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Is there anything that enables a "telnet-like" functionality for UDP? I know the difference between TCP and UDP, and why telnet itself won't work - but I'm wondering if there is something similar to the telnet client, from the end-user perspective. E.g. udp-telnet [ip] [sending-port] [receiving-port] which then prints out wether a packet made it back or not.

Having a tool like this would proove helpful for testing out firewall settings for OpenVPN which uses UDP connections.

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Note that by the very nature of UDP, it's impossible for a client to know whether a packet was delivered -- the server does not send an acknowledgement of received data on the protocol level. – josh3736 Jun 4 '12 at 17:06
Also note that in UDP, order of delivery is not guaranteed. That means that the following packets: "cd /". "cd temp", "rm -rf *" could lead to results you might not expect or enjoy. The way routing works, it's likely that the first packet arrives after the second and possibly third--the first often gets held up for a few seconds while routes are resolved--other packets can slip by before it's requeued. – Bill K Jun 4 '12 at 17:38
up vote 21 down vote accepted

You can use netcat - just start it, and type something inside, and pres the return key.

nc -u <host> <port>

And on the other side you can listen with netcat too (you should see the written text), or just start a tcpdump, and see packets coming in.

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On the other side you could use : tcpdump 'port <port>' don't forget to type some messages after starting nc -u on the client ! – Svenn Dhert Nov 7 '15 at 21:07

You can use netcat or nc. It supports TCP and UDP. Use the -u for UDP sockets. You can read man nc for more details.

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In Windows, I use PortQry for this:

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