How to Examining Local Network Activities in a system

Problem: Want to examine network use occurring on your local machine.

Solution: To print a summary of network use,

# netstat --inet             connected sockets
# netstat --inet --listening  Server sockets
# netstat --inet --all        Both
 
To assign dynamically assigned ports for RPC processes,

# rpcinfo -p [host]

To list network connections for all processes:

# lsof -i[tcp][udp][@host][:port]

To list all open files for specific processes:

# lsof -p pid
# lsof -c command
# lsof -u username

To list all open files for all processes

# lsof

You can also select processes by ID (-p), command name (-c), or username (-u)
# lsof -a -c myprog -u tony

Note: Programs like ps, netstat, and lsof obtain information from the linux kernel via the /proc filesystem.
      
The most important files for networking are /proc/net/tcp and /proc/net/udp, both consulted by netstat. Kernel parameters related to networking can be found in the /proc/sys/net directory.

Information for individual processes is located in /proc/<pid> directories, where <pid> is the process id, for example, the file
 /proc/12345/cmdline contains the orginal command line that invoked the (current running) process 12345.