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To know which service use which udp port:
grep /udp /etc/services
To know which udp port is currently in use (which mean a local process is listening on this port):
To know which port is used on remote machine:
nmap -PU remote_machine
Typical services can be found in a file like /etc/services (at least that's what it is on Debian). This should be the actual mapping for the system you're using. You can also go to iana.org and look up the assigned ports there.
Many services (such as mail), although assigned a port by IANA, can always be reconfigured to use a different port. You need to look at the configuration files of the various services to see what ports they will actually use. In some cases where the port may be hard-coded, you will need to look at the source for your particular service.
There are thousands of little-used servers which will open up ports at just about any address; in such cases you need to look at the configuration and the source.
You can use various network toolkits to see what ports are open for 'listening'.
/etc/services is present on AIX, HP-UX, BSD and many unices and unix-like systems. It's the standard way I believe (althgouth I don't know the iso number or the norm it fulfills).
Anyway, the question was indeed vague. 'configuring' UDP port doesn't really mean anything, and neither does 'finding out'. Maybe you could tell us what you are trying to do, or elaborate on what kind of configuration you are asking for?
A port isn't listening to anything. An application is listening on a port.
The UDP port is the phone number. The phone number isn't listening to anything. If nobody is home, nobody is listening to the phone. What you need is somebody answering the phone when the other one calls and that someone should talk the same language as the caller.
That somebody is an application. Ekiga, for instance, is an application that talks SIP and would gladly answer if someone called on port 5060 to talk SIP.