Introduction

Firewalld is a firewall management solution available for many Linux distributions which acts as a frontend for the iptables packet filtering system provided by the Linux kernel. In this guide, we will cover how to set up a firewall for your server and show you the basics of managing the firewall with the firewall-cmd administrative tool (if you'd rather use iptables with CentOS, follow this guide).

Note: There is a chance that you may be working with a newer version of firewalld than was available at the time of this writing, or that your server was set up slightly differently than the example server used throughout this guide. Thus, the behavior of some of the commands explained in this guide may vary depending on your specific configuration.

Basic Concepts in Firewalld

Before we begin talking about how to actually use the firewall-cmd utility to manage your firewall configuration, we should get familiar with a few basic concepts that the tool introduces.

Zones

The firewalld daemon manages groups of rules using entities called "zones". Zones are basically sets of rules dictating what traffic should be allowed depending on the level of trust you have in the networks your computer is connected to. Network interfaces are assigned a zone to dictate the behavior that the firewall should allow.

For computers that might move between networks frequently (like laptops), this kind of flexibility provides a good method of changing your rules depending on your environment. You may have strict rules in place prohibiting most traffic when operating on a public WiFi network, while allowing more relaxed restrictions when connected to your home network. For a server, these zones are not as immediately important because the network environment rarely, if ever, changes.

Regardless of how dynamic your network environment may be, it is still useful to be familiar with the general idea behind each of the predefined zones for firewalld. In order from least trusted to most trusted, the predefined zones within firewalld are:

• drop: The lowest level of trust. All incoming connections are dropped without reply and only outgoing connections are possible.
• block: Similar to the above, but instead of simply dropping connections, incoming requests are rejected with an icmp-host-prohibited or icmp6-adm-prohibited message.
• public: Represents public, untrusted networks. You don't trust other computers but may allow selected incoming connections on a case-by-case basis.
• external: External networks in the event that you are using the firewall as your gateway. It is configured for NAT masquerading so that your internal network remains private but reachable.
• internal: The other side of the external zone, used for the internal portion of a gateway. The computers are fairly trustworthy and some additional services are available.
• dmz: Used for computers located in a DMZ (isolated computers that will not have access to the rest of your network). Only certain incoming connections are allowed.
• work: Used for work machines. Trust most of the computers in the network. A few more services might be allowed.
• home: A home environment. It generally implies that you trust most of the other computers and that a few more services will be accepted.
• trusted: Trust all of the machines in the network. The most open of the available options and should be used sparingly.

To use the firewall, we can create rules and alter the properties of our zones and then assign our network interfaces to whichever zones are most appropriate.

Rule Permanence

In firewalld, rules can be designated as either permanent or immediate. If a rule is added or modified, by default, the behavior of the currently running firewall is modified. At the next boot, the old rules will be reverted.

Most firewall-cmd operations can take the --permanent flag to indicate that the non-ephemeral firewall should be targeted. This will affect the rule set that is reloaded upon boot. This separation means that you can test rules in your active firewall instance and then reload if there are problems. You can also use the --permanent flag to build out an entire set of rules over time that will all be applied at once when the reload command is issued.

Install and Enable Your Firewall to Start at Boot

firewalld is installed by default on some Linux distributions, including many images of CentOS 7. However, it may be necessary for you to install firewalld yourself:

sudo yum install firewalld

After you install firewalld, you can enable the service and reboot your server. Keep in mind that enabling firewalld will cause the service to start up at boot. It is best practice to create your firewall rules and take the opportunity to test them before configuring this behavior in order to avoid potential issues.

sudo systemctl enable firewalld
sudo reboot

When the server restarts, your firewall should be brought up, your network interfaces should be put into the zones you configured (or fall back to the configured default zone), and any rules associated with the zone(s) will be applied to the associated interfaces.

We can verify that the service is running and reachable by typing:

sudo firewall-cmd --state

output
running

This indicates that our firewall is up and running with the default configuration.

Getting Familiar with the Current Firewall Rules

Before we begin to make modifications, we should familiarize ourselves with the default environment and rules provided by the daemon.

Exploring the Defaults

We can see which zone is currently selected as the default by typing:

firewall-cmd --get-default-zone

output
public

Since we haven't given firewalld any commands to deviate from the default zone, and none of our interfaces are configured to bind to another zone, that zone will also be the only "active" zone (the zone that is controlling the traffic for our interfaces). We can verify that by typing:

firewall-cmd --get-active-zones

output
public
  interfaces: eth0 eth1

Here, we can see that our example server has two network interfaces being controlled by the firewall (eth0 and eth1). They are both currently being managed according to the rules defined for the public zone.

How do we know what rules are associated with the public zone though? We can print out the default zone's configuration by typing:

sudo firewall-cmd --list-all

output
public (default, active)
  target: default
  icmp-block-inversion: no
  interfaces: eth0 eth1
  sources: 
  services: ssh dhcpv6-client
  ports: 
  protocols: 
  masquerade: no
  forward-ports: 
  source-ports: 
  icmp-blocks: 
  rich rules: 

We can tell from the output that this zone is both the default and active and that the eth0 and eth1 interfaces are associated with this zone (we already knew all of this from our previous inquiries). However, we can also see that this zone allows for the normal operations associated with a DHCP client (for IP address assignment) and SSH (for remote administration).

Exploring Alternative Zones

Now we have a good idea about the configuration for the default and active zone. We can find out information about other zones as well.

To get a list of the available zones, type:

firewall-cmd --get-zones

output
block dmz drop external home internal public trusted work

We can see the specific configuration associated with a zone by including the --zone= parameter in our --list-all command:

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

output
home
  interfaces: 
  sources: 
  services: dhcpv6-client ipp-client mdns samba-client ssh
  ports: 
  masquerade: no
  forward-ports: 
  icmp-blocks: 
  rich rules:

You can output all of the zone definitions by using the --list-all-zones option. You will probably want to pipe the output into a pager for easier viewing:

sudo firewall-cmd --list-all-zones | less

Selecting Zones for your Interfaces

Unless you have configured your network interfaces otherwise, each interface will be put in the default zone when the firewall is booted.

Changing the Zone of an Interface

You can transition an interface between zones during a session by using the --zone= parameter in combination with the --change-interface= parameter. As with all commands that modify the firewall, you will need to use sudo.

For instance, we can transition our eth0 interface to the "home" zone by typing this:

sudo firewall-cmd --zone=home --change-interface=eth0

output
success

We can verify that this was successful by asking for the active zones again:

firewall-cmd --get-active-zones

output
home
  interfaces: eth0
public
  interfaces: eth1

Adjusting the Default Zone

If all of your interfaces can best be handled by a single zone, it's probably easier to just select the best default zone and then use that for your configuration.

You can change the default zone with the --set-default-zone= parameter. This will immediately change any interface that had fallen back on the default to the new zone:

sudo firewall-cmd --set-default-zone=home

output
success

Setting Rules for your Applications

The basic way of defining firewall exceptions for the services you wish to make available is easy. We'll run through the basic idea here.

Adding a Service to your Zones

The easiest method is to add the services or ports you need to the zones you are using. Again, you can get a list of the available services with the --get-services option:

firewall-cmd --get-services

output
RH-Satellite-6 amanda-client amanda-k5-client bacula bacula-client bitcoin bitcoin-rpc bitcoin-testnet bitcoin-testnet-rpc ceph ceph-mon cfengine condor-collector ctdb dhcp dhcpv6 dhcpv6-client dns docker-registry dropbox-lansync elasticsearch freeipa-ldap freeipa-ldaps freeipa-replication freeipa-trust ftp ganglia-client ganglia-master high-availability http https imap imaps ipp ipp-client ipsec iscsi-target kadmin kerberos kibana klogin kpasswd kshell ldap ldaps libvirt libvirt-tls managesieve mdns mosh mountd ms-wbt mssql mysql nfs nrpe ntp openvpn ovirt-imageio ovirt-storageconsole ovirt-vmconsole pmcd pmproxy pmwebapi pmwebapis pop3 pop3s postgresql privoxy proxy-dhcp ptp pulseaudio puppetmaster quassel radius rpc-bind rsh rsyncd samba samba-client sane sip sips smtp smtp-submission smtps snmp snmptrap spideroak-lansync squid ssh synergy syslog syslog-tls telnet tftp tftp-client tinc tor-socks transmission-client vdsm vnc-server wbem-https xmpp-bosh xmpp-client xmpp-local xmpp-server

<?xml version="1.0" encoding="utf-8"?>
<service>
  <short>SSH</short>
  <description>Secure Shell (SSH) is a protocol for logging into and executing commands on remote machines. It provides secure encrypted communications. If you plan on accessing your machine remotely via SSH over a firewalled interface, enable this option. You need the openssh-server package installed for this option to be useful.</description>
  <port protocol="tcp" port="22"/>
</service>

For instance, if we are running a web server serving conventional HTTP traffic, we can allow this traffic for interfaces in our "public" zone for this session by typing:

sudo firewall-cmd --zone=public --add-service=http

You can leave out the --zone= if you wish to modify the default zone. We can verify the operation was successful by using the --list-all or --list-services operations:

sudo firewall-cmd --zone=public --list-services

output
dhcpv6-client http ssh

Once you have tested that everything is working as it should, you will probably want to modify the permanent firewall rules so that your service will still be available after a reboot. We can make our "public" zone change permanent by typing:

sudo firewall-cmd --zone=public --permanent --add-service=http

output
success

You can verify that this was successful by adding the --permanent flag to the --list-services operation. You need to use sudo for any --permanent operations:

sudo firewall-cmd --zone=public --permanent --list-services

output
dhcpv6-client http ssh

Your "public" zone will now allow HTTP web traffic on port 80. If your web server is configured to use SSL/TLS, you'll also want to add the https service. We can add that to the current session and the permanent rule-set by typing:

sudo firewall-cmd --zone=public --add-service=https
sudo firewall-cmd --zone=public --permanent --add-service=https

What If No Appropriate Service Is Available?

The firewall services that are included with the firewalld installation represent many of the most common requirements for applications that you may wish to allow access to. However, there will likely be scenarios where these services do not fit your requirements.

In this situation, you have two options.

Opening a Port for your Zones

The easiest way to add support for your specific application is to open up the ports that it uses in the appropriate zone(s). This is as easy as specifying the port or port range, and the associated protocol for the ports you need to open.

For instance, if our application runs on port 5000 and uses TCP, we could add this to the "public" zone for this session using the --add-port= parameter. Protocols can be either tcp or udp:

sudo firewall-cmd --zone=public --add-port=5000/tcp

output
success

We can verify that this was successful using the --list-ports operation:

sudo firewall-cmd --zone=public --list-ports

output
5000/tcp

It is also possible to specify a sequential range of ports by separating the beginning and ending port in the range with a dash. For instance, if our application uses UDP ports 4990 to 4999, we could open these up on "public" by typing:

sudo firewall-cmd --zone=public --add-port=4990-4999/udp

After testing, we would likely want to add these to the permanent firewall. You can do that by typing:

sudo firewall-cmd --zone=public --permanent --add-port=5000/tcp
sudo firewall-cmd --zone=public --permanent --add-port=4990-4999/udp
sudo firewall-cmd --zone=public --permanent --list-ports

output
success
success
5000/tcp 4990-4999/udp

Defining a Service

Opening ports for your zones is easy, but it can be difficult to keep track of what each one is for. If you ever decommission a service on your server, you may have a hard time remembering which ports that have been opened are still required. To avoid this situation, it is possible to define a service.

Services are simply collections of ports with an associated name and description. Using services is easier to administer than ports, but requires a bit of upfront work. The easiest way to start is to copy an existing script (found in /usr/lib/firewalld/services) to the /etc/firewalld/services directory where the firewall looks for non-standard definitions.

For instance, we could copy the SSH service definition to use for our "example" service definition like this. The filename minus the .xml suffix will dictate the name of the service within the firewall services list:

sudo cp /usr/lib/firewalld/services/ssh.xml /etc/firewalld/services/example.xml

Now, you can adjust the definition found in the file you copied:

sudo vi /etc/firewalld/services/example.xml

To start, the file will contain the SSH definition that you copied:

<?xml version="1.0" encoding="utf-8"?>
<service>
  <short>SSH</short>
  <description>Secure Shell (SSH) is a protocol for logging into and executing commands on remote machines. It provides secure encrypted communications. If you plan on accessing your machine remotely via SSH over a firewalled interface, enable this option. You need the openssh-server package installed for this option to be useful.</description>
  <port protocol="tcp" port="22"/>
</service>

The majority of this definition is actually metadata. You will want to change the short name for the service within the <short> tags. This is a human-readable name for your service. You should also add a description so that you have more information if you ever need to audit the service. The only configuration you need to make that actually affects the functionality of the service will likely be the port definition where you identify the port number and protocol you wish to open. This can be specified multiple times.

For our "example" service, imagine that we need to open up port 7777 for TCP and 8888 for UDP. By entering INSERT mode by pressing i, we can modify the existing definition with something like this:

<?xml version="1.0" encoding="utf-8"?>
<service>
  <short>Example Service</short>
  <description>This is just an example service.  It probably shouldn't be used on a real system.</description>
  <port protocol="tcp" port="7777"/>
  <port protocol="udp" port="8888"/>
</service>

Press ESC, then enter :x to save and close the file.

Reload your firewall to get access to your new service:

sudo firewall-cmd --reload

You can see that it is now among the list of available services:

firewall-cmd --get-services

output
RH-Satellite-6 amanda-client amanda-k5-client bacula bacula-client bitcoin bitcoin-rpc bitcoin-testnet bitcoin-testnet-rpc ceph ceph-mon cfengine condor-collector ctdb dhcp dhcpv6 dhcpv6-client dns docker-registry dropbox-lansync elasticsearch example freeipa-ldap freeipa-ldaps freeipa-replication freeipa-trust ftp ganglia-client ganglia-master high-availability http https imap imaps ipp ipp-client ipsec iscsi-target kadmin kerberos kibana klogin kpasswd kshell ldap ldaps libvirt libvirt-tls managesieve mdns mosh mountd ms-wbt mssql mysql nfs nrpe ntp openvpn ovirt-imageio ovirt-storageconsole ovirt-vmconsole pmcd pmproxy pmwebapi pmwebapis pop3 pop3s postgresql privoxy proxy-dhcp ptp pulseaudio puppetmaster quassel radius rpc-bind rsh rsyncd samba samba-client sane sip sips smtp smtp-submission smtps snmp snmptrap spideroak-lansync squid ssh synergy syslog syslog-tls telnet tftp tftp-client tinc tor-socks transmission-client vdsm vnc-server wbem-https xmpp-bosh xmpp-client xmpp-local xmpp-server

You can now use this service in your zones as you normally would.

Creating Your Own Zones

While the predefined zones will probably be more than enough for most users, it can be helpful to define your own zones that are more descriptive of their function.

For instance, you might want to create a zone for your web server, called "publicweb". However, you might want to have another zone configured for the DNS service you provide on your private network. You might want a zone called "privateDNS" for that.

When adding a zone, you must add it to the permanent firewall configuration. You can then reload to bring the configuration into your running session. For instance, we could create the two zones we discussed above by typing:

sudo firewall-cmd --permanent --new-zone=publicweb
sudo firewall-cmd --permanent --new-zone=privateDNS

You can verify that these are present in your permanent configuration by typing:

sudo firewall-cmd --permanent --get-zones

output
block dmz drop external home internal privateDNS public publicweb trusted work

As stated before, these won't be available in the current instance of the firewall yet:

firewall-cmd --get-zones

output
block dmz drop external home internal public trusted work

Reload the firewall to bring these new zones into the active configuration:

sudo firewall-cmd --reload
firewall-cmd --get-zones

output
block dmz drop external home internal privateDNS public publicweb trusted work

Now, you can begin assigning the appropriate services and ports to your zones. It's usually a good idea to adjust the active instance and then transfer those changes to the permanent configuration after testing. For instance, for the "publicweb" zone, you might want to add the SSH, HTTP, and HTTPS services:

sudo firewall-cmd --zone=publicweb --add-service=ssh
sudo firewall-cmd --zone=publicweb --add-service=http
sudo firewall-cmd --zone=publicweb --add-service=https
sudo firewall-cmd --zone=publicweb --list-all

output
publicweb
  target: default
  icmp-block-inversion: no
  interfaces: 
  sources: 
  services: ssh http https
  ports: 
  protocols: 
  masquerade: no
  forward-ports: 
  source-ports: 
  icmp-blocks: 
  rich rules:

Likewise, we can add the DNS service to our "privateDNS" zone:

sudo firewall-cmd --zone=privateDNS --add-service=dns
sudo firewall-cmd --zone=privateDNS --list-all

output
privateDNS
  interfaces: 
  sources: 
  services: dns
  ports: 
  masquerade: no
  forward-ports: 
  icmp-blocks: 
  rich rules:

We could then change our interfaces over to these new zones to test them out:

sudo firewall-cmd --zone=publicweb --change-interface=eth0
sudo firewall-cmd --zone=privateDNS --change-interface=eth1

At this point, you have the opportunity to test your configuration. If these values work for you, you will want to add the same rules to the permanent configuration. You can do that by re-applying the rules with the --permanent flag:

sudo firewall-cmd --zone=publicweb --permanent --add-service=ssh
sudo firewall-cmd --zone=publicweb --permanent --add-service=http
sudo firewall-cmd --zone=publicweb --permanent --add-service=https
sudo firewall-cmd --zone=privateDNS --permanent --add-service=dns

After permanently applying these your rules, you can restart your network and reload your firewall service:

sudo systemctl restart network
sudo systemctl reload firewalld

Validate that the correct zones were assigned:

firewall-cmd --get-active-zones

output
privateDNS
  interfaces: eth1
publicweb
  interfaces: eth0

And validate that the appropriate services are available for both of the zones:

sudo firewall-cmd --zone=publicweb --list-services

output
http https ssh

sudo firewall-cmd --zone=privateDNS --list-services

output
dns

You have successfully set up your own zones! If you want to make one of these zones the default for other interfaces, remember to configure that behavior with the --set-default-zone= parameter:

sudo firewall-cmd --set-default-zone=publicweb

Conclusion

You should now have a fairly good understanding of how to administer the firewalld service on your CentOS system for day-to-day use.

The firewalld service allows you to configure maintainable rules and rule-sets that take into consideration your network environment. It allows you to seamlessly transition between different firewall policies through the use of zones and gives administrators the ability to abstract the port management into more friendly service definitions. Acquiring a working knowledge of this system will allow you to take advantage of the flexibility and power that this tool provides.