Quick HOWTO : IPTables in Linux

Features:

 1. Firewall for Linux

 2. Interface to Netfilter, which is loaded by the kernel

 3. Operates primarily @ layers 3 & 4 of the OSI model

 4. Modular

 5. Provides Network Address Translation (NAT)

 6. IPTables can also access other layers (2, 5-7), with modules

1. grep -i config_netfilter /boot/config*

Note: Save rules in: /etc/sysconfig/iptables so that when IPTables is restarted, the rules will be applied OR, update /etc/sysconfig/iptables-config to save the rules automatically

/sbin/iptables - primary ACL modifier utility

/sbin/iptables-restore - restores rules to current IPTables instance

/sbin/iptables-save - saves rules to STDOUT, by default, or to a file

IPTables includes 3 default tables, which you cannot remove:

 1. NAT

 2. Mangle

 3. Filter (Default) - filters inbound/outbound traffic

Note: Each table, includes chains, which include Access Control Entries (ACEs)

Usage:

 1. iptables -L

Note: The Filter table includes 3 chains:

 1. INPUT - applies to traffic destined to a service that our system is bound to

 2. FORWARD - applies to traffic being routed through the system

 3. OUTPUT - applies to traffic sourced from our system, heading outbound

Examples:

 1. Filter inbound traffic to remote RH5 system to SSH

      a. iptables -A INPUT -p tcp --dport 22 -j ACCEPT

      b. iptables -A INPUT -j DROP

 2. Filter outbound traffic to ANY remote SSH port

     a. iptables -A OUTPUT -p tcp --dport 22 -j DROP

 3. Flush ALL rules from OUTPUT chain of the Filter table

     a. iptables -F OUTPUT

 4. Save rules to file, then flush rules

     a. iptables-save > iptables.rules.1

 5. Reinstate flushed rules

     a. iptables-restore iptables.rules.1