IPtables redirect /port forwading question problems

Question

Im trying to setup a transparent proxy using IPtables but im not having much luck.

What I was hoping to do was forward all outgoing traffic to port 9040 and block everything else. Any ideas?

iptables -t nat -A OUTPUT ! -o lo -p tcp -m owner --uid-owner Mark -m tcp -j REDIRECT --to-ports 9040 

iptables -A OUTPUT -m --uid-owner Mark -j DROP

I receive the following error -

(WARNING): UDP write to 68.32.239.182:21755 failed: Operation not permitted

Thank-you

Answer 1

The problem is that your rule is too restrictive.

iptables -t nat -A OUTPUT ! -o lo -p tcp -m owner --uid-owner Mark -m tcp -j REDIRECT --to-ports 9040 
iptables -A OUTPUT -m --uid-owner Mark -j DROP

The first rule redirects all TCP traffic to port 9040, without trying to discern whether it is something proxy-able (e.g., HTTP) or non-proxy-able (e.g., SSH).

The second rule drops all traffic, TCP, UDP, whathaveyou.

You should delete the second rule, and set the OUTPUT chain's policy to ACCEPT. That will allow all traffic to go through. (Although it will not fix the problem of non-proxy-able traffic gets redirected to the proxy).