Configure firewalld on Ubuntu

What is firewalld?

A firewall is a way to protect machines from any unwanted traffic from outside. It enables users to control incoming network traffic on host machines by defining a set of firewall rules. These rules are used to sort the incoming traffic and either block it or allow it through.

firewalld is a firewall service daemon that provides a dynamic customizable host-based firewall with a D-Bus interface. Being dynamic, it enables creating, changing, and deleting the rules without the necessity to restart the firewall daemon each time the rules are changed.

firewalld uses the concepts of zones and services, that simplify the traffic management.

Zones are predefined sets of rules. Network interfaces and sources can be assigned to a zone. The traffic allowed depends on the network your computer is connected to and the security level this network is assigned. Firewall services are predefined rules that cover all necessary settings to allow incoming traffic for a specific service and they apply within a zone.

Services use one or more ports or addresses for network communication. Firewalls filter communication based on ports. To allow network traffic for a service, its ports must be open. firewalld blocks all traffic on ports that are not explicitly set as open. Some zones, such as trusted, allow all traffic by default.

Checking the firewalld status

The firewall service, firewalld, is installed on the system by default. Use the firewalld CLI interface to check that the service is running.

To see the status of the service:

sudo firewall-cmd --state

For more information about the service status, use the systemctl status sub-command:

sudo systemctl status firewalld

Furthermore, it is important to know how firewalld is set up and which rules are in force before you try to edit the settings. To display the firewall settings, see Viewing current firewalld settings.

Viewing firewalld settings using CLI

With the CLI client, it is possible to get different views of the current firewall settings. The --list-all option shows a complete overview of the firewalld settings.

firewalld uses zones to manage the traffic. If a zone is not specified by the --zone option, the command is effective in the default zone assigned to the active network interface and connection.

To list all the relevant information for the default zone:

firewall-cmd --list-all

To specify the zone for which to display the settings, add the --zone=zone-name argument to the firewall-cmd --list-all command, for example:

firewall-cmd --list-all --zone=home

To see the settings for particular information, such as services or ports, use a specific option. See the firewalld manual pages or get a list of the options using the command help:

firewall-cmd --help

For example, to see which services are allowed in the current zone:

firewall-cmd --list-services
samba-client ssh dhcpv6-client

Listing the settings for a certain subpart using the CLI tool can sometimes be difficult to interpret. For example, you allow the SSH service and firewalld opens the necessary port (22) for the service. Later, if you list the allowed services, the list shows the SSH service, but if you list open ports, it does not show any. Therefore, it is recommended to use the --list-all option to make sure you receive a complete information.

Installing firewalls

Run this command on the command line:

sudo dnf install firewalld

Starting firewalls

Start firewalld, by entering the following commands:

sudo systemctl unmask firewalld
sudo systemctl start firewalld

To make firewalld start automatically at system start:

sudo systemctl enable firewalld

Stopping firewalld

To stop firewalld, enter the following command as root:

sudo systemctl stop firewalld

Prevent firewalld from starting automatically at system start, enter the following command as root:

sudo systemctl disable firewalld

Make sure firewalld is not started by accessing the firewalld D-Bus interface and also if other services require firewalld, enter the following command as root:

sudo systemctl mask firewalld

Runtime and permanent settings

Any changes made while firewalld is running will be lost when firewalld is restarted. When firewalld is restarted, the settings revert to their permanent values.

These changes are said to be made in runtime mode.

To make the changes persistent across reboots, apply them again using the --permanent option. Alternatively, to make changes persistent while firewalld is running, use the --runtime-to-permanent firewall-cmd option.

If you make changes while firewalld is running using only the --permanent option, they do not become effective until firewalld is restarted. However, restarting firewalld briefly stops the networking traffic, causing disruption to your system.

Changing settings in runtime and permanent configuration using CLI

Using the CLI, you can only modify either runtime or permanent mode. To modify the firewall settings in permanent mode, use the –permanent option with the firewall-cmd command.

sudo firewall-cmd --permanent <other options>

Without this option, the command modifies runtime mode. To change settings in both modes, you can use two methods:

Change runtime settings and then make them permanent as follows:

1. Change the runtime settings:

firewall-cmd <other options>

2. Use --runtime-to-permanent to make the changes permanent.

firewall-cmd --runtime-to-permanent

Set permanent settings and reload the settings into runtime mode:

1. Make the changes in permanent mode:

firewall-cmd --permanent <other options>

2. Reload the settings:

firewall-cmd --reload

The first method allows you to test the settings before you apply them to permanent mode.

It is possible that an incorrect setting will result in a user locking themselves out of a machine. To prevent this, use the --timeout option. Using this option means that after a specified amount of time, any change reverts to its previous state. You can not use the --permanent option with the --timeout option.

For example, to add the SSH service for 15 minutes use this command:

sudo firewall-cmd --add-service=ssh --timeout 15m

The SSH service will be available until access is removed after 15 minutes.

Controlling ports using firewalld

Ports are logical devices that enable an operating system to receive and distinguish network traffic and forward it accordingly to system services. These are usually represented by a daemon that listens on the port, that is it waits for any traffic coming to this port.

Normally, system services listen on standard ports that are reserved for them. The httpd daemon, for example, listens on port 80. However, system administrators may configure daemons to listen on different ports to enhance security.

Opening a port

Through open ports, the system is accessible from the outside, which represents a security risk. Generally, keep ports closed and only open them if they are required for certain services.

Opening a port using the command line

Get a list of allowed ports in the current zone:

firewall-cmd --list-ports

Add a port to the allowed ports to open it for incoming traffic:

sudo firewall-cmd --add-port=port-number/port-type

Make the new settings persistent:

sudo firewall-cmd --runtime-to-permanent

The port types are either tcp, udp, sctp, or dccp. The type must match the type of network communication.

Closing a port

When an open port is no longer needed, close that port in firewalld. It is highly recommended to close all unnecessary ports as soon as they are not used because leaving a port open represents a security risk.

Closing a port using the command line

To close a port, remove it from the list of allowed ports:

List all allowed ports:

firewall-cmd --list-ports

This command will only give you a list of ports that have been opened as ports. You will not be able to see any open ports that have been opened as a service. Therefore, you should consider using the --list-all option instead of --list-ports.

Remove the port from the allowed ports to close it for the incoming traffic:

sudo firewall-cmd --remove-port=port-number/port-type

Make the new settings persistent:

sudo firewall-cmd --runtime-to-permanent