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How to configure Red Hat Linux/CentOS as a router?

Postby chandranjoy » Wed Sep 01, 2010 8:09 pm

This is applicable for Red Hat Enterprise Linux/CentOS 3 and above.

There are a few ways to set up a Linux machine to route. Here is a relatively straight forward and common method. This method requires that the system use iptables for Network Address Translation (NAT).

To enable packet forwarding:

echo "1" > /proc/sys/net/ipv4/ip_forward

To make this permanent, set the variable net.ipv4.ip_forward = 1 in the /etc/sysctl.conf file. For example:

# Controls IP packet forwarding
net.ipv4.ip_forward = 1

Next, have iptables handle Network Address Translation:

/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

where eth0 is "outside" or in public connection. Iptables ruleset should be tuned to be more restrictive. Do not forget to save iptables settings with:

service iptables save

See additional articles in the Knowledgebase for additional iptables configuration and tips.

To view the routing table use:

netstat -rn

To view iptables ruleset use:

iptables -L
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