In my Mac(, it will send ICMP packet to my house's router ( with the info Destination unreachable (Port unreachable) enter image description here

and in the Wireshark packet detail panel, there displays the packet structure:


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and more details:

the payload is IP:UDP (from router to mac), and you see the router port is 53, it related to DNS.

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and in my macOS, there is no application listening the port:

$ netstat -an | grep 55166 get nothing.

What's the protocol use the packet? and what's the intension of the packet?

  • Src port is 53. It means DNS. DNS is udp. As i think, it is connecting via DNS
    – infra
    Aug 27, 2020 at 4:08
  • Unfortunately, questions about home networking and consumer-grade devices are explicitly off-topic here. You could try to ask this question on Super User.
    – Ron Maupin
    Aug 27, 2020 at 13:01

2 Answers 2


ICMP is used to signal network layer (IP) or transport layer (TCP, UDP, ...) errors.

In this case, the ICMP sender informs the initial sender that a transport layer port cannot be reached. The ICMP payload contains parts of the problem packet with the network and transport layer headers. In your case that seems to have been a DNS reply.

Note that host configurations and issues are off-topic here, so we can't dive into the actual reason for the rejection.


Your displaying packet is a message sent via ICMP.

the message means from router's DNS response to your mac unreachable. ->

You can check your firewall on your macOS, or use Wireshark to capture the while process.

set the filter in your Capture filter:

(udp) and (host and (port 53)

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