Simple, if it’s not installed already then run the following commands
sudo apt-get install iptables sudo /etc/init.d/iptables start
The safest and best way of configuring iptables, in my opinion, is to have two files. The first is a temporary/test set that you will save to first, the second is the actual rule set that will be loaded to iptables.
So, first we’ll create an empty temp rules file
sudo touch /etc/iptables.temp.rules
Add some simple rules to it:
*filter # Allows all loopback traffic and drop all traffic to 127/8 that doesn't use lo -A INPUT -i lo -j ACCEPT -A INPUT ! -i lo -d 127.0.0.0/8 -j REJECT # Accepts all established inbound connections -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT # Allows all outbound traffic -A OUTPUT -j ACCEPT #SSH -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT #HTTP -A INPUT -p tcp -m tcp --dport 80 -j ACCEPT #HTTPS -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT #SMTP -A INPUT -p tcp -m tcp --dport 25 -j ACCEPT #IMAP -A INPUT -p tcp -m tcp --dport 143 -j ACCEPT #POP3 -A INPUT -p tcp -m tcp --dport 110 -j ACCEPT #PING -A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT # Log -I INPUT 5 -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7 # Reject all other inbound - default deny unless explicitly allowed policy -A INPUT -j REJECT -A FORWARD -j REJECT COMMIT
Next we apply this rule set to the currently running iptables instance
sudo iptables-restore < /etc/iptables.temp.rules
Check to make sure they loaded correctly
sudo iptables -L
If everything looks OK and ready to go then we simply save, this time to the secondary file that I mentioned earlier
sudo iptables-save > /etc/iptables.up.rules
Network interface configuration
Open up the following file with your favourite editor
Find the following lines:
auto lo iface lo inet loopback
And add this to the end
pre-up iptables-restore < /etc/iptables.up.rules
So that it becomes
auto lo iface lo inet loopback pre-up iptables-restore < /etc/iptables.up.rules
This will restore your custom set of iptables rules when it instantiates the network devices.
When you need to add more rules in the future, simply add them to your iptables.temp.rules set, load them in to iptables as shown, if everything looks good then save to iptables.up.rules
Please note; this is only an extremely basic rule set and will need to be improved upon by you or another sys admin in your team to properly secure your server.