IPTables

Howto setup iptables on CentOS/Fedora

Rules are always read from /etc/sysconfig/iptables, but if any "save" option is enable in /etc/sysconfig/iptables, the rules added from the command line are keep in memory, but won't be available on system reboot.

It happens in Fedora that rules in /etc/sysconfig/iptables are overridden until iptables daemon is restarted.

To make the system load our rules on reboot and every time, you must ensure the rules that /etc/iptables.rules file does exist, an has the rules you just created.

How to setup a gateway with 2 Virtual NICs on GNU/Debian

For any given host with 2 virtual NICs (IP aliases)

• WAN = eth0 -> 10.11.12.10
• LAN = eth0:1 -> 192.168.10.10

The steps to setup the host as gateway are

• Add to iptables the proper nat rules in order to ensure SNAT traffic pass through the "Out to the world" NIC.

*nat
:PREROUTING ACCEPT [891:110412]
:POSTROUTING ACCEPT [92:11129]
:OUTPUT ACCEPT [241:27547]
-A POSTROUTING -o eth0 -j MASQUERADE 
-A POSTROUTING -o eth0 -j SNAT --to-source 10.11.12.10
COMMIT

• Because both network IPs are in the same NIC, then add to the filter section a forward in both directions:

-A FORWARD -i eth0 -j ACCEPT 
-A FORWARD -o eth0 -j ACCEPT

• Ensure ip4 traffic is being forwarded on kernel enabling it on /etc/sysctl

net.ipv4.ip_forward = 1

• Load the previous modification:

sysctl -p