How can i test a udp connection

Asked 9 years, 3 months ago. Active 1 year, 6 months ago. Viewed k times. Both servers are internet facing. I am using netcat to have a certain port listening.

I then use nmap to check for that port to see if it is open, but it doesn't appear to be. Iptables is turned off. Improve this question. Lock Lock 1, 6 6 gold badges 25 25 silver badges 33 33 bronze badges. I've respondend to the "Testing UDP port connectivity" question. Add a comment. Active Oldest Votes.

Improve this answer. Anthony Geoghegan 2, 1 1 gold badge 21 21 silver badges 33 33 bronze badges. Luke Luke 5, 3 3 gold badges 41 41 silver badges 55 55 bronze badges.

Think I'm understanding it now. So a netstat on the server with the listening udp port will never show the remote host Only a tcpdump should show the remote requests? Both netstat and tcpdump have the ability to dump data on you, the latter in a more human-readable form. Check out their man pages for details.

To test if udp port is responding, use netcat. An example from the man page : nc -v -u -z -w 3 example. This response gave me a false-positive, where, in contrast, Sasha's answer exhibits what I expect.

Sasha Sasha 5 5 silver badges 4 4 bronze badges. I like this clever answer, because it feeds my own understandings and misunderstandings! Too many factors. This response gives a really simple, definitive call and response which either works or doesn't work, change config, rinse, and repeat.

This solution is wrong according to nc man page last lines : CAVEATS UDP port scans using the -uz combination of flags will always report success irrespective of the target machine's state.

It used raw IP packets to check which ports and operating system are available and running. It also checks for the firewall in case if it blocks the port. Various tools are available in the market to scan the port and check which port is open and close. These tools run on the client and server machine simultaneously.

They are available for cross platforms, including Windows, Mac, and Linux. This command is used to scan the UDP port. Ports to be scanned need to be specified where —sU activates UDP port scan. In this example, the port range is from 1 to at the node Also, instead of scanning the range of port, we can specify a specific port number.

This will scan the port and check if the port is open or closed. UDP port scan attack can occur when some attacker sends packets on your machine. This varies on the destination port. This can let attacker determine which server application service you are running and which operating system you have. You need not worry about this attack unless you have a firewall on your system to protect from the attackers.

These tools use local application. Note: When you run the nc -ul command on the server, it will only connect for the first connection coming to it.

You can't, as I found out, switch between servers pinging it without stopping and restarting nc -ul. Testing open UDP ports with nmap is fraught with perils -- there's no three-way handshake to indicate openness.

Unless the listening process responds to whatever nmap sends, there's no way for nmap to differentiate between an open port that isn't responding and a filtered port.

Much easier is just to listen on one end with netcat and use netcat at the other end to send packets, and see they arrive at the other end. Do it both ways just be sure. You can also tcpdump to see the packets getting to where they need to go.