diff --git a/lib/ns/Makefile.in b/lib/ns/Makefile.in
index e785969b51..96228b3500 100644
--- a/lib/ns/Makefile.in
+++ b/lib/ns/Makefile.in
@@ -28,7 +28,7 @@ CINCLUDES =	-I. -I${top_srcdir}/lib/ns -Iinclude \
 		${NS_INCLUDES} ${DNS_INCLUDES} ${ISC_INCLUDES} \
 		@OPENSSL_INCLUDES@ @DST_GSSAPI_INC@
 
-CDEFINES =
+CDEFINES =	-DNAMED_PLUGINDIR=\"${plugindir}\"
 
 CWARNINGS =
 
diff --git a/lib/ns/hooks.c b/lib/ns/hooks.c
index 83c390de3a..327ae6909e 100644
--- a/lib/ns/hooks.c
+++ b/lib/ns/hooks.c
@@ -13,6 +13,8 @@
 
 #include <config.h>
 
+#include <errno.h>
+#include <stdio.h>
 #include <string.h>
 
 #if HAVE_DLFCN_H
@@ -21,10 +23,12 @@
 #include <windows.h>
 #endif
 
+#include <isc/errno.h>
 #include <isc/list.h>
 #include <isc/log.h>
 #include <isc/mem.h>
 #include <isc/mutex.h>
+#include <isc/print.h>
 #include <isc/result.h>
 #include <isc/platform.h>
 #include <isc/util.h>
@@ -58,6 +62,41 @@ struct ns_plugin {
 static ns_hooklist_t default_hooktable[NS_HOOKPOINTS_COUNT];
 LIBNS_EXTERNAL_DATA ns_hooktable_t *ns__hook_table = &default_hooktable;
 
+isc_result_t
+ns_plugin_expandpath(const char *src, char *dst, size_t dstsize) {
+	int result;
+
+#ifndef WIN32
+	/*
+	 * On Unix systems, differentiate between paths and filenames.
+	 */
+	if (strchr(src, '/') != NULL) {
+		/*
+		 * 'src' is an absolute or relative path.  Copy it verbatim.
+		 */
+		result = snprintf(dst, dstsize, "%s", src);
+	} else {
+		/*
+		 * 'src' is a filename.  Prepend default plugin directory path.
+		 */
+		result = snprintf(dst, dstsize, "%s/%s", NAMED_PLUGINDIR, src);
+	}
+#else
+	/*
+	 * On Windows, always copy 'src' do 'dst'.
+	 */
+	result = snprintf(dst, dstsize, "%s", src);
+#endif
+
+	if (result < 0) {
+		return (isc_errno_toresult(errno));
+	} else if ((size_t)result >= dstsize) {
+		return (ISC_R_NOSPACE);
+	} else {
+		return (ISC_R_SUCCESS);
+	}
+}
+
 #if HAVE_DLFCN_H && HAVE_DLOPEN
 static isc_result_t
 load_symbol(void *handle, const char *modpath,
@@ -253,7 +292,7 @@ load_plugin(isc_mem_t *mctx, const char *modpath, ns_plugin_t **pluginp) {
 	CHECK(load_symbol(handle, modpath, "plugin_version",
 			  (void **)&version_func));
 
-	version = version_func(NULL);
+	version = version_func();
 	if (version < (NS_PLUGIN_VERSION - NS_PLUGIN_AGE) ||
 	    version > NS_PLUGIN_VERSION)
 	{
diff --git a/lib/ns/include/ns/hooks.h b/lib/ns/include/ns/hooks.h
index d2d310183c..5ad1074872 100644
--- a/lib/ns/include/ns/hooks.h
+++ b/lib/ns/include/ns/hooks.h
@@ -315,6 +315,30 @@ ns_plugin_destroy_t plugin_destroy;
 ns_plugin_register_t plugin_register;
 ns_plugin_version_t plugin_version;
 
+isc_result_t
+ns_plugin_expandpath(const char *src, char *dst, size_t dstsize);
+/*%<
+ * Prepare the plugin location to be passed to dlopen() based on the plugin
+ * path or filename found in the configuration file ('src').  Store the result
+ * in 'dst', which is 'dstsize' bytes large.
+ *
+ * On Unix systems, two classes of 'src' are recognized:
+ *
+ *   - If 'src' is an absolute or relative path, it will be copied to 'dst'
+ *     verbatim.
+ *
+ *   - If 'src' is a filename (i.e. does not contain a path separator), the
+ *     path to the directory into which named plugins are installed will be
+ *     prepended to it and the result will be stored in 'dst'.
+ *
+ * On Windows, 'src' is always copied to 'dst' verbatim.
+ *
+ * Returns:
+ *\li	#ISC_R_SUCCESS	Success
+ *\li	#ISC_R_NOSPACE	'dst' is not large enough to hold the output string
+ *\li	Other result	snprintf() returned a negative value
+ */
+
 isc_result_t
 ns_plugin_register(const char *modpath, const char *parameters,
 		   const void *cfg, const char *cfg_file,
diff --git a/lib/ns/tests/Kyuafile b/lib/ns/tests/Kyuafile
index ca15d6fb84..adf0b88ca7 100644
--- a/lib/ns/tests/Kyuafile
+++ b/lib/ns/tests/Kyuafile
@@ -3,4 +3,5 @@ test_suite('bind9')
 
 tap_test_program{name='listenlist_test'}
 tap_test_program{name='notify_test'}
+tap_test_program{name='plugin_test'}
 tap_test_program{name='query_test'}
diff --git a/lib/ns/tests/Makefile.in b/lib/ns/tests/Makefile.in
index 345f900bec..e6d3049ede 100644
--- a/lib/ns/tests/Makefile.in
+++ b/lib/ns/tests/Makefile.in
@@ -17,7 +17,7 @@ VERSION=@BIND9_VERSION@
 
 CINCLUDES =	-I. -Iinclude ${NS_INCLUDES} ${DNS_INCLUDES} ${ISC_INCLUDES} \
 		@OPENSSL_INCLUDES@ @CMOCKA_CFLAGS@
-CDEFINES =	-DTESTS="\"${top_builddir}/lib/ns/tests/\""
+CDEFINES =	-DTESTS="\"${top_builddir}/lib/ns/tests/\"" -DNAMED_PLUGINDIR=\"${plugindir}\"
 
 ISCLIBS =	../../isc/libisc.@A@ @OPENSSL_LIBS@
 ISCDEPLIBS =	../../isc/libisc.@A@
@@ -33,12 +33,14 @@ OBJS =		nstest.@O@
 SRCS =		nstest.c \
 		listenlist_test.c \
 		notify_test.c \
+		plugin_test.c \
 		query_test.c
 
 SUBDIRS =
 TARGETS =	listenlist_test@EXEEXT@ \
 		notify_test@EXEEXT@ \
-		query_test
+		plugin_test@EXEEXT@ \
+		query_test@EXEEXT@
 
 @BIND9_MAKE_RULES@
 
@@ -52,6 +54,11 @@ notify_test@EXEEXT@: notify_test.@O@ nstest.@O@ ${NSDEPLIBS} ${ISCDEPLIBS} ${DNS
 		${LDFLAGS} -o $@ notify_test.@O@ nstest.@O@ \
 		${NSLIBS} ${DNSLIBS} ${ISCLIBS} ${LIBS}
 
+plugin_test@EXEEXT@: plugin_test.@O@ nstest.@O@ ${NSDEPLIBS} ${ISCDEPLIBS} ${DNSDEPLIBS}
+	${LIBTOOL_MODE_LINK} ${PURIFY} ${CC} ${CFLAGS} \
+		${LDFLAGS} -o $@ plugin_test.@O@ nstest.@O@ \
+		${NSLIBS} ${DNSLIBS} ${ISCLIBS} ${LIBS}
+
 query_test@EXEEXT@: query_test.@O@ nstest.@O@ ${NSDEPLIBS} ${ISCDEPLIBS} ${DNSDEPLIBS}
 	${LIBTOOL_MODE_LINK} ${PURIFY} ${CC} ${CFLAGS} \
 		${LDFLAGS} -o $@ query_test.@O@ nstest.@O@ \
diff --git a/lib/ns/tests/plugin_test.c b/lib/ns/tests/plugin_test.c
new file mode 100644
index 0000000000..04ce737681
--- /dev/null
+++ b/lib/ns/tests/plugin_test.c
@@ -0,0 +1,205 @@
+/*
+ * 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.
+ */
+
+#include <config.h>
+
+#if HAVE_CMOCKA
+
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <setjmp.h>
+
+#define UNIT_TESTING
+#include <cmocka.h>
+
+#include <limits.h>
+#include <stdbool.h>
+#include <string.h>
+
+#include <isc/mem.h>
+#include <isc/result.h>
+#include <isc/types.h>
+#include <isc/util.h>
+
+#include <ns/hooks.h>
+
+#include "nstest.h"
+
+static int
+_setup(void **state) {
+	isc_result_t result;
+
+	UNUSED(state);
+
+	result = ns_test_begin(NULL, false);
+	assert_int_equal(result, ISC_R_SUCCESS);
+
+	return (0);
+}
+
+static int
+_teardown(void **state) {
+	if (*state != NULL) {
+		isc_mem_free(mctx, *state);
+	}
+
+	ns_test_end();
+
+	return (0);
+}
+
+/*%
+ * Structure containing parameters for run_full_path_test().
+ */
+typedef struct {
+	const ns_test_id_t id;	/* libns test identifier */
+	const char *input;	/* source string - plugin name or path */
+	size_t output_size;	/* size of target char array to allocate */
+	isc_result_t result;	/* expected return value */
+	const char *output;	/* expected output string */
+} ns_plugin_expandpath_test_params_t;
+
+/*%
+ * Perform a single ns_plugin_expandpath() check using given parameters.
+ */
+static void
+run_full_path_test(const ns_plugin_expandpath_test_params_t *test,
+		   void **state)
+{
+	char **target = (char **)state;
+	isc_result_t result;
+
+	REQUIRE(test != NULL);
+	REQUIRE(test->id.description != NULL);
+	REQUIRE(test->input != NULL);
+	REQUIRE(test->result != ISC_R_SUCCESS || test->output != NULL);
+
+	/*
+	 * Prepare a target buffer of given size.  Store it in 'state' so that
+	 * it can get cleaned up by _teardown() if the test fails.
+	 */
+	*target = isc_mem_allocate(mctx, test->output_size);
+
+	/*
+	 * Call ns_plugin_expandpath().
+	 */
+	result = ns_plugin_expandpath(test->input,
+					 *target, test->output_size);
+
+	/*
+	 * Check return value.
+	 */
+	if (result != test->result) {
+		fail_msg("# test \"%s\" on line %d: "
+			 "expected result %d (%s), got %d (%s)",
+			 test->id.description, test->id.lineno,
+			 test->result, isc_result_totext(test->result),
+			 result, isc_result_totext(result));
+	}
+
+	/*
+	 * Check output string if return value indicates success.
+	 */
+	if (result == ISC_R_SUCCESS && strcmp(*target, test->output) != 0) {
+		fail_msg("# test \"%s\" on line %d: "
+			 "expected output \"%s\", got \"%s\"",
+			 test->id.description, test->id.lineno,
+			 test->output, *target);
+	}
+
+	isc_mem_free(mctx, *target);
+}
+
+/* test ns_plugin_expandpath() */
+static void
+ns_plugin_expandpath_test(void **state) {
+	size_t i;
+
+	const ns_plugin_expandpath_test_params_t tests[] = {
+		{
+			NS_TEST_ID("correct use with an absolute path"),
+			.input = "/usr/lib/named/foo.so",
+			.output_size = PATH_MAX,
+			.result = ISC_R_SUCCESS,
+			.output = "/usr/lib/named/foo.so",
+		},
+		{
+			NS_TEST_ID("correct use with a relative path"),
+			.input = "../../foo.so",
+			.output_size = PATH_MAX,
+			.result = ISC_R_SUCCESS,
+			.output = "../../foo.so",
+		},
+		{
+			NS_TEST_ID("correct use with a filename"),
+			.input = "foo.so",
+			.output_size = PATH_MAX,
+			.result = ISC_R_SUCCESS,
+#ifndef WIN32
+			.output = NAMED_PLUGINDIR "/foo.so",
+#else
+			.output = "foo.so",
+#endif
+		},
+		{
+			NS_TEST_ID("no space at all in target buffer"),
+			.input = "/usr/lib/named/foo.so",
+			.output_size = 0,
+			.result = ISC_R_NOSPACE,
+		},
+		{
+			NS_TEST_ID("target buffer too small to fit input"),
+			.input = "/usr/lib/named/foo.so",
+			.output_size = 1,
+			.result = ISC_R_NOSPACE,
+		},
+		{
+			NS_TEST_ID("target buffer too small to fit NULL byte"),
+			.input = "/foo.so",
+			.output_size = 7,
+			.result = ISC_R_NOSPACE,
+		},
+#ifndef WIN32
+		{
+			NS_TEST_ID("target buffer too small to fit full path"),
+			.input = "foo.so",
+			.output_size = 7,
+			.result = ISC_R_NOSPACE,
+		},
+#endif
+	};
+
+	for (i = 0; i < sizeof(tests) / sizeof(tests[0]); i++) {
+		run_full_path_test(&tests[i], state);
+	}
+}
+
+int
+main(void) {
+	const struct CMUnitTest tests[] = {
+		cmocka_unit_test_setup_teardown(ns_plugin_expandpath_test,
+						_setup, _teardown),
+	};
+
+	return (cmocka_run_group_tests(tests, NULL, NULL));
+}
+#else /* HAVE_CMOCKA */
+
+#include <stdio.h>
+
+int
+main(void) {
+	printf("1..0 # Skipped: cmocka not available\n");
+	return (0);
+}
+
+#endif
diff --git a/lib/ns/win32/libns.def b/lib/ns/win32/libns.def
index bc0d891754..d47deaa6d5 100644
--- a/lib/ns/win32/libns.def
+++ b/lib/ns/win32/libns.def
@@ -75,6 +75,7 @@ ns_log_init
 ns_log_setcontext
 ns_notify_start
 ns_plugin_check
+ns_plugin_expandpath
 ns_plugin_register
 ns_plugins_create
 ns_plugins_free
diff --git a/util/copyrights b/util/copyrights
index ea719293ed..65174fb236 100644
--- a/util/copyrights
+++ b/util/copyrights
@@ -2510,6 +2510,7 @@
 ./lib/ns/tests/notify_test.c			C	2017,2018,2019
 ./lib/ns/tests/nstest.c				C	2017,2018,2019
 ./lib/ns/tests/nstest.h				C	2017,2018,2019
+./lib/ns/tests/plugin_test.c			C	2019
 ./lib/ns/tests/query_test.c			C	2017,2018,2019
 ./lib/ns/tests/testdata/notify/notify1.msg	X	2017,2018,2019
 ./lib/ns/update.c				C	2017,2018,2019