diff --git a/resolv/Makefile b/resolv/Makefile
index 0f1fcc1014..0b8dbf2552 100644
--- a/resolv/Makefile
+++ b/resolv/Makefile
@@ -47,6 +47,7 @@ tests += \
   tst-ns_name \
   tst-ns_name_compress \
   tst-res_hconf_reorder \
+  tst-res_hnok \
   tst-res_use_inet6 \
   tst-resolv-basic \
   tst-resolv-edns \
@@ -172,6 +173,7 @@ $(objpfx)tst-resolv-canonname: \
 $(objpfx)tst-ns_name: $(objpfx)libresolv.so
 $(objpfx)tst-ns_name.out: tst-ns_name.data
 $(objpfx)tst-ns_name_compress: $(objpfx)libresolv.so
+$(objpfx)tst-res_hnok: $(objpfx)libresolv.so
 
 
 # This test case uses the deprecated RES_USE_INET6 resolver option.
diff --git a/resolv/tst-res_hnok.c b/resolv/tst-res_hnok.c
new file mode 100644
index 0000000000..9c92303821
--- /dev/null
+++ b/resolv/tst-res_hnok.c
@@ -0,0 +1,153 @@
+/* Tests for res_hnok and related functions.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <resolv.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/test-driver.h>
+
+/* Bits which indicate which functions are supposed to report
+   success.  */
+enum
+  {
+    hnok = 1,
+    dnok = 2,
+    mailok = 4,
+    ownok = 8,
+    allnomailok = hnok | dnok | ownok,
+    allok = hnok | dnok | mailok | ownok
+  };
+
+/* A string of 60 characters.  */
+#define STRING60 "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
+
+/* A string of 63 characters (maximum label length).  */
+#define STRING63 STRING60 "zzz"
+
+/* Combines a test name with the expected results.  */
+struct test_case
+{
+  const char *dn;
+  unsigned int result;          /* Combination of the *ok flags.  */
+};
+
+static const struct test_case tests[] =
+  {
+    { "", allok },
+    { ".", allok },
+    { "www", allnomailok },
+    { "example", allnomailok },
+    { "example.com", allok },
+    { "www.example.com", allok },
+    { "www.example.com.", allok },
+    { "*.example.com", dnok | mailok | ownok },
+    { "-v", dnok },
+    { "-v.example.com", mailok | dnok },
+    { "**.example.com", dnok | mailok },
+    { STRING63, allnomailok },
+    { STRING63 ".example.com", allok },
+    { STRING63 "." STRING63 "." STRING63 "." STRING60 "z", allok },
+    { "hostmaster@mail.example.com", dnok | mailok },
+    { "with whitespace", 0 },
+    { "with\twhitespace", 0 },
+    { "with\nwhitespace", 0 },
+    { "with.whitespace ", 0 },
+    { "with.whitespace\t", 0 },
+    { "with.whitespace\n", 0 },
+    { "with\\ whitespace", 0 },
+    { "with\\\twhitespace", 0 },
+    { "with\\\nwhitespace", 0 },
+    { "with.whitespace\\ ", 0 },
+    { "with.whitespace\\\t", 0 },
+    { "with.whitespace\\\n", 0 },
+  };
+
+/* Run test case *TEST with FUNC (named FUNCNAME) and report an error
+   if the result does not match the result flag at BIT.  */
+static void
+one_test (const struct test_case *test, const char *funcname,
+          int (*func) (const char *), unsigned int bit)
+{
+  int expected = (test->result & bit) != 0;
+  int actual = func (test->dn);
+  if (actual != expected)
+    {
+      support_record_failure ();
+      printf ("error: %s (\"%s\"): expected=%d, actual=%d\n",
+              funcname, test->dn, expected, actual);
+    }
+}
+
+/* Run 255 tests using all the bytes from 1 to 255, surround the byte
+   with the strings PREFIX and SUFFIX, and check that FUNC (named
+   FUNCNAME) accepts only those bytes listed in ACCEPTED.  */
+static void
+one_char (const char *prefix, const char *accepted, const char *suffix,
+          const char *funcname, int (*func) (const char *))
+{
+  for (int ch = 1; ch <= 255; ++ch)
+    {
+      char dn[1024];
+      snprintf (dn, sizeof (dn), "%s%c%s", prefix, ch, suffix);
+      int expected = strchr (accepted, ch) != NULL;
+      int actual = func (dn);
+      if (actual != expected)
+        {
+          support_record_failure ();
+          printf ("error: %s (\"%s\"): expected=%d, actual=%d\n",
+                  funcname, dn, expected, actual);
+        }
+    }
+}
+
+static int
+do_test (void)
+{
+  for (const struct test_case *test = tests; test < array_end (tests); ++test)
+    {
+      if (test_verbose)
+        printf ("info: testing domain name [[[%s]]] (0x%x)\n",
+                test->dn, test->result);
+      one_test (test, "res_hnok", res_hnok, hnok);
+      one_test (test, "res_dnok", res_dnok, dnok);
+      one_test (test, "res_mailok", res_mailok, mailok);
+      one_test (test, "res_ownok", res_ownok, ownok);
+    }
+
+  one_char
+    ("", "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.",
+     "", "res_hnok", res_hnok);
+  one_char
+    ("middle",
+     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.-_",
+     "suffix", "res_hnok", res_hnok);
+  one_char
+    ("middle",
+     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.-_"
+     "!\"#$%&'()*+,/:;<=>?@[\\]^`{|}~",
+     "suffix.example", "res_mailok", res_mailok);
+  one_char
+    ("mailbox.middle",
+     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.-_",
+     "suffix.example", "res_mailok", res_mailok);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
