[CentOS] Centos firewall/router issue

[John R Pierce]

Dustin Krysak wrote:
> Good day everyone. Today has been "one of those..." days. So I have a
> feeling this is just a brain lapse here.
>
> At any rate, I am setting up a firewall/router using centos 4.4
> (minimal install, yummed up to date).
>
> I have in fact enables V4 ip forwarding (by editing /etc/sysctl.conf)
> and rebooted the system. I use fwbuilder for my rules, and at this
> point have a next to nothing set of rules. a DHCP server is also
> residing on this router.
> ...
> Now the issue is, I can not get out of the internal network to the
> internet for any traffic. Pings (by ip and name), web sites, etc.


sounds like you're trying to do a 'NAT' (network address translation) 
network, where you're local network is on private IP space (192.168.x.y 
or 10.x.x.x or whatever), and you're sharing a single public IP.   this 
requires several very specific and moderately complex rules to implement 
what Linux calls 'MASQUERADE', and you need to load the ip_conntrack 
module into the kernel, and probably a few more.


at a BARE minimum, you need some variant on these two rules...

   iptables --table nat --append POSTROUTING --out-interface $WAN -j MASQUERADE
   iptables --append FORWARD --in-interface $LAN -j ACCEPT


(where $WAN, $LAN are eth0, eth1, or whatever your internet and local 
interfaces are)


if you google for: LINUX MASQUERADE, you'll find all sorts of confusing 
info on this :)