Manually manipulating iptables in CentOS and Redhat

The iptables rules in redhat based distro is being kept by default in /etc/sysconfig/iptables and /etc/sysconfig/ip6tables. To manipulate the firewall, just add or remove rules from this file, and restart iptables services. For example, we want to allow tftp port, which is port 69 udp:

1. Edit /etc/sysconfig/iptables

• # vi /etc/sysconfig/iptables

2. Add the following lines, before the final LOG and DROP lines for INPUT chain:

• -A INPUT -m state --state NEW -m udp -p udp --dport 69 -j ACCEPT

3. Save and close the file
4. Restart iptables service:

• # /etc/init.d/iptables restart

5. Check your new iptables rules, where -L is to list all rules in the selected chain, and -n is for printing port in numeric output:

• # sudo iptables -L -n

6. And you can see that

   "ACCEPT     udp  --  0.0.0.0/0            0.0.0.0/0           state NEW udp dpt:69"

   line is in the iptables file.
7. To save the new rules permanently, just run:

• # /etc/init.d/iptables save 

To block any particular port, you just need to edit /etc/sysconfig/iptables, remove the ACCEPT line that contain that port, and restart iptables, and you are done :)