feat(mdns): allow specifying iface for mDNS

cmd/serf/command/agent/command.go

@@ -88,6 +88,7 @@ func (c *Command) readConfig() *Config {
 		"tag pair, specified as key=value")
 	cmdFlags.StringVar(&cmdConfig.Discover, "discover", "", "mDNS discovery name")
 	cmdFlags.StringVar(&cmdConfig.Interface, "iface", "", "interface to bind to")
+	cmdFlags.StringVar(&cmdConfig.MDNS.Interface, "mdns-iface", "", "interface to use for mDNS")
 	cmdFlags.StringVar(&cmdConfig.TagsFile, "tags-file", "", "tag persistence file")
 	cmdFlags.BoolVar(&cmdConfig.EnableSyslog, "syslog", false,
 		"enable logging to syslog facility")
@@ -176,6 +177,18 @@ func (c *Command) readConfig() *Config {
 		return nil
 	}
 
+	if config.MDNS.Interface != "" {
+		if config.Discover == "" {
+			c.Ui.Error("mDNS interface specified without enabling mDNS discovery")
+			return nil
+		}
+
+		if _, err := net.InterfaceByName(config.MDNS.Interface); err != nil {
+			c.Ui.Error(fmt.Sprintf("Invalid mDNS network interface: %s", err))
+			return nil
+		}
+	}
+
 	// Backward compatibility hack for 'Role'
 	if config.Role != "" {
 		c.Ui.Output("Deprecation warning: 'Role' has been replaced with 'Tags'")
@@ -432,7 +445,9 @@ func (c *Command) startAgent(config *Config, agent *Agent,
 		local := agent.Serf().Memberlist().LocalNode()
 
 		// Get the bind interface if any
-		iface, _ := config.NetworkInterface()
+		iface, _ := config.MDNSNetworkInterface()
+
+		c.logger.Printf("[INFO] agent: Starting mDNS listener on interface %s", iface.Name)
 
 		_, err := NewAgentMDNS(agent, logOutput, config.ReplayOnJoin,
 			config.NodeName, config.Discover, iface, local.Addr, int(local.Port))
@@ -734,7 +749,10 @@ Options:
                            -bind if the interface is known but not the address.
                            If both are provided, then Serf verifies that the
                            interface has the bind address that is provided. This
-                           flag also sets the multicast device used for -discover.
+                           flag also sets the multicast device used for -discover,
+                           if mdns-iface is not specified.
+  -mdns-iface              Network interface to use for mDNS. If not provided, the
+                           -iface value is used.
   -advertise=0.0.0.0       Address to advertise to the other cluster members
   -config-file=foo         Path to a JSON file to read configuration from.
                            This can be specified multiple times.

cmd/serf/command/agent/config.go

@@ -45,6 +45,12 @@ func DefaultConfig() *Config {
 
 type dirEnts []os.FileInfo
 
+type MDNSConfig struct {
+	// Interface is used to provide a binding interface to use for mDNS.
+	// if not set, iface will be used.
+	Interface string `mapstructure:"interface"`
+}
+
 // Config is the configuration that can be set for an Agent. Some of these
 // configurations are exposed as command-line flags to `serf agent`, whereas
 // many of the more advanced configurations can only be set by creating
@@ -154,10 +160,12 @@ type Config struct {
 	// allows Serf agents to join each other with zero configuration.
 	Discover string `mapstructure:"discover"`
 
+	MDNS MDNSConfig `mapstructure:"mdns"`
+
 	// Interface is used to provide a binding interface to use. It can be
 	// used instead of providing a bind address, as Serf will discover the
 	// address of the provided interface. It is also used to set the multicast
-	// device used with `-discover`.
+	// device used with `-discover`, if `mdns-iface` is not set
 	Interface string `mapstructure:"interface"`
 
 	// ReconnectIntervalRaw is the string reconnect interval time. This interval
@@ -292,6 +300,16 @@ func (c *Config) NetworkInterface() (*net.Interface, error) {
 	return net.InterfaceByName(c.Interface)
 }
 
+func (c *Config) MDNSNetworkInterface() (*net.Interface, error) {
+	if c.MDNS.Interface == "" && c.Interface == "" {
+		return nil, nil
+	} else if c.MDNS.Interface != "" {
+		return net.InterfaceByName(c.MDNS.Interface)
+	} else {
+		return net.InterfaceByName(c.Interface)
+	}
+}
+
 // DecodeConfig reads the configuration from the given reader in JSON
 // format and decodes it into a proper Config structure.
 func DecodeConfig(r io.Reader) (*Config, error) {
@@ -436,6 +454,11 @@ func MergeConfig(a, b *Config) *Config {
 	if b.Interface != "" {
 		result.Interface = b.Interface
 	}
+
+	if b.MDNS.Interface != "" {
+		result.MDNS.Interface = b.MDNS.Interface
+	}
+
 	if b.ReconnectInterval != 0 {
 		result.ReconnectInterval = b.ReconnectInterval
 	}