How To Setup a Firewall with UFW

Introduction

One of the first lines of defense in securing your system is a functioning firewall. In the past, this was often done through complicated and arcane utilities. There is a lot of functionality built into these utilities, iptables being the most popular nowadays, but they require a decent effort on behalf of the user to learn and understand them. Firewall rules are not something you want yourself second-guessing.

To this end, UFW is a considerably easier-to-use alternative.

What is UFW?

UFW, or Uncomplicated Firewall, is a front-end to iptables. Its main goal is to make managing your firewall drop-dead simple and to provide an easy-to-use interface. It’s well-supported and popular in the Linux community and installed by default in KaOS. As such, it’s a great way to get started securing your system.

Check the Status

You can check the status of UFW by typing:

sudo ufw status

Right now, it will probably tell you it is inactive. Whenever ufw is active, you’ll get a listing of the current rules that looks similar to this:

Status: active
To               Action      From
--               ------      ----
22               ALLOW       Anywhere

Using IPv6 with UFW

If your system is configured for IPv6, ensure that UFW is configured to support IPv6 so that will configure both your IPv4 and IPv6 firewall rules. To do this, open the UFW configuration with this command:

kate /etc/default/ufw

Then make sure “IPV6” is set to “yes”, like so:

IPV6=yes

Save and quit.

Starting UFW the first time

sudo ufw enable

Now UFW will configure the firewall for both IPv4 and IPv6, when appropriate. To activate UFW on every boot:

sudo systemctl enable ufw

Set Up Defaults

One of the things that will make setting up any firewall easier is to define some default rules for allowing and denying connections. UFW’s defaults are to deny all incoming connections and allow all outgoing connections. This means anyone trying to reach your system would not be able to connect, while any application within the system would be able to reach the outside world. To set the defaults used by UFW, you would use the following commands:

sudo ufw default deny incoming

and

sudo ufw default allow outgoing

Note

If you want to be a little bit more restrictive, you can also deny all outgoing requests as well. The necessity of this is debatable, but if you have a public-facing system/server, it could help prevent against any kind of remote shell connections. It does make your firewall more cumbersome to manage because you’ll have to set up rules for all outgoing connections as well. You can set this as the default with the following:

sudo ufw default deny outgoing

Allow Connections

The syntax is pretty simple. You change the firewall rules by issuing commands in the terminal. If we turned on our firewall now, it would deny all incoming connections. If you’re connected over SSH to your system, that would be a problem because you would be locked out of your server. Let’s enable SSH connections to our system to prevent that from happening:

sudo ufw allow ssh

As you can see, the syntax for adding services is pretty simple. UFW comes with some defaults for common uses. Our SSH command above is one example. It’s basically just shorthand for:

sudo ufw allow 22/tcp

This command allows a connection on port 22 using the TCP protocol. If our SSH system/server is running on port 2222, we could enable connections with the following command:

sudo ufw allow 2222/tcp

Other Connections We Might Need

Now is a good time to allow some other connections we might need. If we’re securing a web server with FTP access, we might need these commands:

sudo ufw allow www
sudo ufw allow ftp

or respectively

sudo ufw allow 80/tcp
sudo ufw allow 21/tcp

You mileage will vary on what ports and services you need to open. There will probably be a bit of testing necessary. In addition, you want to make sure you leave your SSH connection allowed.

Port Ranges

You can also specify port ranges with UFW. To allow ports 1000 through 2000, use the command:

sudo ufw allow 1000:2000/tcp

If you want UDP:

sudo ufw allow 1000:2000/udp

To use kdeconnect-kde behind a firewall make sure to open the needed ports:

sudo ufw allow 1714:1764/tcp
sudo ufw allow 1714:1764/udp

IP Addresses

You can also specify IP addresses. For example, if I wanted to allow connections from a specific IP address (say my work or home address), I’d use this command:

sudo ufw allow from 192.168.255.255

Denying Connections

Our default set up is to deny all incoming connections. This makes the firewall rules easier to administer since we are only selectively allowing certain ports and IP addresses through. However, if you want to flip it and open up all your systems ports (not recommended), you could allow all connections and then restrictively deny ports you didn’t want to give access to by replacing allow with deny in the commands above. For example:

sudo ufw allow 80/tcp

would allow access to port 80 while:

sudo ufw deny 80/tcp

would deny access to port 80.

Deleting Rules

There are two options to delete rules. The most straightforward one is to use the following syntax:

sudo ufw delete allow ssh

As you can see, we use the command delete and input the rules you want to eliminate after that. Other examples include:

sudo ufw delete allow 80/tcp

or

sudo ufw delete allow 1000:2000/tcp

This can get tricky when you have rules that are long and complex.

A simpler, two-step alternative is to type:

sudo ufw status numbered

which will have UFW list out all the current rules in a numbered list. Then, we issue the command:

sudo ufw delete [number]

where [number] is the line number from the previous command.

Restart

After we’ve gotten UFW to where we want it, we can restart it on using this command (remember: if you’re connecting via SSH, make sure you’ve set your SSH port, commonly port 22, to be allowed to receive connections):

sudo systemctl restart ufw

You should see the command prompt again if it all went well. You can check the status of your rules now by typing:

sudo ufw status

or

sudo ufw status verbose

for the most thorough display.

To turn UFW off, use the following command:

sudo ufw disable

Reset Everything

If, for whatever reason, you need to reset your systems’ rules to their default settings, you can do this by typing this command:

sudo ufw reset

Conclusion

You should now have a system that is configured properly to restrict access to a subset of ports or IP addresses.