Where is the documentation on how to configure a plain Linux box (probably running Ubuntu), with two NIC's so that eth0 is the input from the Internet and eth1 is the output to a wireless router?
While the traffic is passing through, I'd like to analyze it.
Use an existing firewall distribution such as smoothwall or ipcop. That will save you a ton of trouble and headache later. If you really want to get into the nitty-gritty of advance router configuration, check out LARTC.
this is nothing special, all you need is a good firewall script or framework that will NAT your traffic.
Take a look at this link to get started: http://ubuntuforums.org/showthread.php?t=713874
Searching google will return countless hits as this topic has been thoroughly investigated.
I use shorewall running on ubuntu. Very easy to set up and maintain, but it still has all the options you will likely need: http://www.shorewall.net/two-interface.htm
Here's a script I use on some debian systems:
#!/bin/sh
## This script starts an NAT internet gateway
## Settings
# Internal Interface
INTERNAL=eth1
# External Interface
EXTERNAL=eth0
## Ensure the modules are loaded
# Load the NAT module
modprobe iptable_nat
# Load the transparent FTP modules
modprobe ip_nat_ftp
modprobe ip_conntrack_ftp
# Load the transparent IRC modules
modprobe ip_nat_irc
modprobe ip_conntrack_irc
modprobe ipt_MASQUERADE
modprobe ipt_REDIRECT
modprobe ipt_REJECT
modprobe iptable_filter
modprobe ip_conntrack
modprobe ip_tables
## Flush the routing tables
iptables --flush
iptables --table nat --flush
iptables --delete-chain
iptables --table nat --delete-chain
## Set up the NAT table
iptables -t nat -A POSTROUTING -o $EXTERNAL -j MASQUERADE
iptables --append FORWARD --in-interface $INTERNAL -j ACCEPT
# Ensure that IP forwarding is on
echo 1 > /proc/sys/net/ipv4/ip_forward
