Upgrading to DD-WRT on NETGEAR WGR614L or WNR3500L: Demystified

So... you bought a WGR614L or WGR614v8 router, or perhaps the WNR3500 Version L, U or 2 router.  You've also been hearing a lot about this "firmware upgrade" and "DD-WRT" stuff but have no idea where to begin.

If you look around, you will see there are all kinds of firmware versions out there.  Once in a while, you'll hear about people have upgraded to the wrong thing and it "bricked" their router.  You hear that one firmware does one thing and another firmware does something different; some do it better while some others don't, and so on.  It can be confusing and even a bit intimidating.

Port-Mirroring / Span Port / Monitor Port with iptables on NETGEAR WGR614L

I have a NETGEAR WGR614L and I needed to attach a tcp monitor pc (tcpdump) to one of its LAN ports. The fact the the router is a switch makes that not so trivial a task. 

The switch only forwards packets to the port where the destinaion MAC address is known.  As such, each connected network device only receives the packets which are destined for that specific device.

Expensive switches provide a functionality which is called “Port-Mirroring," “Span Port,” or “Monitor Port."  This feature copies all packets from another port to that monitor port.  Sadly, our WGR614L doesn't have that functionality built-in.