Introduction to FirewallD on CentOS
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FirewallD is frontend controller for iptables used to implement persistent network traffic rules. It provides command line and graphical interfaces and is available in the repositories of most Linux distributions. Working with FirewallD has two main differences compared to directly controlling iptables:
- FirewallD uses zones and services instead of chain and rules.
- It manages rulesets dynamically, allowing updates without breaking existing sessions and connections.
NoteFirewallD is a wrapper for iptables to allow easier management of iptables rules–it is not an iptables replacement. While iptables commands are still available to FirewallD, it’s recommended to use only FirewallD commands with FirewallD.
This guide will introduce you to FirewallD, its notions of zones and services, and show you some basic configuration steps.
Installing and Managing FirewallD
FirewallD is included by default with CentOS 7 but it’s inactive. Controlling it is the same as with other systemd units.
To start the service and enable FirewallD on boot:
sudo systemctl start firewalld sudo systemctl enable firewalld
To stop and disable it:
sudo systemctl stop firewalld sudo systemctl disable firewalld
Check the firewall status. The output should say either
sudo firewall-cmd --state
To view the status of the FirewallD daemon:
sudo systemctl status firewalld
firewalld.service - firewalld - dynamic firewall daemon Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled) Active: active (running) since Thu 2019-08-08 15:11:24 IST; 23h ago Docs: man:firewalld(1) Main PID: 2577 (firewalld) CGroup: /system.slice/firewalld.service └─2577 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid
To reload a FirewallD configuration:
sudo firewall-cmd --reload
Firewalld is configured with XML files. Except for very specific configurations, you won’t have to deal with them and firewall-cmd should be used instead.
Configuration files are located in two directories:
/usr/lib/FirewallDholds default configurations like default zones and common services. Avoid updating them because those files will be overwritten by each firewalld package update.
/etc/firewalldholds system configuration files. These files will overwrite a default configuration.
Firewalld uses two configuration sets: Runtime and Permanent. Runtime configuration changes are not retained on reboot or upon restarting FirewallD whereas permanent changes are not applied to a running system.
firewall-cmd commands apply to runtime configuration but using the
--permanent flag will establish a persistent configuration. To add and activate a permanent rule, you can use one of two methods.
Add the rule to both the permanent and runtime sets.
sudo firewall-cmd --zone=public --add-service=http --permanent sudo firewall-cmd --zone=public --add-service=http
Add the rule to the permanent set and reload FirewallD.
sudo firewall-cmd --zone=public --add-service=http --permanent sudo firewall-cmd --reload
NoteThe reload command drops all runtime configurations and applies a permanent configuration. Because firewalld manages the ruleset dynamically, it won’t break an existing connection and session.
Zones are pre-constructed rulesets for various trust levels you would likely have for a given location or scenario (e.g. home, public, trusted, etc.). Different zones allow different network services and incoming traffic types while denying everything else. After enabling FirewallD for the first time, Public will be the default zone.
Zones can also be applied to different network interfaces. For example, with separate interfaces for both an internal network and the Internet, you can allow DHCP on an internal zone but only HTTP and SSH on external zone. Any interface not explicitly set to a specific zone will be attached to the default zone.
To view the default zone:
sudo firewall-cmd --get-default-zone
To change the default zone:
sudo firewall-cmd --set-default-zone=internal
To see the zones used by your network interface(s):
sudo firewall-cmd --get-active-zones
public interfaces: eth0
To get all configurations for a specific zone:
sudo firewall-cmd --zone=public --list-all
public (active) target: default icmp-block-inversion: no interfaces: eth0 sources: services: ssh dhcpv6-client http ports: 12345/tcp protocols: masquerade: no forward-ports: source-ports: icmp-blocks: rich rules:
To get all configurations for all zones:
sudo firewall-cmd --list-all-zones
trusted target: ACCEPT icmp-block-inversion: no interfaces: sources: services: ports: protocols: masquerade: no forward-ports: source-ports: icmp-blocks: rich rules: ... work target: default icmp-block-inversion: no interfaces: sources: services: ssh dhcpv6-client ports: protocols: masquerade: no forward-ports: source-ports: icmp-blocks: rich rules:
Working with Services
FirewallD can allow traffic based on predefined rules for specific network services. You can create your own custom service rules and add them to any zone. The configuration files for the default supported services are located at
/usr/lib/firewalld/services and user-created service files would be in
To view the default available services:
sudo firewall-cmd --get-services
As an example, to enable or disable the HTTP service:
sudo firewall-cmd --zone=public --add-service=http --permanent sudo firewall-cmd --zone=public --remove-service=http --permanent
Allowing or Denying an Arbitrary Port/Protocol
As an example: Allow or disable TCP traffic on port 12345.
sudo firewall-cmd --zone=public --add-port=12345/tcp --permanent sudo firewall-cmd --zone=public --remove-port=12345/tcp --permanent
The example rule below forwards traffic from port 80 to port 12345 on the same server.
sudo firewall-cmd --zone="public" --add-forward-port=port=80:proto=tcp:toport=12345
To forward a port to a different server:
Activate masquerade in the desired zone.
sudo firewall-cmd --zone=public --add-masquerade
Add the forward rule. This example forwards traffic from local port 80 to port 8080 on a remote server located at the IP address: 198.51.100.0.
sudo firewall-cmd --zone="public" --add-forward-port=port=80:proto=tcp:toport=8080:toaddr=198.51.100.0
To remove the rules, substitute
--remove. For example:
sudo firewall-cmd --zone=public --remove-masquerade
Constructing a Ruleset with FirewallD
As an example, here is how you would use FirewallD to assign basic rules to your Linode if you were running a web server.
Assign the dmz zone as the default zone to eth0. Of the default zones offered, dmz (demilitarized zone) is the most desirable to start with for this application because it allows only SSH and ICMP.
sudo firewall-cmd --set-default-zone=dmz sudo firewall-cmd --zone=dmz --add-interface=eth0
Add permanent service rules for HTTP and HTTPS to the dmz zone:
sudo firewall-cmd --zone=dmz --add-service=http --permanent sudo firewall-cmd --zone=dmz --add-service=https --permanent
Reload FirewallD so the rules take effect immediately:
sudo firewall-cmd --reload
If you now run
firewall-cmd --zone=dmz --list-all, this should be the output:
dmz (default) interfaces: eth0 sources: services: http https ssh ports: masquerade: no forward-ports: icmp-blocks: rich rules:
This tells us that the dmz zone is our default which applies to the eth0 interface, all network sources and ports. Incoming HTTP (port 80), HTTPS (port 443) and SSH (port 22) traffic is allowed and since there are no restrictions on IP versioning, this will apply to both IPv4 and IPv6. Masquerading and port forwarding are not allowed. We have no ICMP blocks, so ICMP traffic is fully allowed, and no rich rules. All outgoing traffic is allowed.
Services and ports are fine for basic configuration but may be too limiting for advanced scenarios. Rich Rules and Direct Interface allow you to add fully custom firewall rules to any zone for any port, protocol, address and action.
Rich rules syntax is extensive but fully documented in the
firewalld.richlanguage(5) man page (or see
man firewalld.richlanguage in your terminal). Use
--remove-rich-rule with firewall-cmd command to manage them.
Here are some common examples:
Allow all IPv4 traffic from host 192.0.2.0.
sudo firewall-cmd --zone=public --add-rich-rule 'rule family="ipv4" source address=192.0.2.0 accept'
Deny IPv4 traffic over TCP from host 192.0.2.0 to port 22.
sudo firewall-cmd --zone=public --add-rich-rule 'rule family="ipv4" source address="192.0.2.0" port port=22 protocol=tcp reject'
Allow IPv4 traffic over TCP from host 192.0.2.0 to port 80, and forward it locally to port 6532.
sudo firewall-cmd --zone=public --add-rich-rule 'rule family=ipv4 source address=192.0.2.0 forward-port port=80 protocol=tcp to-port=6532'
Forward all IPv4 traffic on port 80 to port 8080 on host 198.51.100.0 (masquerade should be active on the zone).
sudo firewall-cmd --zone=public --add-rich-rule 'rule family=ipv4 forward-port port=80 protocol=tcp to-port=8080 to-addr=198.51.100.0'
To list your current Rich Rules in the public zone:
sudo firewall-cmd --zone=public --list-rich-rules
iptables Direct Interface
For the most advanced usage, or for iptables experts, FirewallD provides a direct interface that allows you to pass raw iptables commands to it. Direct Interface rules are not persistent unless the
--permanent is used.
To see all custom chains or rules added to FirewallD:
firewall-cmd --direct --get-all-chains firewall-cmd --direct --get-all-rules
Discussing iptables syntax details goes beyond the scope of this guide. If you want to learn more, you can review our iptables guide.
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