Transparent Proxy for IPv6 traffic under Linux

Question

When maintaining networks, it is often an expedient thing to do to run a transparent proxy. By transparent proxy I mean a proxy that 'hijacks' outgoing connections and runs them through a local service. Specifically I run a linux firewall with squid configured so that all tcp/ip connections fowarded on port 80 are proxied by squid.

This is achived using the iptables 'nat' table, using IPv4.

But iptables for IPv6 does not have a 'nat' table, so I cannot use the same implementation. What is a technique I can use to transparently proxy traffic for IPv6 connections?

Answer 1

A viable way to do this is with the TPROXY rule in iptables, documentation is available here:

• http://wiki.squid-cache.org/Features/Tproxy4#IPv6_Support
• http://www.mjmwired.net/kernel/Documentation/networking/tproxy.txt

This should be supported Squid (>= version 3.2). Using --enable-linux-netfilter and the iptables -t mangle -j TPROXY rule.

Answer 2

iptables has a QUEUE target, which you can use to deliver packets to userspace. I am not sure, but perhaps something could be implemented there.

Past that, you could take a stab at adding something to the kernel to do redirection.

Answer 3

You can't. Quoting from squid-cache.org:

NAT simply does not exist in IPv6. By Design.

Given that transparency/interception is actually a feature gained by secretly twisting NAT routes inside out and back on themselves. It's quite logical that a protocol without NAT cannot do transparency and interception that way.

Answer 4

Here's an implementation:

http://www.suse.de/~krahmer/ip6nat/

Answer 5

Another sort of ugly hack:

• MARK all traffic with iptables (seems, there is CONNMARK target for IPv6)
• route all marked traffic to tun device
• do user-space NAT in the daemon listening at tun device
• ...

Answer 6

Write your own implementation of NAT in IPv6 stack.