You need to increase the security of your linux system by finding and closing open ports.

47.1.1. When to use firewalld, nftables, or iptables

The following is a brief overview in which scenario you should use one of the following utilities:

• firewalld: Use the firewalld utility for simple firewall use cases. The utility is easy to use and covers the typical use cases for these scenarios.
• nftables: Use the nftables utility to set up complex and performance critical firewalls, such as for a whole network.
• iptables: The iptables utility on Red Hat Enterprise Linux uses the nf_tables kernel API instead of the legacy back end. The nf_tables API provides backward compatibility so that scripts that use iptables commands still work on Red Hat Enterprise Linux. For new firewall scripts, Red Hat recommends to use nftables.

To avoid that the different firewall services influence each other, run only one of them on a RHEL host, and disable the other services.

47.1.2. Zones

⁠firewalld can be used to separate networks into different zones according to the level of trust that the user has decided to place on the interfaces and traffic within that network. A connection can only be part of one zone, but a zone can be used for many network connections.

NetworkManager notifies firewalld of the zone of an interface. You can assign zones to interfaces with:

• NetworkManager
• firewall-config tool
• firewall-cmd command-line tool
• The RHEL web console

The latter three can only edit the appropriate NetworkManager configuration files. If you change the zone of the interface using the web console, firewall-cmd or firewall-config, the request is forwarded to NetworkManager and is not handled by ⁠firewalld.

The predefined zones are stored in the /usr/lib/firewalld/zones/ directory and can be instantly applied to any available network interface. These files are copied to the /etc/firewalld/zones/ directory only after they are modified. The default settings of the predefined zones are as follows:

block Any incoming network connections are rejected with an icmp-host-prohibited message for IPv4 and icmp6-adm-prohibited for IPv6. Only network connections initiated from within the system are possible. dmz For computers in your demilitarized zone that are publicly-accessible with limited access to your internal network. Only selected incoming connections are accepted. drop Any incoming network packets are dropped without any notification. Only outgoing network connections are possible. external For use on external networks with masquerading enabled, especially for routers. You do not trust the other computers on the network to not harm your computer. Only selected incoming connections are accepted. home For use at home when you mostly trust the other computers on the network. Only selected incoming connections are accepted. internal For use on internal networks when you mostly trust the other computers on the network. Only selected incoming connections are accepted. public For use in public areas where you do not trust other computers on the network. Only selected incoming connections are accepted. trusted All network connections are accepted. work For use at work where you mostly trust the other computers on the network. Only selected incoming connections are accepted.

One of these zones is set as the default zone. When interface connections are added to NetworkManager, they are assigned to the default zone. On installation, the default zone in firewalld is set to be the public zone. The default zone can be changed.

The network zone names should be self-explanatory and to allow users to quickly make a reasonable decision. To avoid any security problems, review the default zone configuration and disable any unnecessary services according to your needs and risk assessments.

Additional resources

• The firewalld.zone(5) man page.

47.1.3. Predefined services

A service can be a list of local ports, protocols, source ports, and destinations, as well as a list of firewall helper modules automatically loaded if a service is enabled. Using services saves users time because they can achieve several tasks, such as opening ports, defining protocols, enabling packet forwarding and more, in a single step, rather than setting up everything one after another.

Service configuration options and generic file information are described in the firewalld.service(5) man page. The services are specified by means of individual XML configuration files, which are named in the following format: service-name.xml. Protocol names are preferred over service or application names in firewalld.

Services can be added and removed using the graphical firewall-config tool, firewall-cmd, and firewall-offline-cmd.

Alternatively, you can edit the XML files in the /etc/firewalld/services/ directory. If a service is not added or changed by the user, then no corresponding XML file is found in /etc/firewalld/services/. The files in the /usr/lib/firewalld/services/ directory can be used as templates if you want to add or change a service.

Additional resources

• The firewalld.service(5) man page

47.1.4. Starting firewalld

Procedure

1. To start firewalld, enter the following command as root:

   # systemctl unmask firewalld
   # systemctl start firewalld

2. To ensure firewalld starts automatically at system start, enter the following command as root:

   # systemctl enable firewalld

47.1.5. Stopping firewalld

Procedure

1. To stop firewalld, enter the following command as root:

   # systemctl stop firewalld

2. To prevent firewalld from starting automatically at system start:

   # systemctl disable firewalld

3. To make sure firewalld is not started by accessing the firewalld D-Bus interface and also if other services require firewalld:

   # systemctl mask firewalld

47.1.6. Verifying the permanent firewalld configuration

In certain situations, for example after manually editing firewalld configuration files, administrators want to verify that the changes are correct. This section describes how to verify the permanent configuration of the firewalld service.

Prerequisites

• The firewalld service is running.

Procedure

1. Verify the permanent configuration of the firewalld service:

   # firewall-cmd --check-config
   success

   If the permanent configuration is valid, the command returns success. In other cases, the command returns an error with further details, such as the following:

   # firewall-cmd --check-config
   Error: INVALID_PROTOCOL: 'public.xml': 'tcpx' not from {'tcp'|'udp'|'sctp'|'dccp'}

47.2.1. 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.

Procedure

1. To see the status of the service:

   # firewall-cmd --state

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

   # 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

47.2.2. 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.

You can start the graphical firewall configuration tool using the command-line.

Prerequisites

• You installed the firewall-config package.

Procedure

• To start the graphical firewall configuration tool using the command-line:

  $ 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.

47.2.3. 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.

Procedure

• 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
  ... [trimmed for clarity]

• 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

• To see which services are allowed in the current zone:

  # firewall-cmd --list-services
  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.

47.3.1. Disabling all traffic in case of emergency using CLI

In an emergency situation, such as a system attack, it is possible to disable all network traffic and cut off the attacker.

Procedure

1. To immediately disable networking traffic, switch panic mode on:

   # firewall-cmd --panic-on

   Enabling panic mode stops all networking traffic. For this reason, it should be used only when you have the physical access to the machine or if you are logged in using a serial console.

2. Switching off panic mode reverts the firewall to its permanent settings. To switch panic mode off, enter:

   # firewall-cmd --panic-off

Verification

• To see whether panic mode is switched on or off, use:

  # firewall-cmd --query-panic

47.3.2. Controlling traffic with predefined services using CLI

The most straightforward method to control traffic is to add a predefined service to firewalld. This opens all necessary ports and modifies other settings according to the service definition file.

Procedure

1. Check that the service is not already allowed:

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

2. List all predefined services:

   # firewall-cmd --get-services
   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 ...
   [trimmed for clarity]

3. Add the service to the allowed services:

   # firewall-cmd --add-service=<service-name>

4. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

47.3.3. Controlling traffic with predefined services using GUI

This procedure describes how to control the network traffic with predefined services using graphical user interface.

Prerequisites

• You installed the firewall-config package

Procedure

1. To enable or disable a predefined or custom service:

   1. Start the firewall-config tool and select the network zone whose services are to be configured.
   2. Select the Zones tab and then the Services tab below.
   3. Select the check box for each type of service you want to trust or clear the check box to block a service in the selected zone.

2. To edit a service:

   1. Start the firewall-config tool.
   2. Select Permanent from the menu labeled Configuration. Additional icons and menu buttons appear at the bottom of the Services window.
   3. Select the service you want to configure.

The Ports, Protocols, and Source Port tabs enable adding, changing, and removing of ports, protocols, and source port for the selected service. The modules tab is for configuring Netfilter helper modules. The Destination tab enables limiting traffic to a particular destination address and Internet Protocol (IPv4 or IPv6).

It is not possible to alter service settings in the Runtime mode.

47.3.4. Adding new services

Services can be added and removed using the graphical firewall-config tool, firewall-cmd, and firewall-offline-cmd. Alternatively, you can edit the XML files in /etc/firewalld/services/. If a service is not added or changed by the user, then no corresponding XML file are found in /etc/firewalld/services/. The files /usr/lib/firewalld/services/ can be used as templates if you want to add or change a service.

Service names must be alphanumeric and can, additionally, include only _ (underscore) and - (dash) characters.

Procedure

To add a new service in a terminal, use firewall-cmd, or firewall-offline-cmd in case of not active firewalld.

1. Enter the following command to add a new and empty service:

   $ firewall-cmd --new-service=service-name --permanent

2. To add a new service using a local file, use the following command:

   $ firewall-cmd --new-service-from-file=service-name.xml --permanent

   You can change the service name with the additional --name=service-name option.

3. As soon as service settings are changed, an updated copy of the service is placed into /etc/firewalld/services/.

   As root, you can enter the following command to copy a service manually:

   # cp /usr/lib/firewalld/services/service-name.xml /etc/firewalld/services/service-name.xml

firewalld loads files from /usr/lib/firewalld/services in the first place. If files are placed in /etc/firewalld/services and they are valid, then these will override the matching files from /usr/lib/firewalld/services. The overridden files in /usr/lib/firewalld/services are used as soon as the matching files in /etc/firewalld/services have been removed or if firewalld has been asked to load the defaults of the services. This applies to the permanent environment only. A reload is needed to get these fallbacks also in the runtime environment.

47.3.5. Opening ports using GUI

To permit traffic through the firewall to a certain port, you can open the port in the GUI.

Prerequisites

• You installed the firewall-config package

Procedure

1. Start the firewall-config tool and select the network zone whose settings you want to change.
2. Select the Ports tab and click the Add button on the right-hand side. The Port and Protocol window opens.
3. Enter the port number or range of ports to permit.
4. Select tcp or udp from the list.

47.3.6. Controlling traffic with protocols using GUI

To permit traffic through the firewall using a certain protocol, you can use the GUI.

Prerequisites

• You installed the firewall-config package

Procedure

1. Start the firewall-config tool and select the network zone whose settings you want to change.
2. Select the Protocols tab and click the Add button on the right-hand side. The Protocol window opens.
3. Either select a protocol from the list or select the Other Protocol check box and enter the protocol in the field.

47.3.7. Opening source ports using GUI

To permit traffic through the firewall from a certain port, you can use the GUI.

Prerequisites

• You installed the firewall-config package

Procedure

1. Start the firewall-config tool and select the network zone whose settings you want to change.
2. Select the Source Port tab and click the Add button on the right-hand side. The Source Port window opens.
3. Enter the port number or range of ports to permit. Select tcp or udp from the list.

47.4.1. 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.

Procedure

To get a list of open ports in the current zone:

1. List all allowed ports:

   # firewall-cmd --list-ports

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

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

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

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

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

47.4.2. 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.

Procedure

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

1. 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.

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

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

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

47.5.1. Listing zones

This procedure describes how to list zones using the command line.

Procedure

1. To see which zones are available on your system:

   # firewall-cmd --get-zones

   The firewall-cmd --get-zones command displays all zones that are available on the system, but it does not show any details for particular zones.

2. To see detailed information for all zones:

   # firewall-cmd --list-all-zones
   

3. To see detailed information for a specific zone:

   # firewall-cmd --zone=zone-name --list-all

47.5.2. Modifying firewalld settings for a certain zone

The Controlling traffic with predefined services using cli and Controlling ports using cli explain how to add services or modify ports in the scope of the current working zone. Sometimes, it is required to set up rules in a different zone.

Procedure

• To work in a different zone, use the --zone=zone-name option. For example, to allow the SSH service in the zone public:

  # firewall-cmd --add-service=ssh --zone=public

47.5.3. Changing the default zone

System administrators assign a zone to a networking interface in its configuration files. If an interface is not assigned to a specific zone, it is assigned to the default zone. After each restart of the firewalld service, firewalld loads the settings for the default zone and makes it active.

Procedure

To set up the default zone:

1. Display the current default zone:

   # firewall-cmd --get-default-zone

2. Set the new default zone:

   # firewall-cmd --set-default-zone zone-name

   Following this procedure, the setting is a permanent setting, even without the --permanent option.

47.5.4. Assigning a network interface to a zone

It is possible to define different sets of rules for different zones and then change the settings quickly by changing the zone for the interface that is being used. With multiple interfaces, a specific zone can be set for each of them to distinguish traffic that is coming through them.

Procedure

To assign the zone to a specific interface:

1. List the active zones and the interfaces assigned to them:

   # firewall-cmd --get-active-zones

2. Assign the interface to a different zone:

   # firewall-cmd --zone=zone_name --change-interface=interface_name --permanent

47.5.5. Assigning a zone to a connection using nmcli

This procedure describes how to add a firewalld zone to a NetworkManager connection using the nmcli utility.

Procedure

1. Assign the zone to the NetworkManager connection profile:

   # nmcli connection modify profile connection.zone zone_name

2. Activate the connection:

   # nmcli connection up profile

47.5.6. Manually assigning a zone to a network connection in an ifcfg file

When the connection is managed by NetworkManager, it must be aware of a zone that it uses. For every network connection, a zone can be specified, which provides the flexibility of various firewall settings according to the location of the computer with portable devices. Thus, zones and settings can be specified for different locations, such as company or home.

Procedure

• To set a zone for a connection, edit the /etc/sysconfig/network-scripts/ifcfg-connection_name file and add a line that assigns a zone to this connection:

  ZONE=zone_name

47.5.7. Creating a new zone

To use custom zones, create a new zone and use it just like a predefined zone. New zones require the --permanent option, otherwise the command does not work.

Procedure

1. Create a new zone:

   # firewall-cmd --permanent --new-zone=zone-name

2. Check if the new zone is added to your permanent settings:

   # firewall-cmd --get-zones

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

47.5.8. Zone configuration files

Zones can also be created using a zone configuration file. This approach can be helpful when you need to create a new zone, but want to reuse the settings from a different zone and only alter them a little.

A firewalld zone configuration file contains the information for a zone. These are the zone description, services, ports, protocols, icmp-blocks, masquerade, forward-ports and rich language rules in an XML file format. The file name has to be zone-name.xml where the length of zone-name is currently limited to 17 chars. The zone configuration files are located in the /usr/lib/firewalld/zones/ and /etc/firewalld/zones/ directories.

The following example shows a configuration that allows one service (SSH) and one port range, for both the TCP and UDP protocols:

<?xml version="1.0" encoding="utf-8"?>
<zone>
  <short>My Zone</short>
  <description>Here you can describe the characteristic features of the zone.</description>
  <service name="ssh"/>
  <port protocol="udp" port="1025-65535"/>
  <port protocol="tcp" port="1025-65535"/>
</zone>

To change settings for that zone, add or remove sections to add ports, forward ports, services, and so on.

Additional resources

• firewalld.zone manual page

47.5.9. Using zone targets to set default behavior for incoming traffic

For every zone, you can set a default behavior that handles incoming traffic that is not further specified. Such behavior is defined by setting the target of the zone. There are four options:

• ACCEPT: Accepts all incoming packets except those disallowed by specific rules.
• REJECT: Rejects all incoming packets except those allowed by specific rules. When firewalld rejects packets, the source machine is informed about the rejection.
• DROP: Drops all incoming packets except those allowed by specific rules. When firewalld drops packets, the source machine is not informed about the packet drop.
• default: Similar behavior as for REJECT, but with special meanings in certain scenarios. For details, see the Options to Adapt and Query Zones and Policies section in the firewall-cmd(1) man page.

Procedure

To set a target for a zone:

1. List the information for the specific zone to see the default target:

   # firewall-cmd --zone=zone-name --list-all

2. Set a new target in the zone:

   # firewall-cmd --permanent --zone=zone-name --set-target=<default|ACCEPT|REJECT|DROP>

Additional resources

• firewall-cmd(1) man page

47.6.1. Adding a source

To route incoming traffic into a specific zone, add the source to that zone. The source can be an IP address or an IP mask in the classless inter-domain routing (CIDR) notation.

In case you add multiple zones with an overlapping network range, they are ordered alphanumerically by zone name and only the first one is considered.

• To set the source in the current zone:

  # firewall-cmd --add-source=<source>

• To set the source IP address for a specific zone:

  # firewall-cmd --zone=zone-name --add-source=<source>

The following procedure allows all incoming traffic from 192.168.2.15 in the trusted zone:

Procedure

1. List all available zones:

   # firewall-cmd --get-zones

2. Add the source IP to the trusted zone in the permanent mode:

   # firewall-cmd --zone=trusted --add-source=192.168.2.15

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

47.6.2. Removing a source

Removing a source from the zone cuts off the traffic coming from it.

Procedure

1. List allowed sources for the required zone:

   # firewall-cmd --zone=zone-name --list-sources

2. Remove the source from the zone permanently:

   # firewall-cmd --zone=zone-name --remove-source=<source>

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

47.6.3. Adding a source port

To enable sorting the traffic based on a port of origin, specify a source port using the --add-source-port option. You can also combine this with the --add-source option to limit the traffic to a certain IP address or IP range.

Procedure

• To add a source port:

  # firewall-cmd --zone=zone-name --add-source-port=<port-name>/<tcp|udp|sctp|dccp>

47.6.4. Removing a source port

By removing a source port you disable sorting the traffic based on a port of origin.

Procedure

• To remove a source port:

  # firewall-cmd --zone=zone-name --remove-source-port=<port-name>/<tcp|udp|sctp|dccp>

47.6.5. Using zones and sources to allow a service for only a specific domain

To allow traffic from a specific network to use a service on a machine, use zones and source. The following procedure allows only HTTP traffic from the 192.0.2.0/24 network while any other traffic is blocked.

When you configure this scenario, use a zone that has the default target. Using a zone that has the target set to ACCEPT is a security risk, because for traffic from 192.0.2.0/24, all network connections would be accepted.

Procedure

1. List all available zones:

   # firewall-cmd --get-zones
   block dmz drop external home internal public trusted work

2. Add the IP range to the internal zone to route the traffic originating from the source through the zone:

   # firewall-cmd --zone=internal --add-source=192.0.2.0/24

3. Add the http service to the internal zone:

   # firewall-cmd --zone=internal --add-service=http

4. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

Verification

• Check that the internal zone is active and that the service is allowed in it:

  # firewall-cmd --zone=internal --list-all
  internal (active)
    target: default
    icmp-block-inversion: no
    interfaces:
    sources: 192.0.2.0/24
    services: cockpit dhcpv6-client mdns samba-client ssh http
    ...

Additional resources

• firewalld.zones(5) man page

47.7.1. The relationship between policy objects and zones

Policy objects allow the user to attach firewalld’s primitives’ such as services, ports, and rich rules to the policy. You can apply the policy objects to traffic that passes between zones in a stateful and unidirectional manner.

# firewall-cmd --permanent --new-policy myOutputPolicy

# firewall-cmd --permanent --policy myOutputPolicy --add-ingress-zone HOST

# firewall-cmd --permanent --policy myOutputPolicy --add-egress-zone ANY

HOST and ANY are the symbolic zones used in the ingress and egress zone lists.

• The HOST symbolic zone allows policies for the traffic originating from or has a destination to the host running firewalld.
• The ANY symbolic zone applies policy to all the current and future zones. ANY symbolic zone acts as a wildcard for all zones.

47.7.2. Using priorities to sort policies

Multiple policies can apply to the same set of traffic, therefore, priorities should be used to create an order of precedence for the policies that may be applied.

To set a priority to sort the policies:

# firewall-cmd --permanent --policy mypolicy --set-priority -500

In the above example -500 is a lower priority value but has higher precedence. Thus, -500 will execute before -100. Higher priority values have precedence over lower values.

The following rules apply to policy priorities:

• Policies with negative priorities apply before rules in zones.
• Policies with positive priorities apply after rules in zones.
• Priority 0 is reserved and hence is unusable.

47.7.3. Using policy objects to filter traffic between locally hosted Containers and a network physically connected to the host

The policy objects feature allows users to filter their container and virtual machine traffic.

Procedure

1. Create a new policy.

   # firewall-cmd --permanent --new-policy podmanToHost

2. Block all traffic.

   # firewall-cmd --permanent --policy podmanToHost --set-target REJECT
   
   # firewall-cmd --permanent --policy podmanToHost --add-service dhcp
   
   # firewall-cmd --permanent --policy podmanToHost --add-service dns

   Red Hat recommends that you block all traffic to the host by default and then selectively open the services you need for the host.

3. Define the ingress zone to use with the policy.

   # firewall-cmd --permanent --policy podmanToHost --add-ingress-zone podman

4. Define the egress zone to use with the policy.

   # firewall-cmd --permanent --policy podmanToHost --add-egress-zone ANY

Verification

• Verify information about the policy.

  # firewall-cmd --info-policy podmanToHost

47.7.4. Setting the default target of policy objects

You can specify --set-target options for policies. The following targets are available:

• ACCEPT - accepts the packet
• DROP - drops the unwanted packets
• REJECT - rejects unwanted packets with an ICMP reply

• CONTINUE (default) - packets will be subject to rules in following policies and zones.

  # firewall-cmd --permanent --policy mypolicy --set-target CONTINUE

Verification

• Verify information about the policy

  # firewall-cmd --info-policy mypolicy

47.8.1. The different NAT types: masquerading, source NAT, destination NAT, and redirect

These are the different network address translation (NAT) types:

Masquerading and source NAT (SNAT)

Use one of these NAT types to change the source IP address of packets. For example, Internet Service Providers do not route private IP ranges, such as 10.0.0.0/8. If you use private IP ranges in your network and users should be able to reach servers on the Internet, map the source IP address of packets from these ranges to a public IP address.

Both masquerading and SNAT are very similar. The differences are:

• Masquerading automatically uses the IP address of the outgoing interface. Therefore, use masquerading if the outgoing interface uses a dynamic IP address.
• SNAT sets the source IP address of packets to a specified IP and does not dynamically look up the IP of the outgoing interface. Therefore, SNAT is faster than masquerading. Use SNAT if the outgoing interface uses a fixed IP address.

47.8.2. Configuring IP address masquerading

The following procedure describes how to enable IP masquerading on your system. IP masquerading hides individual machines behind a gateway when accessing the Internet.

Procedure

1. To check if IP masquerading is enabled (for example, for the external zone), enter the following command as root:

   # firewall-cmd --zone=external --query-masquerade

   The command prints yes with exit status 0 if enabled. It prints no with exit status 1 otherwise. If zone is omitted, the default zone will be used.

2. To enable IP masquerading, enter the following command as root:

   # firewall-cmd --zone=external --add-masquerade

3. To make this setting persistent, pass the --permanent option to the command.

4. To disable IP masquerading, enter the following command as root:

   # firewall-cmd --zone=external --remove-masquerade
   

   To make this setting permanent, pass the --permanent option to the command.

47.10.1. Listing and blocking ICMP requests

Listing ICMP requests

The ICMP requests are described in individual XML files that are located in the /usr/lib/firewalld/icmptypes/ directory. You can read these files to see a description of the request. The firewall-cmd command controls the ICMP requests manipulation.

• To list all available ICMP types:

  # firewall-cmd --get-icmptypes

• The ICMP request can be used by IPv4, IPv6, or by both protocols. To see for which protocol the ICMP request has used:

  # firewall-cmd --info-icmptype=<icmptype>

• The status of an ICMP request shows yes if the request is currently blocked or no if it is not. To see if an ICMP request is currently blocked:

  # firewall-cmd --query-icmp-block=<icmptype>

Blocking or unblocking ICMP requests

When your server blocks ICMP requests, it does not provide the information that it normally would. However, that does not mean that no information is given at all. The clients receive information that the particular ICMP request is being blocked (rejected). Blocking the ICMP requests should be considered carefully, because it can cause communication problems, especially with IPv6 traffic.

• To see if an ICMP request is currently blocked:

  # firewall-cmd --query-icmp-block=<icmptype>

• To block an ICMP request:

  # firewall-cmd --add-icmp-block=<icmptype>

• To remove the block for an ICMP request:

  # firewall-cmd --remove-icmp-block=<icmptype>

Blocking ICMP requests without providing any information at all

Normally, if you block ICMP requests, clients know that you are blocking it. So, a potential attacker who is sniffing for live IP addresses is still able to see that your IP address is online. To hide this information completely, you have to drop all ICMP requests.

• To block and drop all ICMP requests:

• Set the target of your zone to DROP:

  # firewall-cmd --permanent --set-target=DROP

Now, all traffic, including ICMP requests, is dropped, except traffic which you have explicitly allowed.

To block and drop certain ICMP requests and allow others:

1. Set the target of your zone to DROP:

   # firewall-cmd --permanent --set-target=DROP

2. Add the ICMP block inversion to block all ICMP requests at once:

   # firewall-cmd --add-icmp-block-inversion

3. Add the ICMP block for those ICMP requests that you want to allow:

   # firewall-cmd --add-icmp-block=<icmptype>

4. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

The block inversion inverts the setting of the ICMP requests blocks, so all requests, that were not previously blocked, are blocked because of the target of your zone changes to DROP. The requests that were blocked are not blocked. This means that if you want to unblock a request, you must use the blocking command.

To revert the block inversion to a fully permissive setting:

1. Set the target of your zone to default or ACCEPT:

   # firewall-cmd --permanent --set-target=default

2. Remove all added blocks for ICMP requests:

   # firewall-cmd --remove-icmp-block=<icmptype>

3. Remove the ICMP block inversion:

   # firewall-cmd --remove-icmp-block-inversion

4. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

47.10.2. Configuring the ICMP filter using GUI

• To enable or disable an ICMP filter, start the firewall-config tool and select the network zone whose messages are to be filtered. Select the ICMP Filter tab and select the check box for each type of ICMP message you want to filter. Clear the check box to disable a filter. This setting is per direction and the default allows everything.
• To enable inverting the ICMP Filter, click the Invert Filter check box on the right. Only marked ICMP types are now accepted, all other are rejected. In a zone using the DROP target, they are dropped.

47.11.1. Configuring IP set options using CLI

IP sets can be used in firewalld zones as sources and also as sources in rich rules. In Red Hat Enterprise Linux, the preferred method is to use the IP sets created with firewalld in a direct rule.

• To list the IP sets known to firewalld in the permanent environment, use the following command as root:

  # firewall-cmd --permanent --get-ipsets

• To add a new IP set, use the following command using the permanent environment as root:

  # firewall-cmd --permanent --new-ipset=test --type=hash:net
  success

  The previous command creates a new IP set with the name test and the hash:net type for IPv4. To create an IP set for use with IPv6, add the --option=family=inet6 option. To make the new setting effective in the runtime environment, reload firewalld.

• List the new IP set with the following command as root:

  # firewall-cmd --permanent --get-ipsets
  test

• To get more information about the IP set, use the following command as root:

  # firewall-cmd --permanent --info-ipset=test
  test
  type: hash:net
  options:
  entries:

  Note that the IP set does not have any entries at the moment.

• To add an entry to the test IP set, use the following command as root:

  # firewall-cmd --permanent --ipset=test --add-entry=192.168.0.1
  success
  

  The previous command adds the IP address 192.168.0.1 to the IP set.

• To get the list of current entries in the IP set, use the following command as root:

  # firewall-cmd --permanent --ipset=test --get-entries
  192.168.0.1

• Generate a file containing a list of IP addresses, for example:

  # cat > iplist.txt <<EOL
  192.168.0.2
  192.168.0.3
  192.168.1.0/24
  192.168.2.254
  EOL

  The file with the list of IP addresses for an IP set should contain an entry per line. Lines starting with a hash, a semi-colon, or empty lines are ignored.

• To add the addresses from the iplist.txt file, use the following command as root:

  # firewall-cmd --permanent --ipset=test --add-entries-from-file=iplist.txt
  success

• To see the extended entries list of the IP set, use the following command as root:

  # firewall-cmd --permanent --ipset=test --get-entries
  192.168.0.1
  192.168.0.2
  192.168.0.3
  192.168.1.0/24
  192.168.2.254

• To remove the addresses from the IP set and to check the updated entries list, use the following commands as root:

  # firewall-cmd --permanent --ipset=test --remove-entries-from-file=iplist.txt
  success
  # firewall-cmd --permanent --ipset=test --get-entries
  192.168.0.1

• You can add the IP set as a source to a zone to handle all traffic coming in from any of the addresses listed in the IP set with a zone. For example, to add the test IP set as a source to the drop zone to drop all packets coming from all entries listed in the test IP set, use the following command as root:

  # firewall-cmd --permanent --zone=drop --add-source=ipset:test
  success

  The ipset: prefix in the source shows firewalld that the source is an IP set and not an IP address or an address range.

Only the creation and removal of IP sets is limited to the permanent environment, all other IP set options can be used also in the runtime environment without the --permanent option.

Red Hat does not recommend using IP sets that are not managed through firewalld. To use such IP sets, a permanent direct rule is required to reference the set, and a custom service must be added to create these IP sets. This service needs to be started before firewalld starts, otherwise firewalld is not able to add the direct rules using these sets. You can add permanent direct rules with the /etc/firewalld/direct.xml file.

47.12.1. How the priority parameter organizes rules into different chains

You can set the priority parameter in a rich rule to any number between -32768 and 32767, and lower values have higher precedence.

The firewalld service organizes rules based on their priority value into different chains:

• Priority lower than 0: the rule is redirected into a chain with the _pre suffix.
• Priority higher than 0: the rule is redirected into a chain with the _post suffix.
• Priority equals 0: based on the action, the rule is redirected into a chain with the _log, _deny, or _allow the action.

Inside these sub-chains, firewalld sorts the rules based on their priority value.

47.12.2. Setting the priority of a rich rule

The procedure describes an example of how to create a rich rule that uses the priority parameter to log all traffic that is not allowed or denied by other rules. You can use this rule to flag unexpected traffic.

Procedure

1. Add a rich rule with a very low precedence to log all traffic that has not been matched by other rules:

   # firewall-cmd --add-rich-rule='rule priority=32767 log prefix="UNEXPECTED: " limit value="5/m"'

   The command additionally limits the number of log entries to 5 per minute.

2. Optionally, display the nftables rule that the command in the previous step created:

   # nft list chain inet firewalld filter_IN_public_post
   table inet firewalld {
     chain filter_IN_public_post {
       log prefix "UNEXPECTED: " limit rate 5/minute
     }
   }

47.13.1. Configuring lockdown using CLI

This procedure describes how to enable or disable lockdown using the command line.

• To query whether lockdown is enabled, use the following command as root:

  # firewall-cmd --query-lockdown

  The command prints yes with exit status 0 if lockdown is enabled. It prints no with exit status 1 otherwise.

• To enable lockdown, enter the following command as root:

  # firewall-cmd --lockdown-on

• To disable lockdown, use the following command as root:

  # firewall-cmd --lockdown-off

47.13.2. Configuring lockdown allowlist options using CLI

The lockdown allowlist can contain commands, security contexts, users and user IDs. If a command entry on the allowlist ends with an asterisk "*", then all command lines starting with that command will match. If the "*" is not there then the absolute command including arguments must match.

• The context is the security (SELinux) context of a running application or service. To get the context of a running application use the following command:

  $ ps -e --context

  That command returns all running applications. Pipe the output through the grep tool to get the application of interest. For example:

  $ ps -e --context | grep example_program

• To list all command lines that are in the allowlist, enter the following command as root:

  # firewall-cmd --list-lockdown-whitelist-commands

• To add a command command to the allowlist, enter the following command as root:

  # firewall-cmd --add-lockdown-whitelist-command='/usr/bin/python3 -Es /usr/bin/command'

• To remove a command command from the allowlist, enter the following command as root:

  # firewall-cmd --remove-lockdown-whitelist-command='/usr/bin/python3 -Es /usr/bin/command'

• To query whether the command command is in the allowlist, enter the following command as root:

  # firewall-cmd --query-lockdown-whitelist-command='/usr/bin/python3 -Es /usr/bin/command'

  The command prints yes with exit status 0 if true. It prints no with exit status 1 otherwise.

• To list all security contexts that are in the allowlist, enter the following command as root:

  # firewall-cmd --list-lockdown-whitelist-contexts

• To add a context context to the allowlist, enter the following command as root:

  # firewall-cmd --add-lockdown-whitelist-context=context

• To remove a context context from the allowlist, enter the following command as root:

  # firewall-cmd --remove-lockdown-whitelist-context=context

• To query whether the context context is in the allowlist, enter the following command as root:

  # firewall-cmd --query-lockdown-whitelist-context=context

  Prints yes with exit status 0, if true, prints no with exit status 1 otherwise.

• To list all user IDs that are in the allowlist, enter the following command as root:

  # firewall-cmd --list-lockdown-whitelist-uids

• To add a user ID uid to the allowlist, enter the following command as root:

  # firewall-cmd --add-lockdown-whitelist-uid=uid

• To remove a user ID uid from the allowlist, enter the following command as root:

  # firewall-cmd --remove-lockdown-whitelist-uid=uid

• To query whether the user ID uid is in the allowlist, enter the following command:

  $ firewall-cmd --query-lockdown-whitelist-uid=uid

  Prints yes with exit status 0, if true, prints no with exit status 1 otherwise.

• To list all user names that are in the allowlist, enter the following command as root:

  # firewall-cmd --list-lockdown-whitelist-users

• To add a user name user to the allowlist, enter the following command as root:

  # firewall-cmd --add-lockdown-whitelist-user=user

• To remove a user name user from the allowlist, enter the following command as root:

  # firewall-cmd --remove-lockdown-whitelist-user=user

• To query whether the user name user is in the allowlist, enter the following command:

  $ firewall-cmd --query-lockdown-whitelist-user=user

  Prints yes with exit status 0, if true, prints no with exit status 1 otherwise.

47.13.3. Configuring lockdown allowlist options using configuration files

The default allowlist configuration file contains the NetworkManager context and the default context of libvirt. The user ID 0 is also on the list.

+ The allowlist configuration files are stored in the /etc/firewalld/ directory.

<?xml version="1.0" encoding="utf-8"?>
	<whitelist>
	  <selinux context="system_u:system_r:NetworkManager_t:s0"/>
	  <selinux context="system_u:system_r:virtd_t:s0-s0:c0.c1023"/>
	  <user id="0"/>
	</whitelist>

Following is an example allowlist configuration file enabling all commands for the firewall-cmd utility, for a user called user whose user ID is 815:

<?xml version="1.0" encoding="utf-8"?>
	<whitelist>
	  <command name="/usr/libexec/platform-python -s /bin/firewall-cmd*"/>
	  <selinux context="system_u:system_r:NetworkManager_t:s0"/>
	  <user id="815"/>
	  <user name="user"/>
	</whitelist>

This example shows both user id and user name, but only one option is required. Python is the interpreter and is prepended to the command line. You can also use a specific command, for example:

/usr/bin/python3 /bin/firewall-cmd --lockdown-on

In that example, only the --lockdown-on command is allowed.

In Red Hat Enterprise Linux, all utilities are placed in the /usr/bin/ directory and the /bin/ directory is sym-linked to the /usr/bin/ directory. In other words, although the path for firewall-cmd when entered as root might resolve to /bin/firewall-cmd, /usr/bin/firewall-cmd can now be used. All new scripts should use the new location. But be aware that if scripts that run as root are written to use the /bin/firewall-cmd path, then that command path must be added in the allowlist in addition to the /usr/bin/firewall-cmd path traditionally used only for non-root users.

The * at the end of the name attribute of a command means that all commands that start with this string match. If the * is not there then the absolute command including arguments must match.

47.14.1. The difference between intra-zone forwarding and zones with the default target set to ACCEPT

When intra-zone forwarding is enabled, the traffic within a single firewalld zone can flow from one interface or source to another interface or source. The zone specifies the trust level of interfaces and sources. If the trust level is the same, communication between interfaces or sources is possible.

Note that, if you enable intra-zone forwarding in the default zone of firewalld, it applies only to the interfaces and sources added to the current default zone.

The trusted zone of firewalld uses a default target set to ACCEPT. This zone accepts all forwarded traffic, and intra-zone forwarding is not applicable for it.

As for other default target values, forwarded traffic is dropped by default, which applies to all standard zones except the trusted zone.

47.14.2. Using intra-zone forwarding to forward traffic between an Ethernet and Wi-Fi network

You can use intra-zone forwarding to forward traffic between interfaces and sources within the same firewalld zone. For example, use this feature to forward traffic between an Ethernet network connected to enp1s0 and a Wi-Fi network connected to wlp0s20.

Procedure

1. Enable packet forwarding in the kernel:

   # echo "net.ipv4.ip_forward=1" > /etc/sysctl.d/95-IPv4-forwarding.conf
   
   # sysctl -p /etc/sysctl.d/95-IPv4-forwarding.conf

2. Ensure that interfaces between which you want to enable intra-zone forwarding are not assigned to a zone different than the internal zone:

   # firewall-cmd --get-active-zones

3. If the interface is currently assigned to a zone other than internal, reassign it:

   # firewall-cmd --zone=internal --change-interface=interface_name --permanent

4. Add the enp1s0 and wlp0s20 interfaces to the internal zone:

   # firewall-cmd --zone=internal --add-interface=enp1s0 --add-interface=wlp0s20

5. Enable intra-zone forwarding:

   # firewall-cmd --zone=internal --add-forward

Verification

The following verification steps require that the nmap-ncat package is installed on both hosts.

1. Log in to a host that is in the same network as the enp1s0 interface of the host you enabled zone forwarding on.

2. Start an echo service with ncat to test connectivity:

   # ncat -e /usr/bin/cat -l 12345

3. Log in to a host that is in the same network as the wlp0s20 interface.

4. Connect to the echo server running on the host that is in the same network as the enp1s0:

   # ncat <other host> 12345

5. Type something and press Enter, and verify the text is sent back.

Additional resources

• firewalld.zones(5) man page

47.15.1. Introduction to the firewall RHEL System Role

RHEL System Roles is a set of contents for the Ansible automation utility. This content together with the Ansible automation utility provides a consistent configuration interface to remotely manage multiple systems.

The rhel-system-roles.firewall role from the RHEL System Roles was introduced for automated configurations of the firewalld service. The rhel-system-roles package contains this system role, and also the reference documentation.

To apply the firewalld parameters on one or more systems in an automated fashion, use the firewall System Role variable in a playbook. A playbook is a list of one or more plays that is written in the text-based YAML format.

You can use an inventory file to define a set of systems that you want Ansible to configure.

With the firewall role you can configure many different firewalld parameters, for example:

• Zones.
• The services for which packets should be allowed.
• Granting, rejection, or dropping of traffic access to ports.
• Forwarding of ports or port ranges for a zone.

47.15.2. Forwarding incoming traffic from one local port to a different local port

With the firewall role you can remotely configure firewalld parameters with persisting effect on multiple managed hosts.

Perform this procedure on the Ansible control node.

Prerequisites

• You have prepared the control node and the managed nodes
• You are logged in to the control node as a user who can run playbooks on the managed nodes.
• The account you use to connect to the managed nodes has sudo permissions on the them.
• The hosts or host groups on which you want run this playbook are listed in the Ansible inventory file.

Procedure

1. Create a playbook file, for example ~/port_forwarding.yml, with the following content:

   ---
   - name: Configure firewalld
     hosts: managed-node-01.example.com
     tasks:
     - name: Forward incoming traffic on port 8080 to 443
       include_role:
         name: rhel-system-roles.firewall
   
       vars:
         firewall:
           - { forward_port: 8080/tcp;443;, state: enabled, runtime: true, permanent: true }

2. Run the playbook:

   # ansible-playbook ~/port_forwarding.yml

Verification

• On the managed host, display the firewalld settings:

  # firewall-cmd --list-forward-ports

Additional resources

• /usr/share/ansible/roles/rhel-system-roles.firewall/README.md

47.15.3. Configuring ports using System Roles

You can use the RHEL firewall System Role to open or close ports in the local firewall for incoming traffic and make the new configuration persist across reboots. The example describes how to configure the default zone to permit incoming traffic for the HTTPS service.

Perform this procedure on the Ansible control node.

Prerequisites

• You have prepared the control node and the managed nodes
• You are logged in to the control node as a user who can run playbooks on the managed nodes.
• The account you use to connect to the managed nodes has sudo permissions on the them.
• The hosts or host groups on which you want run this playbook are listed in the Ansible inventory file.

Procedure

1. Create a playbook file, for example ~/opening-a-port.yml, with the following content:

   ---
   - name: Configure firewalld
     hosts: managed-node-01.example.com
     tasks:
     - name: Allow incoming HTTPS traffic to the local host
       include_role:
         name: rhel-system-roles.firewall
   
       vars:
         firewall:
           - port: 443/tcp
             service: http
             state: enabled
             runtime: true
             permanent: true

   The permanent: true option makes the new settings persistent across reboots.

2. Run the playbook:

   # ansible-playbook ~/opening-a-port.yml

Verification

• On the managed node, verify that the 443/tcp port associated with the HTTPS service is open:

  # firewall-cmd --list-ports
  443/tcp

Additional resources

• /usr/share/ansible/roles/rhel-system-roles.firewall/README.md

47.15.4. Configuring a DMZ firewalld zone by using the firewalld RHEL System Role

As a system administrator, you can use the firewall System Role to configure a dmz zone on the enp1s0 interface to permit HTTPS traffic to the zone. In this way, you enable external users to access your web servers.

Perform this procedure on the Ansible control node.

Prerequisites

• You have prepared the control node and the managed nodes
• You are logged in to the control node as a user who can run playbooks on the managed nodes.
• The account you use to connect to the managed nodes has sudo permissions on the them.
• The hosts or host groups on which you want run this playbook are listed in the Ansible inventory file.

Procedure

1. Create a playbook file, for example ~/configuring-a-dmz.yml, with the following content:

   ---
   - name: Configure firewalld
     hosts: managed-node-01.example.com
     tasks:
     - name: Creating a DMZ with access to HTTPS port and masquerading for hosts in DMZ
       include_role:
         name: rhel-system-roles.firewall
   
       vars:
         firewall:
           - zone: dmz
             interface: enp1s0
             service: https
             state: enabled
             runtime: true
             permanent: true

2. Run the playbook:

   # ansible-playbook ~/configuring-a-dmz.yml

Verification

• On the managed node, view detailed information about the dmz zone:

  # firewall-cmd --zone=dmz --list-all
  dmz (active)
    target: default
    icmp-block-inversion: no
    interfaces: enp1s0
    sources:
    services: https ssh
    ports:
    protocols:
    forward: no
    masquerade: no
    forward-ports:
    source-ports:
    icmp-blocks:

Additional resources

• /usr/share/ansible/roles/rhel-system-roles.firewall/README.md