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In my company, all http and https must connect to a proxy server, which means we must set proxy in our web browser.

Our department has an internal network (192.168.0.xxx). I use a Linux server as router, use iptables to setup NAT.

iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

Now, the Linux router works well. We can set proxy to access Internet.

My question is: I want to use the Linux router to connect the proxy server, then the computers in our internal network (192.168.0.xxx) could visit Internet without setting proxy.

Is this possible?

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Try this rule:

iptables -t nat -A PREROUTING -i eth0 -s ! 192.168.0.2 -p tcp --dport 80 -j DNAT --to 192.168.0.2:3128
iptables -t nat -A POSTROUTING -o eth0 -s 192.168.0.0/24 -d 192.168.0.2 -j SNAT --to 192.168.0.1
iptables -A FORWARD -s 192.168.0.0/24 -d 192.168.0.2 -i eth0 -o eth0 -p tcp --dport 3128 -j ACCEPT

where:

192.168.0.2 - proxy server IP (Squid, etc);
192.168.0.1 - router IP (where started iptables);
192.168.0.0/24 - your local network

I could be wrong, check carefully.

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Without having tested it I think the solution @ymn is valid for HTTP, but I don't think you can get around specifying an equivalent line for HTTPS - indeed any such solution should throw up certificate errors etc because you are, in-effect, doing a man-in-the-middle attack on your own network.

(You may be able to get round this - to some degree - but its not a good idea as you are stripping the security SSL gives. Have a look at sslstrip - http://www.thoughtcrime.org/software/sslstrip/)

Maybe a better middle ground might be to try doing proxy autoconfiguration (WPAD with DHCP) - http://wiki.squid-cache.org/SquidFaq/ConfiguringBrowsers#Fully_Automatically_Configuring_Browsers_for_WPAD

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There is already a squid server on my router now. – siyuan Feb 2 '13 at 7:10
    
Thank you! There is already a squid server on my router now. Suppose my company's proxy is proxy.com:85, we setup a squid 192.168.0.5:3128. Now, we can access Internet by setting proxy as 192.168.0.5:3128 or proxy.com:85. What I want is to access Internet without setting proxy, because some program can't set proxy, such as some online update. – siyuan Feb 2 '13 at 7:18

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