Create a firewall with iptables rules

Iptables are administration tools for ip4/ip6 packet filtering and nat.
The builten iptables rules for ubuntu are to allow traffic from every where to any where, to check run the command:

sudo iptables -L

To setup a firewall we will configure the iptables to block all the traffic from outside to the pc and then will allow the certain traffic to some ports as for our need.
First delete the iptables builten rules with:

sudo iptables --fulsh

Now we will apply the general policy for our firewall, to block the all traffic from outside internet and allow form pc to the outside:

iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT
iptables -P FORWARD DROP

If we running some servers like web server allow the people to connect to port 80 which is web server port, we will open this port with iptables rule:

iptables -A INPUT -p tcp --sport 80 -j ACCEPT

Allow intranet (local LAN ) traffic between them:

iptables -A INPUT -s 192.168.0.0/24 -j ACCEPT
iptables -A OUTPUT -s 192.168.0.0/24 -j ACCEPT
iptables -A INPUT -s 10.0.0.0/8 -j ACCEPT
iptables -A OUTPUT -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -s 172.16.0.0/12 -j ACCEPT
iptables -A OUTPUT -s 172.16.0.0/12 -j ACCEPT
iptables -A INPUT -s 172.32.0.0/12 -j ACCEPT
iptables -A OUTPUT -s 172.32.0.0/12 -j ACCEPT

Enable established connections:

iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

These are firewall rules i setup for my pc iptables-rules.sh
Ubuntu doesn't save these applied rules for next reboot and the builten iptables rules will apply again, we should make a script to apply our iptables again in the each boot.
Put the iptables rules in a file like iptables-rules.sh make executable:

sudo chmod a+x iptables.rules.sh

Run the script:

sudo ./iptables-rules.sh

The script will apply all the rules and will setup the firewall.
Save the firewall to a file with command:

sudo iptables-save > rules

The command will save the firewall to file rules, check with:

sudo cat rules

create a directory for example firewall in the /etc:

sudo mkdir /etc/firewall

copy the rules to this directory:

sudo cp rules /etc/firewall

Make a bash script 49firewall an put in a place to execute in every boot or every internet connection and restore the firewall rules, this is 49firewall:

#!/bin/bash
iptables -F
iptables -X
iptables -Z

/sbin/iptables-restore < /etc/firewall/rules

The first three lines will delete the builten iptables rules and last line will restore the iptables rules from the file /etc/firewall/rules.
Continue to iptables commands

Create a firewall with iptables rules.