Q:

telnet counterpart for UDP

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.

A:

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. 

A:

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.

A:

In Windows, I use PortQry for this: http://www.microsoft.com/en-us/download/details.aspx?id=17148

