FREE LOOK

  Adapted form    http://pritambaral.com/2012/04/transparent-proxy-on-linux/ 

This post is targeted at those Linux users behind a network proxy   but cannot set it in an app, or are just plain lazy to go about telling every app to use a proxy. I�ll give a quick rundown of the instructions for those in haste, with geeky details following towards the end. I�m assuming a fairly recent distro here, and I�m targeting the Ubuntu 12.04 release.

What we�ll need:

• A Linux OS (obviously!)
• redsocks (an app, check in your repos, or compile it yourself)
• a text-editor

 On Ubuntu, we have another package that we�ll need, iptables-persistent. But before we install it, let�s set some rules (iptables rules.) Don�t worry, it�s nothing but a bunch a lines in a text file.

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A OUTPUT -d 10.0.0.0/8 -j RETURN
-A OUTPUT -d 127.0.0.0/8 -j RETURN
-A OUTPUT -d 192.168.0.0/16 -j RETURN
-A OUTPUT -o eth0 -p tcp -m tcp --dport 80 -j DNAT --to-destination 127.0.0.1:5123
-A OUTPUT -o eth0 -p tcp -m tcp --dport 443 -j DNAT --to-destination 127.0.0.1:5124
-A OUTPUT -o wlan0 -p tcp -m tcp --dport 80 -j DNAT --to-destination 127.0.0.1:5123
-A OUTPUT -o wlan0 -p tcp -m tcp --dport 443 -j DNAT --to-destination 127.0.0.1:5124
COMMIT

I saved it as redirect.rules and ran this command:

sudo iptables-restore ./redirect.rules

Now we shall install the package iptables-persitent. During the installation, it will ask you whether you want to save the current rules. Yes, you do. The redirection�s been set-up. Time to get the juicer running.
Install redsocks (if you haven�t already.) Save this in the file /etc/redsocks.conf

base {
 log_debug = off;

 log_info = off;

 log = "stderr";

 daemon = on;

 user = redsocks;
 group = redsocks;

 redirector = iptables;
}

redsocks {
 /* `local_ip defaults to 127.0.0.1 for security reasons,
 * use 0.0.0.0 if you want to listen on every interface.
 * `local_* are used as port to redirect to.
 */
 local_ip = 127.0.0.1;
 local_port = 5123;

// `ip and `port are IP and tcp-port of proxy-server
 ip = 10.201.13.50;
 port = 80;

// known types: socks4, socks5, http-connect, http-relay
 type = http-relay;

login = "LDAP ID HERE";
 password = "LDAP PASSWORD HERE";
}

redsocks {
 local_ip = 127.0.0.1;
 local_port = 5124;

ip = 10.201.13.50;
 port = 80;

type = http-connect;

login = "LDAP ID HERE TOO";
 password = "LDAP PASSWORD HERE TOO";
}

Make sure you feed your own LDAP IDs and passwords. AT BOTH LOCATIONS. Feed your password as-it-is, no matter what special character it has. (Unless, of course, it�s a double-quote itself! Bit of a soup there.)
Now either restart your system, or run sudo service redsocks start
Voila! You have the ultimate solution to proxy! You may (or may-not) set a proxy in Gnome, Firefox, wget, gedit, whatever; it will work. This will not interfere with what you have set in Chrome/Firefox/whatever. In fact, I recommend that you explicitly set a proxy wherever you can. You see, redsocks has a knack of getting in a bundle sometimes (too many pending requests�,) in which case, a simple sudo service redsocks restart should suffice.

This also �fixes� those apps which provide no method of setting a proxy whatsoever, e.g, Gwibber. Sadly, unsupported protocols still won�t work. Sorry, no Thunderbird, no torrents, no irc.
PS: This is the exact same method used by the Android app ProxyDroid to provide system-wide proxy on rooted Android devices.