micro/go-micro · error

[ERR] mdns: Failed to bind to any udp port

Error message

[ERR] mdns: Failed to bind to any udp port

What it means

NewServer creates wildcard UDP listeners for mDNS on port 5353 for both IPv4 and IPv6. If neither listen succeeds, the server cannot receive mDNS packets and returns this error. Individual listen errors are discarded, so only the combined failure is reported.

Source

Thrown at internal/util/mdns/server.go:94

	outboundIP net.IP
	wg         sync.WaitGroup

	shutdownLock sync.Mutex

	shutdown bool
}

// NewServer is used to create a new mDNS server from a config.
func NewServer(config *Config) (*Server, error) {
	setCustomPort(config.Port)

	// Create the listeners
	// Create wildcard connections (because :5353 can be already taken by other apps)
	ipv4List, _ := net.ListenUDP("udp4", mdnsWildcardAddrIPv4)
	ipv6List, _ := net.ListenUDP("udp6", mdnsWildcardAddrIPv6)
	if ipv4List == nil && ipv6List == nil {
		return nil, fmt.Errorf("[ERR] mdns: Failed to bind to any udp port")
	}

	if ipv4List == nil {
		ipv4List = &net.UDPConn{}
	}
	if ipv6List == nil {
		ipv6List = &net.UDPConn{}
	}

	// Join multicast groups to receive announcements
	p1 := ipv4.NewPacketConn(ipv4List)
	p2 := ipv6.NewPacketConn(ipv6List)
	_ = p1.SetMulticastLoopback(true)
	_ = p2.SetMulticastLoopback(true)

	if config.Iface != nil {
		if err := p1.JoinGroup(config.Iface, &net.UDPAddr{IP: mdnsGroupIPv4}); err != nil {
			return nil, err

View on GitHub (pinned to 24529f1404)

Solutions

1. Stop competing mDNS services holding :5353 (systemctl stop avahi-daemon) or confirm reuse flags are set on both sides
2. Check OS-level restrictions (SELinux, AppArmor, firewall) that block binding UDP 5353
3. Increase fd limits if descriptors are exhausted (ulimit -n)
4. Run as a user/group with permission to bind multicast ports

Example fix

// before: bind fails because Avahi holds :5353
srv, err := mdns.NewServer(&mdns.Config{Zone: zone})
// after: free the port first
// $ sudo systemctl stop avahi-daemon
srv, err := mdns.NewServer(&mdns.Config{Zone: zone})

l4, err4 := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 5353})
l6, err6 := net.ListenUDP("udp6", &net.UDPAddr{IP: net.IPv6zero, Port: 5353})
if l4 == nil && l6 == nil {
    log.Fatalf("port 5353 unusable: %v %v", err4, err6)
}
if l4 != nil { l4.Close() }
if l6 != nil { l6.Close() }

srv, err := mdns.NewServer(&mdns.Config{Zone: zone})
if err != nil {
    if strings.Contains(err.Error(), "Failed to bind to any udp port") {
        return nil, fmt.Errorf("mDNS server cannot start: is another mDNS daemon holding :5353? %w", err)
    }
    return nil, err
}

When it happens

Trigger: NewServer (used by Register and tests) when both net.ListenUDP("udp4", ...) and net.ListenUDP("udp6", ...) return nil connections.

Common situations: Port 5353 occupied by another mDNS daemon (Avahi, Bonjour) without reuse options; firewalls/SELinux blocking binds; out-of-file-descriptor conditions in test environments.

Related errors

• failed to bind to any unicast udp port
• failed to bind to any multicast udp port
• mdns: error sending multicast response: %v
• mdns: error sending unicast response: %v
• failed to join multicast group on all interfaces

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/545442ad1400ad61. Report an issue: GitHub.