iptables redirects packets to local proxy

Question

consider this scenario:

PC1 wants to telnet to SRV1 tcp 40000.

I would like to put a proxy in the middle that receives the connection, rewrites the destination, sends packet to a local socks proxy (redsocks).

So the scenario would be:

PC1 --> PROXY tcp 20000 [iptables rewrites destination to SRV tcp 40000 --> redsocks 127.0.0.1 tcp 12345] --> SRV1 tcp 40000

I tried:
iptables -t nat -A PREROUTING -p tcp -s $MYSOURCEIP --dport 20000 -j DNAT --to $SRV1:40000

It works, but I know it doesn't go through redsocks. I just changed the destination for the packets but how can I force them to flow into redsocks?

Comments

Use -j REDIRECT --to 12345

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If I get it correctly,  I should redirect to 127.0.0.1:12345 after having changed destination address and port to the packets. Is that possible anyway?

Answer 1

You have to pick where you want to change the destination to. Either x:12345 or y:40000 - can't have it both ways. Seems the socks proxy should be configured to send the traffic to y:40000.

Comments

Without using iptables at all for redirecting, socat can do it all.

For example:
socat TCP4-LISTEN:20000,reuseaddr,fork
SOCKS4:127.0.0.1:A.B.C.D:40000,socksport=12345