At the company I work for we need to implement what I think it's called transparent proxy.
How it's now:
A(lower secured area)--Cisco ASA-----Cisco ASA----B(higher secured area)
What we need:
A(lower secured area)--Cisco ASA---(eth0)Proxy(eth1)---Cisco ASA----B(higher secured area)
We've already set up an alpine linux with squid proxy, added two interfaces for both sides towards the firewalls but hit a wall with the iptables configuration.
The proxy just needs to log traffic and pass through everything, without change to packets on src/dst. We don't need any kind of filtering or blocking, all 1-65535 ports can be allowed.
Read about TPROXY, but couldn't find a good example to try.
I know that there are other design options for an implementations like this, but this is how it must be done.