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Chapter 16. Example: Protecting a LAN and DMZ using an nftables script

Use the nftables framework on a RHEL router to write and install a firewall script that protects the network clients in an internal LAN and a web server in a DMZ from unauthorized access from the internet and from other networks.

Important

This example is only for demonstration purposes and describes a scenario with specific requirements.

Firewall scripts highly depend on the network infrastructure and security requirements. Use this example to learn the concepts of nftables firewalls when you write scripts for your own environment.

16.1. Network conditions

The network in this example has the following conditions:

  • The router is connected to the following networks:

    • The internet through interface enp1s0
    • The internal LAN through interface enp7s0
    • The DMZ through enp8s0
  • The internet interface of the router has both a static IPv4 address (203.0.113.1) and IPv6 address (2001:db8:a::1) assigned.
  • The clients in the internal LAN use only private IPv4 addresses from the range 10.0.0.0/24. Consequently, traffic from the LAN to the internet requires source network address translation (SNAT).
  • The administrator PCs in the internal LAN use the IP addresses 10.0.0.100 and 10.0.0.200.
  • The DMZ uses public IP addresses from the ranges 198.51.100.0/24 and 2001:db8:b::/56.
  • The web server in the DMZ uses the IP addresses 198.51.100.5 and 2001:db8:b::5.
  • The router acts as a caching DNS server for hosts in the LAN and DMZ.

16.2. Security requirements to the firewall script

The following are the requirements to the nftables firewall in the example network:

  • The router must be able to:

    • Recursively resolve DNS queries.
    • Perform all connections on the loopback interface.

  • Clients in the internal LAN must be able to:

    • Query the caching DNS server running on the router.
    • Access the HTTPS server in the DMZ.
    • Access any HTTPS server on the internet.
  • The PCs of the administrators must be able to access the router and every server in the DMZ using SSH.

  • The web server in the DMZ must be able to:

    • Query the caching DNS server running on the router.
    • Access HTTPS servers on the internet to download updates.

  • Hosts on the internet must be able to:

    • Access the HTTPS servers in the DMZ.

  • Additionally, the following security requirements exists:

    • Connection attempts that are not explicitly allowed should be dropped.
    • Dropped packets should be logged.

16.3. Configuring logging of dropped packets to a file

By default, systemd logs kernel messages, such as for dropped packets, to the journal. Additionally, you can configure the rsyslog service to log such entries to a separate file. To ensure that the log file does not grow infinitely, configure a rotation policy.

Prerequisites

  • The rsyslog package is installed.
  • The rsyslog service is running.

Procedure

  1. Create the /etc/rsyslog.d/nftables.conf file with the following content: 

    :msg, startswith, "nft drop" -/var/log/nftables.log
& stop
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    Using this configuration, the rsyslog service logs dropped packets to the /var/log/nftables.log file instead of /var/log/messages.

  2. Restart the rsyslog service: 

    # systemctl restart rsyslog
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  3. Create the /etc/logrotate.d/nftables file with the following content to rotate /var/log/nftables.log if the size exceeds 10 MB: 

    /var/log/nftables.log {
  size +10M
  maxage 30
  sharedscripts
  postrotate
    /usr/bin/systemctl kill -s HUP rsyslog.service >/dev/null 2>&1 || true
  endscript
}
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    The maxage 30 setting defines that logrotate removes rotated logs older than 30 days during the next rotation operation.

16.4. Writing and activating the nftables script

This example is an nftables firewall script that runs on a RHEL router and protects the clients in an internal LAN and a web server in a DMZ. For details about the network and the requirements for the firewall used in the example, see Network conditions and Security requirements to the firewall script.

Warning

This nftables firewall script is only for demonstration purposes. Do not use it without adapting it to your environments and security requirements.

Prerequisites

Procedure

  1. Create the /etc/nftables/firewall.nft script with the following content: 

    # Remove all rules
flush ruleset


# Table for both IPv4 and IPv6 rules
table inet nftables_svc {

  # Define variables for the interface name
  define INET_DEV = enp1s0
  define LAN_DEV  = enp7s0
  define DMZ_DEV  = enp8s0


  # Set with the IPv4 addresses of admin PCs
  set admin_pc_ipv4 {
    type ipv4_addr
    elements = { 10.0.0.100, 10.0.0.200 }
  }


  # Chain for incoming trafic. Default policy: drop
  chain INPUT {
    type filter hook input priority filter
    policy drop

    # Accept packets in established and related state, drop invalid packets
    ct state vmap { established:accept, related:accept, invalid:drop }

    # Accept incoming traffic on loopback interface
    iifname lo accept

    # Allow request from LAN and DMZ to local DNS server
    iifname { $LAN_DEV, $DMZ_DEV } meta l4proto { tcp, udp } th dport 53 accept

    # Allow admins PCs to access the router using SSH
    iifname $LAN_DEV ip saddr @admin_pc_ipv4 tcp dport 22 accept

    # Last action: Log blocked packets
    # (packets that were not accepted in previous rules in this chain)
    log prefix "nft drop IN : "
  }


  # Chain for outgoing traffic. Default policy: drop
  chain OUTPUT {
    type filter hook output priority filter
    policy drop

    # Accept packets in established and related state, drop invalid packets
    ct state vmap { established:accept, related:accept, invalid:drop }

    # Accept outgoing traffic on loopback interface
    oifname lo accept

    # Allow local DNS server to recursively resolve queries
    oifname $INET_DEV meta l4proto { tcp, udp } th dport 53 accept

    # Last action: Log blocked packets
    log prefix "nft drop OUT: "
  }


  # Chain for forwarding traffic. Default policy: drop
  chain FORWARD {
    type filter hook forward priority filter
    policy drop

    # Accept packets in established and related state, drop invalid packets
    ct state vmap { established:accept, related:accept, invalid:drop }

    # IPv4 access from LAN and internet to the HTTPS server in the DMZ
    iifname { $LAN_DEV, $INET_DEV } oifname $DMZ_DEV ip daddr 198.51.100.5 tcp dport 443 accept

    # IPv6 access from internet to the HTTPS server in the DMZ
    iifname $INET_DEV oifname $DMZ_DEV ip6 daddr 2001:db8:b::5 tcp dport 443 accept

    # Access from LAN and DMZ to HTTPS servers on the internet
    iifname { $LAN_DEV, $DMZ_DEV } oifname $INET_DEV tcp dport 443 accept

    # Last action: Log blocked packets
    log prefix "nft drop FWD: "
  }


  # Postrouting chain to handle SNAT
  chain postrouting {
    type nat hook postrouting priority srcnat; policy accept;

    # SNAT for IPv4 traffic from LAN to internet
    iifname $LAN_DEV oifname $INET_DEV snat ip to 203.0.113.1
  }
}
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  2. Include the /etc/nftables/firewall.nft script in the /etc/sysconfig/nftables.conf file: 

    include "/etc/nftables/firewall.nft"
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  3. Enable IPv4 forwarding: 

    # echo "net.ipv4.ip_forward=1" > /etc/sysctl.d/95-IPv4-forwarding.conf
# sysctl -p /etc/sysctl.d/95-IPv4-forwarding.conf
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  4. Enable and start the nftables service: 

    # systemctl enable --now nftables
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Verification

  1. Optional: Verify the nftables rule set: 

    # nft list ruleset
...
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  2. Try to perform an access that the firewall prevents. For example, try to access the router using SSH from the DMZ: 

    # ssh router.example.com
ssh: connect to host router.example.com port 22: Network is unreachable
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  3. Depending on your logging settings, search:

    • The systemd journal for the blocked packets: 

      # journalctl -k -g "nft drop"
Oct 14 17:27:18 router kernel: nft drop IN : IN=enp8s0 OUT= MAC=... SRC=198.51.100.5 DST=198.51.100.1 ... PROTO=TCP SPT=40464 DPT=22 ... SYN ...
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    • The /var/log/nftables.log file for the blocked packets: 

      Oct 14 17:27:18 router kernel: nft drop IN : IN=enp8s0 OUT= MAC=... SRC=198.51.100.5 DST=198.51.100.1 ... PROTO=TCP SPT=40464 DPT=22 ... SYN ...
      Copy to Clipboard
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