Add option to specify custom BPF filter

[buger]

Can be useful in case of non standard network interfaces when like
tunnels or SPAN ports, when IP of interface does not match with IP of
packet.

Can be used to read traffic from multiple ports and interfaces at once
as well.

Example: --input-raw-bpf-filter 'dst port 80'

[michaelvilensky]

Just to notice that in case of using "trackResponse", the user will have to manually add filter of src port equals port.

if t.trackResponse {
	bpf = "(tcp dst port " + strconv.Itoa(int(t.port)) + " and (" + bpfDstHost + ")) or (" + "tcp src port " + strconv.Itoa(int(t.port)) + " and (" + bpfSrcHost + "))"
} else {
	bpf = "tcp dst port " + strconv.Itoa(int(t.port)) + " and (" + bpfDstHost + ")"
}

[buger]

That's true, if we simplify: tcp src port 80 and tcp dst port 80

[michaelvilensky]

Won't work:

sudo ./gor --input-raw-bpf-filter 'tcp dst port 8000' --output-stdout
Version: DEV-1501168199
2017/08/03 12:40:31 Required at least 1 input and 1 output

Also, I'm not sure if you are not making use of port (--input-raw :PORT) in other places in the code.
For instance in file: input_raw.go

i.listener = raw.NewListener(host, port, i.engine, i.trackResponse, i.expire)

[DagW]

"Can be used to read traffic from multiple ports and interfaces at once
as well."

Im not sure how to use this to listen to multiple ports? Mabe someone can bring som clarity to how to could be done?