[CentOS] Iptables Question

Joseph L. Casale JCasale at activenetwerx.com
Wed Dec 10 20:22:36 UTC 2008

I have a squid proxy running transparently, so in my firewall script
I run the following fairly early:

iptables -A PREROUTING -t nat -i $LAN -p tcp -m multiport --dports 80,443 -j REDIRECT --to-port 3128

This is a multihomed server so after this change the masquerading was
removed (as only web access on the lan side of this server was needed).

I now need to masq cleanly one device so that it can bypass the squid
proxy. As order is important, would it be correct to put the following
_in front_ of the PREROUTING command above:

iptables -A POSTROUTING -t nat -o $WAN -j MASQUERADE
iptables -A FORWARD -i $LAN -o $WAN -m mac --mac-source <mac addr> -m state --state NEW,ESTABLISHED,RELATED -p tcp -m multiport --dports 443 -j ACCEPT
iptables -A FORWARD -i $WAN -o $LAN -m state --state RELATED,ESTABLISHED -j ACCEPT

Where is the best place to filter for the mac in this scenario? I am hoping
anything w/o this mac will skip the whole masq setup and enter the PREROUTING
command below, resulting in the traffic being proxied through squid.


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