micro/go-micro · error

mdns: error sending unicast response: %v

Error message

mdns: error sending unicast response: %v

What it means

This error is returned by handleQuery when the mDNS server failed to write a unicast DNS response packet back to the client's address via sendResponse. It wraps the underlying UDP socket write error (which is included via %v). It indicates the query was parsed and an answer was generated, but delivering that answer over the network failed.

Source

Thrown at internal/util/mdns/server.go:314

			// 18.13 pertains to resource records (handled by handleQuestion)

			// 18.14: Name Compression - responses should be compressed (though see
			// caveats in the RFC), so set the Compress bit (part of the dns library
			// API, not part of the DNS packet) to true.
			Compress: true,
			Question: query.Question,
			Answer:   answer,
		}
	}

	if mresp := resp(false); mresp != nil {
		if err := s.sendResponse(mresp, from); err != nil {
			return fmt.Errorf("mdns: error sending multicast response: %v", err)
		}
	}
	if uresp := resp(true); uresp != nil {
		if err := s.sendResponse(uresp, from); err != nil {
			return fmt.Errorf("mdns: error sending unicast response: %v", err)
		}
	}
	return nil
}

// handleQuestion is used to handle an incoming question
//
// The response to a question may be transmitted over multicast, unicast, or
// both.  The return values are DNS records for each transmission type.
func (s *Server) handleQuestion(q dns.Question) (multicastRecs, unicastRecs []dns.RR) {
	records := s.config.Zone.Records(q)
	if len(records) == 0 {
		return nil, nil
	}

	// Handle unicast and multicast responses.
	// TODO(reddaly): The decision about sending over unicast vs. multicast is not
	// yet fully compliant with RFC 6762.  For example, the unicast bit should be

View on GitHub (pinned to 24529f1404)

Solutions

1. Read the wrapped %v cause to identify the socket error (e.g. 'connection refused', 'message too long', 'network is unreachable') and fix the underlying network condition
2. Check that the replying host's network interface is up and multicast/unicast UDP on port 5353 is permitted by the firewall
3. Ensure the mDNS server is not being shut down while queries are still in flight; stop the server cleanly before closing sockets
4. If responses exceed the MTU, reduce the size of records in the zone (fewer TXT entries, fewer IPs) or rely on multicast with truncation handling

Example fix

// before: blindly treating any send failure as fatal
if err := s.sendResponse(uresp, from); err != nil {
	return fmt.Errorf("mdns: error sending unicast response: %v", err)
}
// after: tolerate transient client-gone errors
if err := s.sendResponse(uresp, from); err != nil {
	log.Printf("mdns: skipping unicast response to %v: %v", from, err)
}

// Check socket/network readiness before serving
conn, err := net.ListenMulticastUDP("udp4", nil, mdnsAddr)
if err != nil { return err }
// ensure interface is up
ifaces, _ := net.Interfaces()
up := false
for _, i := range ifaces {
	if i.Flags&net.FlagUp != 0 && i.Flags&net.FlagRunning != 0 { up = true }
}
if !up { return errors.New("no active network interface") }

func isSendErr(err error) bool {
	return err != nil && strings.Contains(err.Error(), "error sending unicast response")
}

err := server.HandleQuery(...)
if err != nil {
	if isSendErr(err) {
		log.Printf("mdns unicast response failed (client may be gone): %v", err)
		return // tolerate; query senders time out and retry via multicast
	}
	return err
}

When it happens

Trigger: Calling s.sendResponse(uresp, from) with a unicast (resp(true)) response inside handleQuery returns a non-nil error, e.g. the socket write to the source address fails (network unreachable, ICMP port unreachable, interface down, or socket closed).

Common situations: The client that sent the query has already shut down its socket or left the network; the response is larger than the path MTU and gets dropped with 'message too long'; a firewall blocks UDP from the mDNS port; the machine's network interface went down mid-query.

Related errors

• failed to bind to any unicast udp port
• failed to bind to any multicast udp port
• [ERR] mdns: Failed to bind to any udp port
• mdns: error sending multicast 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/2ce7838a87b2e1e1. Report an issue: GitHub.