Just knowing which applications are using a given port may not be sufficient information to trace down a problem. Sometimes you need to check the data that is being transferred as well.
Tracing packets with tcpdump
Getting ready
You need to be a root user to run tcpdump. The tcpdump application may not be installed in your system by default. So install it with your package manager:
$ sudo apt-get install tcpdump $ sudo yum install libpcap tcpdump
How to do it...
The tcpdump application is the frontend to Wireshark and other network sniffer programs. The GUI interface supports many of the options we'll describe...