Control of System Accessibility by firewalld

Richard Gregory, Petr Bokoc (pbokoc), Peter Boy (pboy) Версія F34 onwards Last review: 2023-08-23

A firewall is a way to protect machines from any unwanted access from outside. In Fedora, it is installed by default during the installation of the operating system, enabled and configured to provide secure operation even without any additional action by the administrator. It blocks any access other than ssh by default.

How it works

A firewall 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 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.

Additional resources

For more information about using firewalld and configuring zones and services, see firewalld documentation or Fedora wiki:firewalld

Setting up firewalld

All Fedora Editions install, configure and activate the firewall by default. No further action is required. The only exception is Cloud Edition, which relies on the higher level cloud system.

Some third party variations and redistributions may differ. In this case, it is up to the administrator to install and activate the firewall afterwards.

You check if firewalld is set up in a terminal by issuing

systemctl status firewalld

You should get soemething like

● firewalld.service - firewalld - dynamic firewall daemon
     Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; preset>
    Drop-In: /usr/lib/systemd/system/service.d
             └─10-timeout-abort.conf
     Active: active (running) since Sat 2023-08-19 19:05:18 CEST; 3 days ago
     ...

Installing and activating firewalld

In case you get by the command above something like

Unit firewalld.service could not be found.

you have to install it. Run on the command line:

$ sudo dnf install firewalld
$ sudo systemctl unmask firewalld
$ sudo systemctl start firewalld
$ sudo systemctl enable firewalld

This sequence installs, starts and ensures an automatic restart after a system boot.

Adjusting firewalld operations during system maintenance

Sometimes a system administrator has to stop or restart firewalld during system maintenance tasks.

Stop firewalld

$ sudo systemctl stop firewalld

Prevent autostart at system boot

$ sudo systemctl disable firewalld

Start firewalld

$ sudo systemctl start firewalld

Activate autostart at system boot

$ sudo systemctl enable firewalld

Disconnecting the firewall from the d-bus controller

$ sudo systemctl mask firewalld

(Re.)Connect the firewall to the d-bus controller

$ sudo systemctl unmask firewalld

Installing the firewall-config GUI configuration tool

To use the firewall-config GUI configuration tool, install the firewall-config package as root:

$ sudo dnf install firewall-config

Alternatively, in GNOME, use the Super key and type Software to launch the Software Sources application. Type firewall to the search box, which appears after selecting the search button in the top-right corner. Select the Firewall item from the search results, and click on the Install button.

To run firewall-config, use either the firewall-config command or press the Super key to enter the Activities Overview, type firewall, and press Enter.

Managing firewalld

Viewing the current status of `firewalld`

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

$ sudo firewall-cmd --state

For more information about the service status, use the systemctl command

$ sudo systemctl status firewalld
firewalld.service - firewalld - dynamic firewall daemon
   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor pr
   Active: active (running) since Mon 2017-12-18 16:05:15 CET; 50min ago
     Docs: man:firewalld(1)
 Main PID: 705 (firewalld)
    Tasks: 2 (limit: 4915)
   CGroup: /system.slice/firewalld.service
           └─705 /usr/bin/python3 -Es /usr/sbin/firewalld --nofork --nopid

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 current firewalld settings

Viewing allowed services using GUI

To view the list of services using the graphical firewall-config tool, press the Super key to enter the Activities Overview, type firewall, and press Enter. The firewall-config tool appears. You can now view the list of services under the Services tab.

Alternatively, to start the graphical firewall configuration tool using the command-line, enter the following command:

$ firewall-config

The Firewall Configuration window opens. Note that this command can be run as a normal user, but you are prompted for an administrator password occasionally.

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
public
  target: default
  icmp-block-inversion: no
  interfaces:
  sources:
  services: ssh dhcpv6-client
  ports:
  protocols:
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:

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
home
  target: default
  icmp-block-inversion: no
  interfaces:
  sources:
  services: ssh mdns samba-client dhcpv6-client
... [output truncated]

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

Usage: firewall-cmd [OPTIONS...]

General Options
  -h, --help           Prints a short help text and exists
  -V, --version        Print the version string of firewalld
  -q, --quiet          Do not print status messages

Status Options
  --state              Return and print firewalld state
  --reload             Reload firewall and keep state information
... [output truncated]

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.

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

What are ports?

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
```