Understating IPTABLES in Linux

IPTABLES in RHEL6

IPTABLE
By default, RedHat comes with a built-in firewall called iptables
The iptables command is actually the tool used to manage a networking
subsystem within the
Linux kernel called netfilter

Syntax: iptables [options] [chain] -j [target]


Options:
----------------------------------------------------------------------
-A               chain Appends to the chain
-D               chain Deletes from the chain
-I                chain Inserts into the chain
-L               chain Lists all rules
-p                proto Uses the protocol specified
-m               match Matches the extended expression
-s                address Defines a source address
-d                address Defines a destination address

Chains
---------------------------------------------------------------------

INPUT                  Packets coming into the system
OUTPUT              Packets leaving the system
FORWARD          Incoming packets that should be forwarded

Target:
---------------------------------------------------------------------

ACCEPT          Allows the packets
DROP              Drops the packets and gives no response
REJECT           Rejects the packets and sends a rejection response

---------------------------------------------------------------------
If you want to show current iptables setting run these commands:

# iptables --list
# iptables -L

This is an example to create iptable rule

Allow SSH connections over TCP port 22:

# iptables -I INPUT -p tcp -m tcp --dport 22 -j ACCEPT

Default firewall rules that come with Red Hat:
# cat /etc/sysconfig/iptables

example, suppose you need to allow SSH access to your server but you’d
like to restrict the subnet that can connect.

Step 1. You could do something like this:

# iptables -I INPUT  -s 172.168.1.1/24 -p tcp --dport 22 -j ACCEPT

# service iptables save

# service iptables restart

the current status of iptables.
# service iptables status