net: fall back to hosts file if DNS lookup fails, despite order

Fixes #13090

Change-Id: I5612d792dabdff89bd0cec57dc2cacf9be7ebf64
Reviewed-on: https://go-review.googlesource.com/16341
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix.go

@@ -473,12 +473,12 @@ func goLookupIPOrder(name string, order hostLookupOrder) (addrs []IPAddr, err er
 	}
 	sortByRFC6724(addrs)
 	if len(addrs) == 0 {
-		if lastErr != nil {
-			return nil, lastErr
-		}
 		if order == hostLookupDNSFiles {
 			addrs = goLookupIPFiles(name)
 		}
+		if lastErr != nil {
+			return nil, lastErr
+		}
 	}
 	return addrs, nil
 }

src/net/dnsclient_unix_test.go

@@ -378,6 +378,48 @@ func TestGoLookupIPWithResolverConfig(t *testing.T) {
 	}
 }
 
+// Test that goLookupIPOrder falls back to the host file when no DNS servers are available.
+func TestGoLookupIPOrderFallbackToFile(t *testing.T) {
+	if testing.Short() || !*testExternal {
+		t.Skip("avoid external network")
+	}
+
+	// Add a config that simulates no dns servers being available.
+	conf, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := conf.writeAndUpdate([]string{}); err != nil {
+		t.Fatal(err)
+	}
+	conf.tryUpdate(conf.path)
+	// Redirect host file lookups.
+	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	testHookHostsPath = "testdata/hosts"
+
+	for _, order := range []hostLookupOrder{hostLookupFilesDNS, hostLookupDNSFiles} {
+		name := fmt.Sprintf("order %v", order)
+
+		// First ensure that we get an error when contacting a non-existant host.
+		_, err := goLookupIPOrder("notarealhost", order)
+		if err == nil {
+			t.Errorf("%s: expected error while looking up name not in hosts file", name)
+			continue
+		}
+
+		// Now check that we get an address when the name appears in the hosts file.
+		addrs, err := goLookupIPOrder("thor", order) // entry is in "testdata/hosts"
+		if err != nil {
+			t.Errorf("%s: expected to successfully lookup host entry", name)
+			continue
+		}
+		if got, want := addrs, []IPAddr{IPAddr{IP: IP{127, 0, 0, 1}}}; !reflect.DeepEqual(got, want) {
+			t.Errorf("%s: address doesn't match expectation. got %v, want %v", name, got, want)
+		}
+	}
+	defer conf.teardown()
+}
+
 func BenchmarkGoLookupIP(b *testing.B) {
 	testHookUninstaller.Do(uninstallTestHooks)