Adding a default filter

[sio4]

Is your feature request related to a problem? Please describe.
If we run it without a filter, the client forwards all packets to the server and the server writes them all into the wire. If they are in the same segment, it introduce loop. If the switch detected it, the switch will shutdown the port of server. So... people need to be more careful but how about we have an option for this?

Describe the solution you'd like

• set a default filter.
• add an option something like --nofilter to clear default filter.
• or nofilter as filter to do the same.