Suzana Lojic-Skoric wrote:
From: Jörg Harmuth <harmuth@xxxxxxxxx>
To: netfilter@xxxxxxxxxxxxxxxxxxx
Subject: Re: DNS and NAT
Date: Fri, 15 Jul 2005 10:53:17 +0200
Suzana Lojic-Skoric schrieb:
> I don't think proxy can help because it is just caching the web pages,
> it does not change the IP addresses. I'll check if tunneling can help,
> if not then I'll have to change iptables to inspect DNS answer and
> replace the IP in the payload.
No. Introducing a proxy at the right location, is much more than just
caching web sites. It means significant changes to at least to the IP
headers.
Wether a proxy helps you or not depends totally on where you place the
proxy. If you place it on the nat box (like primero said) or between
this nasty dropping box and the nat box, everything is probably fine.
The requests will then go to 10.x.x.x and the answers will originate
from 10.x.x.x. The e.g. google address of 216.239.39.99 is within the
*data* part of the 4th packet - not in the headers (headers are
src=10.y.y.y dst=10.x.x.x). As long as the nasty dropping box doesn't
scan the packets payload for proxy requests and the like and drops them,
everything should work.
I can put the proxy on the NAT machine.
As I said, right now just with the NAT, if I send a DNS request for
the google.com from the client 10.0.0.1 behind the nasty dropping box,
it will go out through the nasty dropping box and the NAT gateway. NAT
will change its 10.x.x.x source and destination from 10.x.x.x to some
outside addresses e.g. 150.x.x.x. The DNS answer comes back to NAT,
it's source and destination gets translated back to 10.x.x.x and
10.0.0.1 destination, and the google address 216.239.39.99 is within
the *data* part. This goes fine through the nasty dropping box back to
the client 10.0.0.1. Client then takes the answer from the data part
of the message, which is 216.239.39.99 and tries to contact it. It
sends an HTTP message to destination 216.239.39.99. This gets dropped
on the nasty dropping box since it is not 10.x.x.x (This is what's
happening when you type in www.google.com in the browser on the client
10.0.0.1).
So the DNS request and answer can get through the internal network,
but what I need is to somehow replace the 216.239.39.99 that is
embedded in the DNS *data* with 10.z.z.z. Also my NAT needs to know
that 10.z.z.z is actually 216.239.39.99. to be able to translate it
for outside.
Do you still think proxy can help?
with a *standard proxy* configured on the browser of client 10.0.0.1
your request for 216.239.39.99 will be in the payload of the proxy
request that has the IP address of your proxy machine in the destination
address field of the network layer ... it should be good for your nasty
dropping box.
From there the HTTP request will be managed from your proxy wich will
answer to your client with a connection completely inside the 10.x.x.x
network.
bye
<f>
