feat(mdns): add option to disable either IPv4 or IPv6 stack

allow the user to disable v4 or v6 if needed. This solves the issue when
a user is using discover option but having only one IP stack

cmd/serf/command/agent/command.go

@@ -89,6 +89,8 @@ func (c *Command) readConfig() *Config {
 	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.BoolVar(&cmdConfig.MDNS.DisableIPv4, "mdns-disable-ipv4", false, "disable IPv4 for mDNS")
+	cmdFlags.BoolVar(&cmdConfig.MDNS.DisableIPv6, "mdns-disable-ipv6", false, "disable IPv6 for mDNS")
 	cmdFlags.StringVar(&cmdConfig.TagsFile, "tags-file", "", "tag persistence file")
 	cmdFlags.BoolVar(&cmdConfig.EnableSyslog, "syslog", false,
 		"enable logging to syslog facility")
@@ -182,6 +184,12 @@ func (c *Command) readConfig() *Config {
 			c.Ui.Error(fmt.Sprintf("Invalid mDNS network interface: %s", err))
 			return nil
 		}
+
+		// Check for a valid mdns ip mode
+		if config.MDNS.DisableIPv4 && config.MDNS.DisableIPv6 {
+			c.Ui.Error("Invalid mDNS configuration: both IPv4 and IPv6 are disabled")
+			return nil
+		}
 	}
 
 	// Backward compatibility hack for 'Role'
@@ -445,7 +453,7 @@ func (c *Command) startAgent(config *Config, agent *Agent,
 		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))
+			config.NodeName, config.Discover, iface, local.Addr, int(local.Port), config.MDNS)
 		if err != nil {
 			c.Ui.Error(fmt.Sprintf("Error starting mDNS listener: %s", err))
 			return nil

cmd/serf/command/agent/config.go

@@ -48,7 +48,9 @@ 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"`
+	Interface   string `mapstructure:"interface"`
+	DisableIPv4 bool   `mapstructure:"disable_ipv4"`
+	DisableIPv6 bool   `mapstructure:"disable_ipv6"`
 }
 
 // Config is the configuration that can be set for an Agent. Some of these
@@ -459,6 +461,14 @@ func MergeConfig(a, b *Config) *Config {
 		result.MDNS.Interface = b.MDNS.Interface
 	}
 
+	if b.MDNS.DisableIPv4 == true {
+		result.MDNS.DisableIPv4 = true
+	}
+
+	if b.MDNS.DisableIPv6 == true {
+		result.MDNS.DisableIPv6 = true
+	}
+
 	if b.ReconnectInterval != 0 {
 		result.ReconnectInterval = b.ReconnectInterval
 	}

cmd/serf/command/agent/mdns.go

@@ -21,18 +21,21 @@ const (
 // AgentMDNS is used to advertise ourself using mDNS and to
 // attempt to join peers periodically using mDNS queries.
 type AgentMDNS struct {
-	agent    *Agent
-	discover string
-	logger   *log.Logger
-	seen     map[string]struct{}
-	server   *mdns.Server
-	replay   bool
-	iface    *net.Interface
+	agent       *Agent
+	discover    string
+	logger      *log.Logger
+	seen        map[string]struct{}
+	server      *mdns.Server
+	replay      bool
+	iface       *net.Interface
+	ipModes     []string
+	disableIPv4 bool
+	disableIPv6 bool
 }
 
 // NewAgentMDNS is used to create a new AgentMDNS
 func NewAgentMDNS(agent *Agent, logOutput io.Writer, replay bool,
-	node, discover string, iface *net.Interface, bind net.IP, port int) (*AgentMDNS, error) {
+	node, discover string, iface *net.Interface, bind net.IP, port int, MDNSConfig MDNSConfig) (*AgentMDNS, error) {
 	// Create the service
 	service, err := mdns.NewMDNSService(
 		node,
@@ -60,13 +63,15 @@ func NewAgentMDNS(agent *Agent, logOutput io.Writer, replay bool,
 
 	// Initialize the AgentMDNS
 	m := &AgentMDNS{
-		agent:    agent,
-		discover: discover,
-		logger:   log.New(logOutput, "", log.LstdFlags),
-		seen:     make(map[string]struct{}),
-		server:   server,
-		replay:   replay,
-		iface:    iface,
+		agent:       agent,
+		discover:    discover,
+		logger:      log.New(logOutput, "", log.LstdFlags),
+		seen:        make(map[string]struct{}),
+		server:      server,
+		replay:      replay,
+		iface:       iface,
+		disableIPv4: MDNSConfig.DisableIPv4,
+		disableIPv6: MDNSConfig.DisableIPv6,
 	}
 
 	// Start the background workers
@@ -123,9 +128,11 @@ func (m *AgentMDNS) run() {
 // poll is invoked periodically to check for new hosts
 func (m *AgentMDNS) poll(hosts chan *mdns.ServiceEntry) {
 	params := mdns.QueryParam{
-		Service:   mdnsName(m.discover),
-		Interface: m.iface,
-		Entries:   hosts,
+		Service:     mdnsName(m.discover),
+		Interface:   m.iface,
+		Entries:     hosts,
+		DisableIPv4: m.disableIPv4,
+		DisableIPv6: m.disableIPv6,
 	}
 	if err := mdns.Query(&params); err != nil {
 		m.logger.Printf("[ERR] agent.mdns: Failed to poll for new hosts: %v", err)