icmp port unreachable error message

12
votes

I am sending UDP packets from one PC to another. I am watching the the whole activity using Wire Shark. I notice that for some time there is smooth transmission of packets from one system to the other. Then suddenly ICMP packets with error 'port unreachable' starts to appear. Then they disappear for some time and there is again smooth transmission of UDP packets. Then these ICMP packets again appear with the same 'port unreachable' error. Then these ICMP packets disappear again and so on. And this continues in a periodic manner.

Can anybody shed some light that what could be the reason for it.

2
icmp
Connection quality is poor? – Adrian
the connection quality is quite good. Not really sure why are these appearing – aaaa
Another service may be using that port(NAT for example) – awiebe
is there any cmd or any other method which can tell me that which ports are free and which ports are being used by other services. – aaaa
Any body any input :( ? – aaaa

2 Answers

6
votes

One odd error that might be associated with a Port Unreachable message is seen when an otherwise normally operating conversation is interrupted by a Port Unreachable message. When you inspect the conversation you observe that the unreachable port was working without a problem. Frames were going to and from the port number when, suddenly - Port Unreachable. This is indicative of an overload condition or process priority configuration problem in the reporting host. The process in question was swapped out of memory and was not able to swap back in quickly enough to avoid the unreachable indication.

Source: https://www.savvius.com/resources/compendium/tcp_ip/unreachable#port_unreachable

4
votes

The reason this occurs is that there is no process on the receiver that waiting on that port.

You need to have a client, that has a socket open, and has done the bind() to that port.