badunk's questions

If a proxy receives the tcp packet, then the destination ip address and port are that of the proxy. In that case, I imagine the only way a proxy can still resolve the intended destination is either through routing the source ip address/port or through the host field in the http header. Is this correct?

In both Fiddler and Charles http proxies, I noticed that the tool accepts both http and https connections through a single port that you can specify. What do these tools do to tell the difference between the two types of connections?

I've got a linux box set up with 2 network cards to inspect traffic going through port 80. One card is used to go out to the internet, the other one is hooked up to a networking switch. The point is to be able to inspect all HTTP and HTTPS traffic on devices hooked up to that switch for debugging purposes.

I've written the following rules for iptables:

nat

-A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j DNAT --to-destination 192.168.2.1:1337
-A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 1337

-A POSTROUTING -s 192.168.2.0/24 -o eth0 -j MASQUERADE

On 192.168.2.1:1337, I've got a transparent http proxy using Charles (http://www.charlesproxy.com/) for recording.

Everything's fine for port 80, but when I add similar rules for port 443 (SSL) pointing to port 1337, I get an error about invalid message through Charles.

I've used SSL proxying on the same computer before with Charles (http://www.charlesproxy.com/documentation/proxying/ssl-proxying/), but have been unsuccessful with doing it transparently for some reason. Some resources I've googled say its not possible - I'm willing to accept that as an answer if someone can explain why.

As a note, I have full access to the described set up including all the clients hooked up to the subnet - so I can accept self-signed certs by Charles. The solution doesn't have to be Charles-specific since in theory, any transparent proxy will do.

Thanks!

Edit: After playing with it a little, I was able to get it working for a specific host. When I modify my iptables to the following (and open 1338 in charles for reverse proxy):

nat

-A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j DNAT --to-destination 192.168.2.1:1337
-A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 1337

-A PREROUTING -i eth1 -p tcp -m tcp --dport 443 -j DNAT --to-destination 192.168.2.1:1338
-A PREROUTING -i eth1 -p tcp -m tcp --dport 443 -j REDIRECT --to-ports 1338

-A POSTROUTING -s 192.168.2.0/24 -o eth0 -j MASQUERADE

I am able to get a response, but with no destination host. In the reverse proxy, if I just specify that everything from 1338 goes to a specific host that I wanted to hit, it performs the hand shake properly and I can turn on SSL proxying to inspect the communication.

The setup is less than ideal because I don't want to assume everything from 1338 goes to that host - any idea why the destination host is being stripped?

Thanks again