[tproxy] proxy app and server app on the same host

John Lauro john.lauro at covenanteyes.com
Sun Jan 18 01:59:02 CET 2015

I have done used it with single interface on the box running tproxy and
the server on the same subnet also with a single interface. (Technically
they have extra interfaces with policy based routing for handling outgoing
connections differently, but probably not strictly required unless you are
doing ANYCAST).

I would think that at a minimum, if you run on the same box and want to
reuse the client's ip on the application, you will have to use a secondary
IP on the same interface, or possibly a loopback interface.  You might
have to setup policy based routing from that secondary IP so it stays
local, just a guess.  Is it ok to bind the app to listen on
or some other secondary IP?

Easier would be if you could loose the client IP info on the app (track
or tag it with the proxy) and just recreate a new session...  but I assume
that is not an option...

----- Original Message -----
> From: "U.Mutlu" <for-forums at mutluit.com>
> To: tproxy at lists.balabit.hu
> Sent: Saturday, January 17, 2015 6:56:57 PM
> Subject: [tproxy] proxy app and server app on the same host
> Hello,
> TPROXY works fine if the server application is on a different host,
> and if that host has the proxy host defined as its gateway.
> I have this special tproxy requirement:
> A transparent proxy program needs to sit between client and server
> program,
> ie. like a MITM, and both (proxy pgm and server pgm) must be on the
> same host.
> The host has only one physical interface (eth0).
> So, I have to move the server app to the proxy host.
> But then TPROXYing no more functions.
> I'm sure it's a routing problem, but I couldn't find a solution yet.
> I know some advanced routing (policy routing), ie. using different
> routine
> tables and assigning rules for them etc., but if everything is on the
> same
> host then somewhere it hangs; the SYN-ACK of the server app doesn't
> receive
> the proxy pgm, it seems to go out to the internet.
> I also tried with alias IPs, virtual subnets, and also with tap
> devices,
> unfortunately the results so far are negative :-(
> The author of haproxy writes at their web site
> http://blog.loadbalancer.org/configure-haproxy-with-tproxy-kernel-for-full-transparent-proxy/
> "We also need to ensure that we have the correct architecture for the
> trick to work. Using the normal HAProxy you can have real servers
> anywhere on
> the internet because the source address always points back at the
> HAProxy
> units IP address. However if the clients source IP address is going
> to be used
> then the HAProxy server MUST BE IN THE PATH of the return traffic.
> The easiest way to do this is to put the backend servers in a
> different subnet
> to the front end clients and make sure that the default gateway
> points back at
> the HAProxy load balancer.
> NB. With clever routing this should be possible on the same subnet
> but I
> haven’t tried that yet!
> "
> So, I'm looking for the above said "clever routing" solution.
> Could someone help me please?
> My environment: recent Linux kernel (3.16), Debian 8, x86_64
> Thx
> U.Mutlu
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