
@@ -92,6 +92,7 @@ static cfg_type_t cfg_type_dnstapoutput;
 static cfg_type_t cfg_type_dyndb;
 static cfg_type_t cfg_type_plugin;
 static cfg_type_t cfg_type_ixfrdifftype;
+static cfg_type_t cfg_type_ixfrratio;
 static cfg_type_t cfg_type_key;
 static cfg_type_t cfg_type_logfile;
 static cfg_type_t cfg_type_logging;
@@ -2165,6 +2166,8 @@ static cfg_clausedef_t zone_clauses[] = {
 	  CFG_ZONE_MASTER | CFG_ZONE_SLAVE | CFG_ZONE_MIRROR | CFG_ZONE_STUB |
 		  CFG_ZONE_REDIRECT },
 	{ "max-ixfr-log-size", &cfg_type_size, CFG_CLAUSEFLAG_ANCIENT },
+	{ "max-ixfr-ratio", &cfg_type_ixfrratio,
+	  CFG_ZONE_MASTER | CFG_ZONE_SLAVE | CFG_ZONE_MIRROR },
 	{ "max-journal-size", &cfg_type_size,
 	  CFG_ZONE_MASTER | CFG_ZONE_SLAVE | CFG_ZONE_MIRROR },
 	{ "max-records", &cfg_type_uint32,
@@ -2699,6 +2702,28 @@ static cfg_type_t cfg_type_sizeorpercent = {
 	doc_parse_size_or_percent, &cfg_rep_string,	  sizeorpercent_enums
 };
 
+/*%
+ * An IXFR size ratio: percentage, or "unlimited".
+ */
+
+static isc_result_t
+parse_ixfrratio(cfg_parser_t *pctx, const cfg_type_t *type, cfg_obj_t **ret) {
+	return (cfg_parse_enum_or_other(pctx, type, &cfg_type_percentage, ret));
+}
+
+static void
+doc_ixfrratio(cfg_printer_t *pctx, const cfg_type_t *type) {
+	UNUSED(type);
+	cfg_print_cstr(pctx, "( unlimited | ");
+	cfg_doc_terminal(pctx, &cfg_type_percentage);
+	cfg_print_cstr(pctx, " )");
+}
+
+static const char *ixfrratio_enums[] = { "unlimited", NULL };
+static cfg_type_t cfg_type_ixfrratio = { "ixfr_ratio", parse_ixfrratio,
+					 NULL,	       doc_ixfrratio,
+					 NULL,	       ixfrratio_enums };
+
 /*%
  * optional_keyvalue
  */