How to Install a Firewall on Ubuntu

So you want to setup a firewall to limit and block unwanted inbound traffic to your Server. Setting up a firewall is really up to you, but we strongly recommend that you use the example below to block traffic to ports that are not commonly used. It’s a good way to deter would-be intruders! You can always modify the rules or disable the firewall later.

Here’s how to create a firewall on your Server:

Check your Server’s default firewall rules by entering the following command:

sudo iptables -L

Examine the output. If you haven’t implemented any firewall rules yet, you should see an empty ruleset, as shown below:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Create a file to hold your firewall rules by entering the following command:

sudo nano /etc/iptables.firewall.rules

Now it’s time to create some firewall rules. We’ve created some basic rules to get you started. Copy and paste the rules shown below in to the iptables.firewall.rules file you just created.

*filter

#  Allow all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT -d 127.0.0.0/8 -j REJECT

#  Accept all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

#  Allow all outbound traffic - you can modify this to only allow certain traffic
-A OUTPUT -j ACCEPT

#  Allow HTTP and HTTPS connections from anywhere (the normal ports for websites and SSL).
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT

#  Allow SSH connections
#
#  The -dport number should be the same port number you set in sshd_config
#
-A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT

#  Allow ping
-A INPUT -p icmp --icmp-type echo-request -j ACCEPT

#  Log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

#  Drop all other inbound - default deny unless explicitly allowed policy
-A INPUT -j DROP
-A FORWARD -j DROP

COMMIT

Edit the rules as necessary. By default, the rules will allow traffic to the following services and ports: HTTP (80), HTTPS (443), SSH (22), and ping. All other ports will be blocked.

Optional: If you plan on using the Server Longview service, add these additional lines above the 

# Drop all other inbound

 section:

#  Allow incoming Longview connections 
-A INPUT -s longview.linode.com -j ACCEPT

# Allow metrics to be provided Longview
-A OUTPUT -d longview.linode.com -j ACCEPT

Save the changes to the firewall rules file by pressing Control-X, and then Y.

Activate the firewall rules by entering the following command:

Recheck your Server’s firewall rules by entering the following command:

sudo iptables-restore < /etc/iptables.firewall.rules

Examine the output. The new ruleset should look like the one shown below:

Chain INPUT (policy ACCEPT)
   target     prot opt source               destination
   ACCEPT     all  --  anywhere             anywhere
   REJECT     all  --  anywhere             127.0.0.0/8          reject-with icmp-port-unreachable
   ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
   ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
   ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:https
   ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh
   ACCEPT     icmp --  anywhere             anywhere
   LOG        all  --  anywhere             anywhere             limit: avg 5/min burst 5 LOG level debug prefix "iptables denied: "
   DROP       all  --  anywhere             anywhere

   Chain FORWARD (policy ACCEPT)
   target     prot opt source               destination
   DROP       all  --  anywhere             anywhere

   Chain OUTPUT (policy ACCEPT)
   target     prot opt source               destination
   ACCEPT     all  --  anywhere             anywhere  

Now you need to ensure that the firewall rules are activated every time you restart your Server.

Start by creating a new script with the following command:

sudo nano /etc/network/if-pre-up.d/firewall

Copy and paste the following lines in to the file you just created:

#!/bin/sh
/sbin/iptables-restore < /etc/iptables.firewall.rules

Press Control-X and then press Y to save the script. Set the script’s permissions by entering the following command:

sudo chmod +x /etc/network/if-pre-up.d/firewall

That’s it! Your firewall rules are in place and protecting your server. Remember, you’ll need to edit the firewall rules later if you install other software or services.