fix: Utilize hoplimit for multicast

If the hoplimit is set, it will be used. If not set, the default
values defined in the [OCF Core Specification](https://openconnectivity.org/specs/OCF_Core_Specification.pdf)
section 12.2.9 will be used.

Default values:
- For "224.0.1.187:5683", the hoplimit is set to 1.
- For "[ff02::158]:5683", the hoplimit is set to 1.
- For "[ff03::158]:5683", the hoplimit is set to 255.
- For "[ff05::158]:5683", the hoplimit is set to 255.

Author: jkralik

client/core/client.go

@@ -80,7 +80,7 @@ func WithTLS(tlsConfig *TLSConfig) OptionFunc {
 
 // DiscoveryConfiguration setup discovery configuration
 type DiscoveryConfiguration struct {
-	MulticastHopLimit    int      // default: 2, min value: 1 - don't pass through router, max value: 255, https://tools.ietf.org/html/rfc2460#section-3
+	MulticastHopLimit    int      // default: {224.0.1.187:5683, ff02::158]:5683} = 1, {[ff03::158]:5683, [ff05::158]:5683} = 255, https://openconnectivity.org/specs/OCF_Core_Specification.pdf 12.2.9
 	MulticastAddressUDP4 []string // default: "[224.0.1.187:5683] (client.DiscoveryAddressUDP4), empty: don't use ipv4 multicast"
 	MulticastAddressUDP6 []string // default: "[ff02::158]:5683", "[ff03::158]:5683", "[ff05::158]:5683]"] (client.DiscoveryAddressUDP6), empty: don't use ipv6 multicast"
 	MulticastOptions     []coapNet.MulticastOption
@@ -151,7 +151,7 @@ func (c *Client) getDeviceConfiguration() DeviceConfiguration {
 
 func DefaultDiscoveryConfiguration() DiscoveryConfiguration {
 	return DiscoveryConfiguration{
-		MulticastHopLimit:    2,
+		MulticastHopLimit:    0, // will be set to 1 or 255 based on address
 		MulticastAddressUDP4: DiscoveryAddressUDP4,
 		MulticastAddressUDP6: DiscoveryAddressUDP6,
 		MulticastOptions: []coapNet.MulticastOption{coapNet.WithMulticastInterfaceError(func(iface *net.Interface, err error) {

client/core/discover.go

@@ -36,9 +36,16 @@ import (
 // See the section 10.4 on the line 2482 of the Core specification:
 // https://openconnectivity.org/specs/OCF_Core_Specification_v2.0.0.pdf
 // https://iotivity.org/documentation/linux/programmers-guide
+const (
+	DiscoveryAddressUDP4Local      = "224.0.1.187:5683"
+	DiscoveryAddressUDP6LinkLocal  = "[ff02::158]:5683"
+	DiscoveryAddressUDP6RealmLocal = "[ff03::158]:5683"
+	DiscoveryAddressUDP6SiteLocal  = "[ff05::158]:5683"
+)
+
 var (
-	DiscoveryAddressUDP4 = []string{"224.0.1.187:5683"}
-	DiscoveryAddressUDP6 = []string{"[ff02::158]:5683", "[ff03::158]:5683", "[ff05::158]:5683"}
+	DiscoveryAddressUDP4 = []string{DiscoveryAddressUDP4Local}
+	DiscoveryAddressUDP6 = []string{DiscoveryAddressUDP6LinkLocal, DiscoveryAddressUDP6RealmLocal, DiscoveryAddressUDP6SiteLocal}
 )
 
 type DiscoveryHandler = func(conn *client.Conn, req *pool.Message)
@@ -89,6 +96,24 @@ func (d *DiscoveryClient) Close() error {
 	return err
 }
 
+// See the section 12.2.9 https://openconnectivity.org/specs/OCF_Core_Specification.pdf
+var defaultHopLimit = map[string]int{
+	DiscoveryAddressUDP4Local:      1,
+	DiscoveryAddressUDP6LinkLocal:  1,
+	DiscoveryAddressUDP6RealmLocal: 255,
+	DiscoveryAddressUDP6SiteLocal:  255,
+}
+
+func getHopLimit(addr string, desiredHopLimit int) int {
+	if desiredHopLimit > 0 {
+		return desiredHopLimit
+	}
+	if v, ok := defaultHopLimit[addr]; ok {
+		return v
+	}
+	return 1
+}
+
 // DialDiscoveryAddresses connects to discovery endpoints.
 func DialDiscoveryAddresses(ctx context.Context, cfg DiscoveryConfiguration, errors func(error)) ([]*DiscoveryClient, error) {
 	v, ok := ctx.Deadline()
@@ -105,15 +130,23 @@ func DialDiscoveryAddresses(ctx context.Context, cfg DiscoveryConfiguration, err
 	msgIDudp6 := msgIDudp4 + ^uint16(0)/2
 
 	for _, address := range cfg.MulticastAddressUDP4 {
-		c, err := newDiscoveryClient("udp4", address, msgIDudp4, timeout, errors, cfg.MulticastOptions)
+		multicastOptions := []net.MulticastOption{
+			net.WithMulticastHoplimit(getHopLimit(address, cfg.MulticastHopLimit)),
+		}
+		multicastOptions = append(multicastOptions, cfg.MulticastOptions...)
+		c, err := newDiscoveryClient("udp4", address, msgIDudp4, timeout, errors, multicastOptions)
 		if err != nil {
 			errors(err)
 			continue
 		}
 		out = append(out, c)
 	}
 	for _, address := range cfg.MulticastAddressUDP6 {
-		c, err := newDiscoveryClient("udp6", address, msgIDudp6, timeout, errors, cfg.MulticastOptions)
+		multicastOptions := []net.MulticastOption{
+			net.WithMulticastHoplimit(getHopLimit(address, cfg.MulticastHopLimit)),
+		}
+		multicastOptions = append(multicastOptions, cfg.MulticastOptions...)
+		c, err := newDiscoveryClient("udp6", address, msgIDudp6, timeout, errors, multicastOptions)
 		if err != nil {
 			errors(err)
 			continue