This is a modified version of original redsocks. 
This variant is useful for anti-GFW (Great Fire Wall). 
This is a modified version of original PREROUTING. 
This is a modified version which is work on OpenWrt or Tomato and DD-WRT. 
It provides the following advanced features: 

Program has following command-line options: 
-c   sets proper path to config file ("./PREROUTING.conf" is default one) 
-t   tests config file syntax 
-p   set a file to write the getpid() into 

Following signals are understood: 
SIGUSR1 dumps list of connected clients to log 
SIGTERM and SIGINT terminates daemon, all active connections are closed 

You can see configuration file example in PREROUTING.conf.example 

redsocks {
 local_ip = 192.168.1.1;
 local_port = 1081;
 ip = 192.168.1.1;
 port = 9050;
 type = socks5; // I use socks5 proxy for GFW'ed IP
 autoproxy = 1; // I want autoproxy feature enabled on this section.
                // The two lines above have same effect as
                //    type = autosocks5;
                // in previous release.
 // timeout is meaningful when 'autoproxy' is non-zero.
 // It specified timeout value when trying to connect to destination
 // directly. Default is 10 seconds. When it is set to 0, default
 // timeout value will be used.
 timeout = 10;
 //type = http-connect;
 //login = username;
 //password = passwd;
}

redsocks {
 local_ip = 192.168.1.1;
 local_port = 1081; //HTTP should be redirect to this port.
 ip = 192.168.1.1;
 port = 8080;
 type = http-relay; // Must be 'htt-relay' for HTTP traffic. 
 autoproxy = 1; // I want autoproxy feature enabled on this section.
 // timeout is meaningful when 'autoproxy' is non-zero.
 // It specified timeout value when trying to connect to destination
 // directly. Default is 10 seconds. When it is set to 0, default
 // timeout value will be used.
 timeout = 10;
}
redsocks {
 local_ip = 192.168.1.1;
 local_port = 1082; // HTTPS should be redirect to this port.
 ip = 192.168.1.1;
 port = 8080;
 type = http-connect; // Must be 'htt-connect' for HTTPS traffic. 
 autoproxy = 1; // I want autoproxy feature enabled on this section.
 // timeout is meaningful when 'autoproxy' is non-zero.
 // It specified timeout value when trying to connect to destination
 // directly. Default is 10 seconds. When it is set to 0, default
 // timeout value will be used.
 timeout = 10;
}

Create new chain
 iptables -t nat -N PREROUTING

Ignore your PREROUTING server's addresses
It's very IMPORTANT, just be careful.
 iptables -t nat -A PREROUTING -d 123.123.123.123 -j RETURN

Ignore LANs and any other addresses you'd like to bypass the proxy
See Wikipedia and RFC5735 for full list of reserved networks.
See ashi009/bestroutetb for a highly optimized CHN route list.
 iptables -t nat -A PREROUTING -d 0.0.0.0/8 -j RETURN
 iptables -t nat -A PREROUTING -d 10.0.0.0/8 -j RETURN
 iptables -t nat -A PREROUTING -d 127.0.0.0/8 -j RETURN
 iptables -t nat -A PREROUTING -d 169.254.0.0/16 -j RETURN
 iptables -t nat -A PREROUTING -d 172.16.0.0/12 -j RETURN
 iptables -t nat -A PREROUTING -d 192.168.0.0/16 -j RETURN
 iptables -t nat -A PREROUTING -d 224.0.0.0/4 -j RETURN
 iptables -t nat -A PREROUTING -d 240.0.0.0/4 -j RETURN

Anything else should be redirected to PREROUTING's local port
 iptables -t nat -A PREROUTING -p tcp -j REDIRECT --to-ports 12345

Apply the rules
 iptables -t nat -A OUTPUT -p tcp -j PREROUTING