Sharing network connection between two nics in one box .centos

[tryLinux]

Hi guys.

what I need to do is:

I have a centos box with two nics

eth0 is connected to the network.

eth1 I want to connect to a router and connect 3 more machines to it.

I want the Centos box to give internet access to the router and the other 3 pc's but I don't want anyone from the outside world to be able to reach the little network.

any ideas?

[atreyu]

Familiarize yourself with the Firewall provided by the Linux kernel (a.k.a. iptables/netfilter). You can do all that you mention with it. There is a graphical utility that can get you started, but a lot of the commands you'll have to work out yourself, experimentation is key here.

The software is probably already installed on your system:

man iptables
man iptables-save

There is an initscript that control starting and stopping it, too, e.g.:

/etc/init.d/iptables stop
/etc/init.d/iptables start

In a nutshell, you run commands using the iptables utility. You insert rules using it. You can then save it to file with the iptables-save command. This file is by default /etc/sysconfig/iptables. There are way too many options available to list here, go read up on it.

Keep in mind that to do routing, you'll need to enable TCP forwarding. You can do this in the /etc/sysctl.conf file.

[Lazydog]

First thing you need to do is turn on FORWARDing on the Linux box.

sysctl -w net.ipv4.ip_forward=1

Then you need to ensure that it is on when the system gets rebooted by changing /etc/sysctl.conf as follows;

in sysctl.conf change

net.ipv4.ip_forward = 0

to

net.ipv4.ip_forward = 1

.

A simple firewall could be;

iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -i eth1 -m state --state NEW -j ACCEPT
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -m state --state NEW -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 - state --state NEW -j ACCEPT

This should get you started with your firewall. More reading is located HERE!