I'm a complete newbie on setting up network with debian so please be patient with me.
Here's my problem
my configuration for networking
[internet]---[router]---[debian server]---[LAN]

we've just done a fresh install,used debian 5.0 before (kernel 2.6) for some unknown reasons it broke(wont recognize the hd) so we decided to do a fresh install using debian 7,it's old anyway.

we backup the etc for squid3 and shorewall,install,copied the old etc it's now up and running. now the problem is I can't connect to steam at all. I can browse and download just fine but I can't connect to steam network at all.
I've trie\\d this thing :
1. connect modem directly to my laptop (no proxy) I can connect to steam and can play
2. tried to disable my firewall (windows softwares) and restart computer and the modem connect using proxy : can't

now I know I'm a stupid and stubborn person ( I can just connect modem directly to play) but I'm dying to find out what's the problem here.is it in shorewall or squid3 and how to fix it?

I've googled how to fix this problem and the answers I got didn't work at all.