Set Up a Firewall with UFW on Debian 10

A properly configured firewall is one of the most important aspects of overall system security.

UFW (Uncomplicated Firewall) is a user-friendly front-end for managing iptables firewall rules. Its main goal is to make managing iptables easier or, as the name says, uncomplicated.

This article describes how to set up a firewall with UFW on Debian 10.

Prerequisites

Only root or user with sudo privileges can manage the system firewall.

Installing UFW

Enter the following command to install the ufw package:

sudo apt updatesudo apt install ufw

Checking UFW Status

The installation will not activate the firewall automatically to avoid a lockout from the server. You can check the status of UFW by typing:

sudo ufw status verbose

The output will look like this:

Status: inactive

If UFW is activated, the output will look similar to the following:

UFW Default Policies

By default, UFW blocks all of the incoming connections and allow all outbound connections. This means that anyone trying to access your server will not be able to connect unless you specifically open the port. The applications and services running on the server will be able to access the outside world.

The default polices are defined in the /etc/default/ufw file and can be changed using the sudo ufw default <policy> <chain> command.

Firewall policies are the foundation for building more detailed and user-defined rules. Generally, the initial UFW Default Policies are a good starting point.

Application Profiles

Most of the applications ship with an application profile that describes the service and contains UFW settings. The profile is automatically created in the /etc/ufw/applications.d directory during the package installation.

To list all application profiles available on your system type:

sudo ufw utf --help

Depending on the packages installed on your system, the output will look similar to the following:

Available applications:
  DNS
  IMAP
  IMAPS
  OpenSSH
  POP3
  POP3S
  Postfix
  Postfix SMTPS
  Postfix Submission
  ...

To find more information about a specific profile and included rules, use the app info command, followed by the profile name. For example to get info about the OpenSSH profile you would use:

sudo ufw app info OpenSSH

Profile: OpenSSH
Title: Secure shell server, an rshd replacement
Description: OpenSSH is a free implementation of the Secure Shell protocol.

Port:
  22/tcp

The output includes the profile name, title, description, and firewall rules.

Allow SSH Connections

Before enabling the UFW firewall first, you need to allow incoming SSH connections.

If you’re connecting to your server from a remote location, and you enable the UFW firewall before explicitly allow incoming SSH connections you will no longer be able to connect to your Debian server.

To configure your UFW firewall to accept SSH connections, run the following command:

sudo ufw allow OpenSSH

Rules updated
Rules updated (v6)

If the SSH server is listening on a port other than the default port 22, you will need to open that port.

For example, your ssh server listens on port 7722, you would execute:

sudo ufw allow 7722/tcp

Enable UFW

Now that the UFW firewall is configured to allow incoming SSH connections, enable it by running:

sudo ufw enable

Command may disrupt existing ssh connections. Proceed with operation (y|n)? y
Firewall is active and enabled on system startup

You will be warned that enabling the firewall may disrupt existing ssh connections. Type “y” and press “Enter”.

Opening Ports

Depending on the applications that run on your server, you’ll need to open the ports on which the services run.

Below are several examples of how to allow incoming connections to some of the most common services:

Open port 80 – HTTP

Allow HTTP connections:

sudo ufw allow http

Instead of the http profile, you can use the port number, 80:

sudo ufw allow 80/tcp

Open port 443 – HTTPS

Allow HTTPS connections:

sudo ufw allow https

You can also use the port number, 443:

sudo ufw allow 443/tcp

Open port 8080

If you run Tomcat or any other application that listens on port 8080 open the port with:

sudo ufw allow 8080/tcp

Opening Port Ranges

With UFW, you can also allow access to port ranges. When opening a range, you must specify the port protocol.

For example, to allow ports from 7100 to 7200 on both tcp and udp, run the following command:

sudo ufw allow 7100:7200/tcpsudo ufw allow 7100:7200/udp

Allowing Specific IP Addresses

To allow access on all ports from a specific IP address, use the ufw allow from command followed by the IP address:

sudo ufw allow from 64.63.62.61

Allowing Specific IP Addresses on Specific port

To allow access on a specific port, let’s say port 22 from your work machine with IP address of 64.63.62.61 use the following command:

sudo ufw allow from 64.63.62.61 to any port 22

Allowing Subnets

The command for allowing connection from a subnet of IP addresses is the same as when using a single IP address. The only difference is that you need to specify the netmask. For example, if you want to allow access for IP addresses ranging from 192.168.1.1 to 192.168.1.254 to port 3360 (MySQL) you can use this command:

sudo ufw allow from 192.168.1.0/24 to any port 3306

Allow Connections to a Specific Network Interface

To allow access on a specific port let’s say port 3360 only to specific network interface eth2, use allow in on and the name of the network interface:

sudo ufw allow in on eth2 to any port 3306

Deny connections

The default policy for all incoming connections is set to deny, which means that UFW will block all incoming connections unless you specifically open the connection.

Let’s say you opened the ports 80 and 443, and your server is under attack from the 23.24.25.0/24 network. To deny all connections from 23.24.25.0/24, use the following command:

sudo ufw deny from 23.24.25.0/24

If you only want to deny access to ports 80 and 443 from 23.24.25.0/24 use:

sudo ufw deny from 23.24.25.0/24 to any port 80sudo ufw deny from 23.24.25.0/24 to any port 443

Writing deny rules is the same as writing allow rules. You only need to replace allow with deny.

Delete UFW Rules

There are two different ways to delete UFW rules. By rule number and by specifying the actual rule.

Deleting UFW rules by rule number is easier, especially if you are new to UFW.

To delete a rule by its number first, you need to find the number of the rule you want to delete. To do that run following command:

sudo ufw status numbered

Status: active

     To                         Action      From
     --                         ------      ----
[ 1] 22/tcp                     ALLOW IN    Anywhere
[ 2] 80/tcp                     ALLOW IN    Anywhere
[ 3] 8080/tcp                   ALLOW IN    Anywhere

To delete rule number 3, the rule that allows connections to port 8080, you can use the following command:

sudo ufw delete 3

The second method is to delete a rule by specifying the actual rule. For example, if you added a rule to open port 8069 you can delete it with:

sudo ufw delete allow 8069

Disable UFW

If for any reason you want to stop UFW and deactivate all rules run:

sudo ufw disable

Later if you want to re-enable UTF and activate all rules just type:

sudo ufw enable

Reset UFW

Resetting UFW will disable UFW, and delete all active rules. This is helpful if you want to revert all of your changes and start fresh.

To reset UFW simply type in the following command:

sudo ufw reset