1. <big>Ubuntu: Stat / Stop / Restart Iptables Firewall Service</big>

   a) ufw command – This command is used for managing a Linux firewall and aims to provide an easy to use interface for the user.
   b) iptables command – This command is used to set up, maintain, and inspect the tables of IPv4 packet filter rules in the Linux kernel.

   Find status of firewall
   Login as root user either by opening the Terminal or login over the ssh based session. Type the following command:
   $ sudo ufw status
   Sample outputs:
   Status: inactive

   Ubuntu stop iptables service command
   Type the following command to unloads firewall and disables firewall on boot:
   $ sudo ufw disable

   Ubuntu start iptables service command
   Type the following command to reloads firewall and enables firewall on boot:
   $ sudo ufw enable

   Ubuntu reload / restart iptables service command
   Type the following command to reloads firewall:
   $ sudo ufw reload

   Alternative method to enable/disable firewall on Ubuntu and other Linux distros
   If you are not using ufw command and/or ufw is not installed, try the following generic methods:
   Get IPv4 iptables status
   $ sudo iptables -L -n -v

   Get IPv6 ip6tables status
   $ sudo ip6tables -L -n -v

   Save IPv4 iptables firewall
   Use the iptables-save command to save current firewall rules:
   $ sudo iptables-save > $HOME/firewall.txt

   Save IPv6 ip6tables firewall
   $ sudo ip6tables-save > $HOME/firewall-6.txt

   Restore IPv4 iptables firewall
   Use the iptables-restore command to restore firewall rules:
   $ sudo iptables-restore < $HOME/firewall.txt

   Restore IPv6 ip6tables firewall
   $ sudo ip6tables-restore < $HOME/firewall-6.txt

   Putting it all together
   To stop Ipv4 based iptables firewall, enter:

sudo iptables-save > $HOME/firewall.txt
sudo iptables -X
sudo iptables -t nat -F
sudo iptables -t nat -X
sudo iptables -t mangle -F
sudo iptables -t mangle -X
sudo iptables -P INPUT ACCEPT
sudo iptables -P FORWARD ACCEPT
sudo iptables -P OUTPUT ACCEPT

To stop Ipv6 based iptables firewall, enter:

sudo ip6tables-save > $HOME/firewall-6.txt
sudo ip6tables -X
sudo ip6tables -t mangle -F
sudo ip6tables -t mangle -X
sudo ip6tables -P INPUT ACCEPT
sudo ip6tables -P FORWARD ACCEPT
sudo ip6tables -P OUTPUT ACCEPT

Where,
- **-F** : Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting all the rules one by one.
- **-X** : Delete the optional user-defined chain specified. There must be no references to the chain. If there are, you must delete or replace the referring rules before the chain can be deleted.
- **-P chainNameHere ACCEPT** : Set the policy for the chain to the given target.
- **-L** : List rules.
- **-v** : Verbose output.
- **-n** : Numeric output. IP addresses and port numbers will be printed in numeric format.

參考：https://www.cyberciti.biz/faq/ubuntu-start-stop-iptables-service/