View source for IPTables

__NOTOC__
=== Howto setup iptables on CentOS/Fedora ===
Rules are always read from <tt>/etc/sysconfig/iptables</tt>, but if any "save" option is enable in <tt>/etc/sysconfig/iptables</tt>, 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 <tt>/etc/sysconfig/iptables</tt> 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 <tt>/etc/iptables.rules</tt> 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 <code>nat</code> rules in order to ensure <code>SNAT</code> 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 <tt>/etc/sysctl</tt>
 net.ipv4.ip_forward = 1
* Load the previous modification:
 sysctl -p