Misconfigured or Missing Services File
You should also verify that the services file in use on your system matches up with the well-known port numbers expected by the application. Some applications will ask the system for the port number associated with TFTP, for example, and if your system’s services file does not have an entry for that application, it will not return a port number to the client. This problem will prevent the client from being able to send any data, since it cannot get the destination port number for the application.
To see the well-known ports used on your system, examine the /etc/services file on a Unix host, or the C:\WinNT \System32\Drivers\Etc\SERVICES file on a Windows NT host.
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