http://justhowto.info/home_open/document-432
6. Transparent Proxy to a Remote Box
Now, the question naturally arises, if we can do all this nifty stuff
redirecting HTTP connections to local ports, could we do the same
thing but to a remote box (e.g., the machine with squid running is
not the same machine as iptables is running on). The answer is yes,
but it takes a little different magic words. If you only want to
redirect to the local box (the normal case), skip this section.
For the purposes of example commands, let's assume we have two boxes
called squid-box and iptables-box, and that they are on the network
local-network. In the commands below, replace these strings with the
actual IP addresses or name of your machines and network.
I will present two different approaches here.
6.1. First method (simpler, but does not work for some esoteric
cases)
First, we need to machine that squid will be running on, squid-box.
You do not need iptables or any special kernel options on this
machine, just squid. You *will*, however, need the 'http_accel'
options as described above. (Previous version of this HOWTO suggested
that you did not need those options. That was a mistake. Sorry to
have confused people...)
Now, the machine that iptables will be running on, iptables-box You
will need to configure the kernel as described in section 3 above,
except that you don't need the REDIRECT target support). Now, for the
iptables commands. You need three:
· iptables -t nat -A PREROUTING -i eth0 -s ! squid-box -p tcp --dport
80 -j DNAT --to squid-box:3128
· iptables -t nat -A POSTROUTING -o eth0 -s local-network -d squid-
box -j SNAT --to iptables-box
· iptables -A FORWARD -s local-network -d squid-box -i eth0 -o eth0
-p tcp --dport 3128 -j ACCEPT
The first one sends the packets to squid-box from iptables-box. The
second makes sure that the reply gets sent back through iptables-box,
instead of directly to the client (this is very important!). The last
one makes sure the iptables-box will forward the appropriate packets
to squid-box. It may not be needed. YMMV. Note that we specified '-i
eth0' and then '-o eth0', which stands for input interface eth0 and
output interface eth0. If your packets are entering and leaving on
different interfaces, you will need to adjust the commands
accordingly.
Add these commands to your appropriate startup scripts under
/etc/rc.d/
(Thanks to Giles Coochey for help writing this section).
6.2. Second method (more complicated, but more general)
Our first shot at this works good, but there is a minor drawback in
that HTTP/1.0 connections without the Host header do not get handled
properly. Connections that are fully or partially HTTP/1.1 compliant
work fine. As most modern web browsers send the Host header, this is
not a problem for most people. However, some small programs or
embedded devices may send only very simple HTTP/1.0 requests. If you
want to support these, we'll need to do a little more work. Namely,
on iptables-box we'll need the following options enabled in the kernel
in addition to what was specified above:
· IP: advanced router
· IP: policy routing
· IP: use netfilter MARK value as routing key
· IP: Netfilter Configuration -> Packet mangling
· IP: Netfilter Configuration -> MARK target support
You'll also need the iproute2 tools. Your distribution probably
already has them installed, but if not, look at
ftp://ftp.inr.ac.ru/ip-routing/
You'll want to use the following set of commands on iptables-box:
· iptables -t mangle -A PREROUTING -j ACCEPT -p tcp --dport 80 -s
squid-box
· iptables -t mangle -A PREROUTING -j MARK --set-mark 3 -p tcp
--dport 80
· ip rule add fwmark 3 table 2
· ip route add default via squid-box dev eth1 table 2
Note that the choice of firewall mark (3) and routing table (2) was
fairly arbitrary. If you are already using policy routing or
firewall marking for some other purpose, make sure you choose
unique numbers here. Otherwise, don't worry about it.
Next, squid-box. Use this command, which should look remarkably
similar to a command we've seen previously.
· iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT
--to-port 3128
As before, add all of these commands to the appropriate startup
scripts.
Here is a brief explanation of how this works: in method one, we used
Network Address Translation to get the packets to the other box. The
result of this is that the packet gets altered. This alteration is
what causes some kinds of clients mentioned above to fail. In method
two, we use a magic thing called policy routing. The first thing we
do is to select the packets we want. Thus, all packets on port 80,
except those coming from squid-box itself, are MARKed. Then, when the
kernel goes to make a routing decision, the MARKed packets aren't
routing using the normal routing table that you access with the
``route'' command but with a special table. This special table has
only one entry, a default gateway to squid-box. Thus, the packet is
sent merrily on it's way without every having been altered. So, even
HTTP/1.0 connections can be handled perfectly. (Thanks to Michal
Svoboda for suggesting and helping to write this section)
6.3. Method One: What if iptables-box is on a dynamic IP?
If the iptables-box is on a dynamic IP address (e.g. a dialup PPP
connection, or a DHCP assigned IP address from a cable modem, etc.),
then you will want to make a slight change to the above commands.
Replace the second command with this one:
· iptables -t nat -A POSTROUTING -o eth0 -s local-network -d squid-
box -j MASQUERADE
This change avoids having to specify the IP address of iptables-box in
the command. Since it will change often, you'd have to change your
commands to reflect it. This will save you a lot of hassle.
7. Transparent Proxy With Bridging
Warning, this is really esoteric stuff. If you need it, you'll know.
If not, skip this section. Thanks to Lewis Shobbrook
(lshobbrook@fasttrack.net.au) for contributing to this section.
If you are trying to setup a transparent proxy on a Linux machine that
has been configured as a bridge, you will need to add one additional
iptables command to what we had in section 5. Specifically, you need
to explicitly allow connections to the machine on port 3128 (or any
other port squid is listening on), otherwise the machine will just
forward them over to the other interface like a good little bridge.
Here's the magic words:
· iptables -A INPUT -i interface -p tcp -d your_bridge_ip -s local-
network --dport 3128 -m state --state NEW,ESTABLISHED -j ACCEPT
Replacing interface with the interface that corresponds to
your_bridge_ip (typically eth0 or eth1). First time bridge users
should also note that you'll probably want to repeat the same
command with ``3128'' replaced by ``telnet'' if you want to
administer your bridge remotely.
Вариант для распечатки
OpenNET: Виртуальная конференция (Public)
on 20-Апр-06, 19:08 
