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
# 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.