NAT Packet Analysis Using Wireshark
One of the most popular questions I get when people get the hang of protocol analysis is the daunting exercise of multitrace analysis. As with anything else the best advice is to start with the basics before tackling anything complicated.
Multitrace analysis is only effective if you truly understand your vendors products, networking and how it relates to the OSI model or packet analysis. I always suggest that you start at layer 1 and work yourself up. The key is to know what fields in the frame or packet changes, or remains the same. Ideally when you figure this out you can use a better capture or display filter
A multitrace capture of a hub, switched, or bridged network is most straight forward since a hub or switch is transparent at layer 1 or 2 and doesn’t change anything in the packet.
When you move up to layer 3 or routing, several things change in the packet such as MAC address, IP TTL and TOS. Of course your mileage will vary, and any device could be configured to muck with more bits in the packet, but I figure I would give you a point of reference.
At layer 4 we get into application gateways, proxy, firewalls and NAT type devices where the following packet fields gets modified; MAC address, IP address, IP TOS, TCP/UDP port numbers, TCP ACK/SEQ values, etc.
Lastly at layer 7, we are dealing with multi-tiered applications and basically everything changes in the packet.
In this video example I do a multitrace analysis of a simple netgear router/NAT/firewall device where I take a trace from the WAN and LAN side to compare. Not to sound like a broken record, but please remember that your devices might behave totally differently and these notes and techniques should only be used as a reference in your environment.