IPTables configuration for Transparent Proxy

Question

I am confuse why my IPTable does not work in Router. what I'm trying to do is redirect any packets from source ip destined to port 80 and 443 to 192.168.1.110:3128. however when I tried this:

 iptables -t nat -A PREROUTING -s 192.168.1.5 -p tcp --dport 80:443 -j DNAT --to-destination 192.168.1.110:3128

does not work. however when I add this,

iptables -t nat -A POSTROUTING-j MASQUARADE

it works. but the problem with masquarade is I do not get the real ip but instead the ip of the router. I need to get the source ip so my proxy server could record all ip connected to it. can some one tell me how to make it work without making POSTROUTING jump to Masquarade?

Answer 1

For real transparent proxying you need to use the TPROXY target (in the mangle table, PREROUTING chain). All other iptables-mechanisms like any NAT, MASQUERADE, REDIRECT rewrite the IP addresses of the packet, which makes it impossible to find out where the packet originally was intended to.

The proxy program has to bind() and listen() on a socket like any other server, but needs some specific socket flags (which requires some Linux capabilities (type of permission) or root). – Once connected, there is some way to get the “intended server” from the OS.

Sorry, I’m a little lazy about the details, but searching for “TPROXY” as keyword will get you going quickly!