diff --git a/Makefile.am b/Makefile.am
index 738f301..54c1831 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -68,6 +68,7 @@ handwritten_tests = \
 	t/geturl_connectivity.pl \
 	t/geturl_response.pl \
 	t/group_hosts_by.pl \
+	t/header_ok.pl \
 	t/interval_expired.pl \
 	t/is-and-extract-ipv4.pl \
 	t/is-and-extract-ipv6.pl \
diff --git a/t/header_ok.pl b/t/header_ok.pl
new file mode 100644
index 0000000..e835757
--- /dev/null
+++ b/t/header_ok.pl
@@ -0,0 +1,67 @@
+use Test::More;
+SKIP: { eval { require Test::Warnings; } or skip($@, 1); }
+eval { require 'ddclient'; } or BAIL_OUT($@);
+my $have_mock = eval { require Test::MockModule; };
+
+my $failmsg;
+my $module;
+if ($have_mock) {
+    $module = Test::MockModule->new('ddclient');
+    # Note: 'mock' is used instead of 'redefine' because 'redefine' is not available in the versions
+    # of Test::MockModule distributed with old Debian and Ubuntu releases.
+    $module->mock('failed', sub { $failmsg //= ''; $failmsg .= sprintf(shift, @_) . "\n"; });
+}
+
+my @test_cases = (
+    {
+        desc => 'malformed not OK',
+        input => 'malformed',
+        want => 0,
+        wantmsg => qr/unexpected/,
+    },
+    {
+        desc => 'HTTP/1.1 200 OK',
+        input => 'HTTP/1.1 200 OK',
+        want => 1,
+    },
+    {
+        desc => 'HTTP/2 200 OK',
+        input => 'HTTP/2 200 OK',
+        want => 1,
+    },
+    {
+        desc => 'HTTP/3 200 OK',
+        input => 'HTTP/3 200 OK',
+        want => 1,
+    },
+    {
+        desc => '401 not OK',
+        input => 'HTTP/1.1 401 bad',
+        want => 0,
+        wantmsg => qr/authentication failed/,
+    },
+    {
+        desc => '403 not OK',
+        input => 'HTTP/1.1 403 bad',
+        want => 0,
+        wantmsg => qr/not authorized/,
+    },
+    {
+        desc => 'other 4xx not OK',
+        input => 'HTTP/1.1 456 bad',
+        want => 0,
+    },
+);
+
+for my $tc (@test_cases) {
+    subtest $tc->{desc} => sub {
+        $failmsg = '';
+        is(ddclient::header_ok('host', $tc->{input}), $tc->{want}, 'return value matches');
+        SKIP: {
+            skip('Test::MockModule not available') if !$have_mock;
+            like($failmsg, $tc->{wantmsg} // qr/^$/, 'fail message matches');
+        }
+    };
+}
+
+done_testing();