Firewalld Advanced

Firewalld is the default firewall management tool for RHEL/CentOS-based distributions. This article dives deep into Rich Rules, Direct Rules, custom zones, port forwarding, IP masquerading, ipsets, and other advanced features to help you build fine-grained network security policies.

Rich Rules

Rich Rules provide more powerful and flexible rule definitions than simple --add-port/--add-service, allowing fine-grained control based on source addresses, destination addresses, ports, and more.

Rich Rules Syntax Structure

rule
  [family="ipv4|ipv6"]
  [source address="address[/mask]" [invert="true"]]
  [destination address="address[/mask]" [invert="true"]]
  [service name="service" | port port="port" protocol="tcp|udp" |
   protocol value="protocol" | icmp-block name="type" | masquerade |
   forward-port port="port" protocol="tcp|udp" to-port="port" [to-addr="address"]]
  [log [prefix="prefix"] [level="level"] [limit value="rate"]]
  [audit]
  [accept|reject|drop]

Common Rich Rules Examples

Allow a Specific IP to Access a Specific Port

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="192.168.1.100" port port="3306" protocol="tcp" accept'

Drop All Traffic from a Specific IP

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.50" drop'

Allow a Subnet to Access SSH

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="192.168.1.0/24" service name="ssh" accept'

Reject HTTP from a Specific IP and Log It

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="203.0.113.0/24" service name="http" log prefix="HTTP-BLOCKED: " level="warning" limit value="5/m" reject'

Allow a Specific IP Range to Access Multiple Ports

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/8" port port="8080-8090" protocol="tcp" accept'

Managing Rich Rules

# View Rich Rules for the current zone
sudo firewall-cmd --list-rich-rules

# Remove a Rich Rule
sudo firewall-cmd --permanent --remove-rich-rule='rule family="ipv4" source address="10.0.0.50" drop'

# Apply changes
sudo firewall-cmd --reload

Tip

Rich Rules are matched in this order: log/audit actions execute first, then drop/reject/accept. Within the same zone, Rich Rules are matched in the order they were added.

Direct Rules

Direct Rules allow you to insert rules directly into the iptables/nftables backend, suitable for complex scenarios that Rich Rules cannot handle.

Caution

Direct Rules are deprecated as of RHEL 9 and may not work on EL 10. Red Hat has explicitly listed the iptables backend and Direct Interface as deprecated in the RHEL 9 release notes.

If you are currently using Direct Rules on EL 9, migrate to Rich Rules or Policy Objects before upgrading to EL 10 to avoid rules silently failing.

Direct Rules Basic Syntax

sudo firewall-cmd --direct --add-rule ipv4 filter INPUT 0 <iptables rule parameters>

Direct Rules Examples

Rate-Limit ICMP

sudo firewall-cmd --permanent --direct --add-rule ipv4 filter INPUT 0 -p icmp --icmp-type echo-request -m limit --limit 1/s --limit-burst 4 -j ACCEPT

Log All Dropped Packets

sudo firewall-cmd --permanent --direct --add-rule ipv4 filter INPUT 99 -j LOG --log-prefix "DROPPED: " --log-level 4

Block a Specific MAC Address

sudo firewall-cmd --permanent --direct --add-rule ipv4 filter INPUT 0 -m mac --mac-source AA:BB:CC:DD:EE:FF -j DROP

Managing Direct Rules

# View all Direct Rules
sudo firewall-cmd --direct --get-all-rules

# Remove a Direct Rule
sudo firewall-cmd --permanent --direct --remove-rule ipv4 filter INPUT 0 -m mac --mac-source AA:BB:CC:DD:EE:FF -j DROP

sudo firewall-cmd --reload

Custom Zones

Firewalld comes with built-in zones like public, trusted, and dmz, but you can also create custom zones to meet specific requirements.

Create a Custom Zone

# Create a zone named webservers
sudo firewall-cmd --permanent --new-zone=webservers
sudo firewall-cmd --reload

Configure a Custom Zone

# Set zone description
sudo firewall-cmd --permanent --zone=webservers --set-description="Web Server Zone"

# Add allowed services
sudo firewall-cmd --permanent --zone=webservers --add-service=http
sudo firewall-cmd --permanent --zone=webservers --add-service=https
sudo firewall-cmd --permanent --zone=webservers --add-port=8080/tcp

# Bind a network interface to the custom zone
sudo firewall-cmd --permanent --zone=webservers --change-interface=eth1

# Bind a source address to the zone
sudo firewall-cmd --permanent --zone=webservers --add-source=10.0.1.0/24

sudo firewall-cmd --reload

View Zone Information

# View all zones
sudo firewall-cmd --get-zones

# View detailed configuration for a specific zone
sudo firewall-cmd --zone=webservers --list-all

# View all active zones
sudo firewall-cmd --get-active-zones

# View the default zone
sudo firewall-cmd --get-default-zone

Delete a Custom Zone

sudo firewall-cmd --permanent --delete-zone=webservers
sudo firewall-cmd --reload

Port Forwarding

Port forwarding redirects traffic arriving at one port to another port or another host.

Local Port Forwarding

Forward local port 80 to port 8080:

sudo firewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toport=8080
sudo firewall-cmd --reload

Forward to a Remote Host

Forward traffic on local port 80 to port 8080 on another host:

# Masquerade must be enabled first
sudo firewall-cmd --permanent --add-masquerade

# Add the port forwarding rule
sudo firewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toport=8080:toaddr=192.168.1.100
sudo firewall-cmd --reload

Port Forwarding with Rich Rules

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" forward-port port="443" protocol="tcp" to-port="8443" to-addr="192.168.1.100"'
sudo firewall-cmd --reload

View Port Forwarding Rules

sudo firewall-cmd --list-forward-ports

Remove a Port Forwarding Rule

sudo firewall-cmd --permanent --remove-forward-port=port=80:proto=tcp:toport=8080
sudo firewall-cmd --reload

IP Masquerading

Masquerading is a special form of SNAT (Source Network Address Translation), typically used to allow internal network machines to access the internet through the firewall host.

Enable Masquerade

sudo firewall-cmd --permanent --add-masquerade
sudo firewall-cmd --reload

Check Masquerade Status

sudo firewall-cmd --query-masquerade

Enable Masquerade for a Specific Zone

sudo firewall-cmd --permanent --zone=internal --add-masquerade
sudo firewall-cmd --reload

Limit Masquerade Scope with Rich Rules

Enable NAT only for a specific source subnet:

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/24" masquerade'
sudo firewall-cmd --reload

Enable Kernel IP Forwarding

Both masquerading and port forwarding require kernel IP forwarding:

# Enable temporarily
sudo sysctl -w net.ipv4.ip_forward=1

# Enable permanently
echo 'net.ipv4.ip_forward = 1' | sudo tee /etc/sysctl.d/ip_forward.conf
sudo sysctl -p /etc/sysctl.d/ip_forward.conf

IPSet

IPSet allows you to define collections of IP addresses and reference them in firewall rules, making it ideal for managing large numbers of IPs.

Create an IPSet

# Create an IPv4 address set
sudo firewall-cmd --permanent --new-ipset=blacklist --type=hash:ip

# Create a set that supports subnets
sudo firewall-cmd --permanent --new-ipset=trusted_nets --type=hash:net

Add IPs to an IPSet

sudo firewall-cmd --permanent --ipset=blacklist --add-entry=203.0.113.50
sudo firewall-cmd --permanent --ipset=blacklist --add-entry=198.51.100.0/24
sudo firewall-cmd --permanent --ipset=trusted_nets --add-entry=10.0.0.0/8
sudo firewall-cmd --reload

Bulk Import from a File

Create an IP list file:

cat <<EOF > /tmp/blacklist.txt
203.0.113.10
203.0.113.20
198.51.100.0/24
EOF

Import into the IPSet:

sudo firewall-cmd --permanent --ipset=blacklist --add-entries-from-file=/tmp/blacklist.txt
sudo firewall-cmd --reload

Use IPSets in Rich Rules

# Block all IPs in the blacklist
sudo firewall-cmd --permanent --add-rich-rule='rule source ipset="blacklist" drop'

# Allow IPs in the whitelist to access SSH
sudo firewall-cmd --permanent --add-rich-rule='rule source ipset="trusted_nets" service name="ssh" accept'

sudo firewall-cmd --reload

Manage IPSets

# View all IPSets
sudo firewall-cmd --get-ipsets

# View IPSet entries
sudo firewall-cmd --ipset=blacklist --get-entries

# Remove an entry
sudo firewall-cmd --permanent --ipset=blacklist --remove-entry=203.0.113.50

# Delete an IPSet
sudo firewall-cmd --permanent --delete-ipset=blacklist
sudo firewall-cmd --reload

Logging

Firewall logging is essential for security auditing and troubleshooting.

Log with Rich Rules

# Log rejected SSH connections
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" service name="ssh" log prefix="SSH-ACCESS: " level="notice" limit value="3/m" accept'

# Log dropped traffic
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="0.0.0.0/0" log prefix="FW-DROP: " level="warning" limit value="10/m" drop'

sudo firewall-cmd --reload

Log Levels

Level	Description
emerg	Emergency
alert	Alert
crit	Critical
error	Error
warning	Warning
notice	Notice
info	Informational
debug	Debug

Rate-Limit Logging

limit value prevents log flooding that could fill up disk space:

Rate Format	Description
1/s	1 entry per second
5/m	5 entries per minute
10/h	10 entries per hour
100/d	100 entries per day

View Firewall Logs

# View via journalctl
sudo journalctl -k | grep "FW-DROP"

# View via dmesg
sudo dmesg | grep "SSH-ACCESS"

# View messages log
sudo grep "FW-DROP" /var/log/messages

Configure LogDenied

Firewalld can globally log all denied connections:

# View current setting
sudo firewall-cmd --get-log-denied

# Enable (options: all, unicast, broadcast, multicast, off)
sudo firewall-cmd --set-log-denied=all

Caution

--set-log-denied=all can generate a large volume of logs in high-traffic environments, potentially affecting performance and disk space. It is recommended to enable it temporarily for debugging and use precise Rich Rules logging during normal operation.

Hands-On: Building a Web Server Firewall Policy

The following example configures a complete firewall policy for a web server, including custom zones, IPSet whitelists, port forwarding, and logging.

1. Create an admin IPSet:

   sudo firewall-cmd --permanent --new-ipset=admin_ips --type=hash:ip
   sudo firewall-cmd --permanent --ipset=admin_ips --add-entry=192.168.1.10
   sudo firewall-cmd --permanent --ipset=admin_ips --add-entry=192.168.1.11

2. Configure basic public zone rules:

   # Allow HTTP and HTTPS
   sudo firewall-cmd --permanent --add-service=http
   sudo firewall-cmd --permanent --add-service=https
   
   # Remove default SSH service (will be restricted via Rich Rule)
   sudo firewall-cmd --permanent --remove-service=ssh

3. Add Rich Rules:

   # Allow SSH only from admin IPs
   sudo firewall-cmd --permanent --add-rich-rule='rule source ipset="admin_ips" service name="ssh" accept'
   
   # Log and drop other SSH attempts
   sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" service name="ssh" log prefix="SSH-DENIED: " level="warning" limit value="3/m" drop'
   
   # Log HTTP access (sampled)
   sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" service name="http" log prefix="HTTP: " level="info" limit value="1/m" accept'

4. Configure port forwarding (HTTP to backend application):

   sudo firewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toport=8080

5. Apply and verify:

   sudo firewall-cmd --reload
   sudo firewall-cmd --list-all
   sudo firewall-cmd --list-rich-rules

By leveraging Rich Rules, IPSets, port forwarding, and logging, you can build fine-grained firewall security policies that effectively defend against network threats while keeping services running smoothly.