Check access to UDP services

To test connectivity with ports accepting TCP connections, I normally use:

$ telnet <machine> <port>

It has the additional feature that once connected, you can interact with the service using the protocol directly. For instance when connecting to a web server you can perform HTTP GETs.

However, with services listening for UDP only, like NTP, telnet doesn't work. You can then use netcat:

$ nc -v -u -z -w 3 123
Connection to 123 port [udp/ntp] succeeded!

Once you get used to using nc, you may even start using it for testing TCP connectivity too:

$ nc -v -z -w 3 80
Connection to 80 port [tcp/http] succeeded!

Easy when you know how :-)

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