Linux firewall: ufw

1. Installation(on Ubuntu):
• sudo apt-get install ufw
2. Configuration script:

• #!/bin/sh
  # disable and reset ufw
  sudo ufw reset --force
  
  # deny everything
  sudo ufw default deny
  
  # allow remote ssh connection from host 1.2.3.4
  sudo ufw allow proto tcp from 1.2.3.4 to any port 22
  
  # allow remote ssh connection from host 1.2.3.5
  sudo ufw allow proto tcp from 1.2.3.5 to any port 22
  
  # trust remote ip 1.2.3.6
  sudo ufw allow from 1.2.3.6
  sudo ufw allow to 1.2.3.6
  
  # allow https server port
  sudo ufw allow proto tcp from any to any port 443 
  
  # allow tcp connection from 1.2.3.7 to port 6666
  sudo ufw allow proto tcp from 1.2.3.7 to any port 6666
  
  # enable ufw
  sudo ufw enable
  
  # list the rules
  sudo ufw status verbose
  

See also:
• Example ufw script
• UFW: How to block outgoing ports expect those needed
• How to set up the ufw firewall on Ubuntu 10.04
• Setting Up a Firewall on Arch Linux with UFW
• Ubuntu Firewall
• Configuring an iptables firewall on Ubuntu Hardy Heron server
• Simple firewall for Ubuntu using iptables