I've searched around for an answer to this question, but it seems like most of what I found wasn't specifically what I wanted to do (mostly redirecting traffic on the same host to squid running on that host).
If at all possible, I'd like to know how to use iptables (or similar, say, SuSEFirewall2 if on opensuse) to mimic the system proxy settings (ie: HTTP_PROXY_HOST in /etc/environment in Ubuntu).
The reason is, not everything uses that setting. Many services require their own proxy settings. I'm hoping to just "catch it all" at the network layer. So if I have phpcurl calling a remote webservice, I don't have to define a proxy there. Same for apt/zypper/yum, and so on. I don't want to have to configure proxy settings 25 different times on the same server for all the various services.
I've tried different variations of this (in this case, sending traffic to a Squid proxy):
iptables -t nat -A OUTPUT -p tcp -o lo –dport 80 -j DNAT –to 192.168.0.34:3128
But it doesn't work. I've set a firewall rule to block all outbound internet connectivity from this host and fired up tcpdump on the squid box to see if any packets get there (they don't). I can set my browser to use the Squid proxy on my laptop and it works, but in case I didn't have squid set up right, I figured I'd at least see the initial traffic sent to squid via tcpdump.
If someone has a better idea of how to handle this, meeting the requirements above, I'm open to suggestions.
Best Answer
You simply can't do this with a iptables alone and a http proxy like squid, because most network protocols simply were not designed with transparent proxy as a feature. They must be explicitly configured to use a proxy. The closest you can get is to use a socks proxy like Redsocks