Openwrt and Netfilter


Not Non-Fungible Token, but Netfilter NFT

It has been a long time coming, but Netfilter have finally arrived in the latest release of OpenWrt (22.03.x). Netfilter brings network parity for IPv6, while improving firewall performance.

The Early Days

A little history, Linux has always had the concept of packet filtering, or a firewall. The original packet filter, ipfw, integrated into Linux in 1994, was based on a BSD ipfirewall. In Linux version 2.2 (1999), ipchains was introduced, but was stateless only. The Linux firewall evolved again with iptables in version 2.4 (2001), where stateful inspection returned.

iptables continued to evolve, but with separate streams, one for IPv4, another for IPv6, and yet another for the Ethernet (ebtables). Having multiple tables impacted performance, and led to kernel code duplication.


Netfilter‘s new packet-filtering utility, nft, replaces the older tools: iptables, ip6tables, arptables and ebtables.

At first look, nft options look quite a bit different from the old iptables.

nft --help
Usage: nft [ options ] [ cmds... ]

Options (general):
  -h, --help                      Show this help
  -v, --version                   Show version information
  -V                              Show extended version information

Options (ruleset input handling):
  -f, --file <filename>           Read input from <filename>
  -D, --define <name=value>       Define variable, e.g. --define foo=
  -i, --interactive               Read input from interactive CLI
  -I, --includepath <directory>   Add <directory> to the paths searched for include files. Default is: /etc
  -c, --check                     Check commands validity without actually applying the changes.
  -o, --optimize                  Optimize ruleset

Options (ruleset list formatting):
  -a, --handle                    Output rule handle.
  -s, --stateless                 Omit stateful information of ruleset.
  -t, --terse                     Omit contents of sets.
  -S, --service                   Translate ports to service names as described in /etc/services.
  -N, --reversedns                Translate IP addresses to names.
  -u, --guid                      Print UID/GID as defined in /etc/passwd and /etc/group.
  -n, --numeric                   Print fully numerical output.
  -y, --numeric-priority          Print chain priority numerically.
  -p, --numeric-protocol          Print layer 4 protocols numerically.
  -T, --numeric-time              Print time values numerically.

A key difference is that nft wants to read a json-like formatted file of rules, rather than just a simple (sometimes not so simple) command line of iptables.

The first useful command is to show the tables defined (on OpenWrt). Netfilter has a new address family, inet which applies to IPv4 and IPv6.

# nft list tables
table inet fw4

Unfortunately, for the new-comer, that doesn’t appear to tell us much. But in fact, it is stating that there is a table of the family type of inet with the name fw4. A more informative command shows the chains and rules in the table (fw4):

# nft list table inet fw4
table inet fw4 {
    chain input {
        type filter hook input priority filter; policy accept;
        iifname "lo" accept comment "!fw4: Accept traffic from loopback"
        ct state established,related accept comment "!fw4: Allow inbound established and related flows"
        tcp flags syn / fin,syn,rst,ack jump syn_flood comment "!fw4: Rate limit TCP syn packets"
        iifname "br-lan" jump input_lan comment "!fw4: Handle lan IPv4/IPv6 input traffic"
        iifname "wan" jump input_wan comment "!fw4: Handle wan IPv4/IPv6 input traffic"

    chain forward {
        type filter hook forward priority filter; policy drop;
        ct state established,related accept comment "!fw4: Allow forwarded established and related flows"
        iifname "br-lan" jump forward_lan comment "!fw4: Handle lan IPv4/IPv6 forward traffic"
        iifname "wan" jump forward_wan comment "!fw4: Handle wan IPv4/IPv6 forward traffic"
        jump handle_reject

    chain output {
        type filter hook output priority filter; policy accept;
        oifname "lo" accept comment "!fw4: Accept traffic towards loopback"
        ct state established,related accept comment "!fw4: Allow outbound established and related flows"
        oifname "br-lan" jump output_lan comment "!fw4: Handle lan IPv4/IPv6 output traffic"
        oifname "wan" jump output_wan comment "!fw4: Handle wan IPv4/IPv6 output traffic"


    chain forward_wan {
        icmpv6 type { destination-unreachable, time-exceeded, echo-request, echo-reply } limit rate 1000/second counter packets 4 bytes 416 accept comment "!fw4: Allow-ICMPv6-Forward"
        icmpv6 type . icmpv6 code { packet-too-big . no-route, parameter-problem . no-route, parameter-problem . admin-prohibited } limit rate 1000/second counter packets 0 bytes 0 accept comment "!fw4: Allow-ICMPv6-Forward"
        meta l4proto esp counter packets 0 bytes 0 jump accept_to_lan comment "!fw4: Allow-IPSec-ESP"
        udp dport 500 counter packets 0 bytes 0 jump accept_to_lan comment "!fw4: Allow-ISAKMP"
        meta nfproto ipv6 tcp dport { 22, 80, 443 } counter packets 1 bytes 80 jump accept_to_lan comment "!fw4: ext_mgmt_fwd"
        meta nfproto ipv6 udp dport { 22, 80, 443 } counter packets 0 bytes 0 jump accept_to_lan comment "!fw4: ext_mgmt_fwd"
        jump reject_to_wan

    chain mangle_forward {
        type filter hook forward priority mangle; policy accept;
        iifname "wan" tcp flags syn tcp option maxseg size set rt mtu comment "!fw4: Zone wan IPv4/IPv6 ingress MTU fixing"
        oifname "wan" tcp flags syn tcp option maxseg size set rt mtu comment "!fw4: Zone wan IPv4/IPv6 egress MTU fixing"

As you can see there are IPv4 and IPv6 rules in the Netfilter table. The nft man page is long but full of good info.

Firewall abstraction of OpenWrt

Fortunately, the developers of OpenWrt have kept the familiar web interface (LuCI) for firewall configuration, and the user doesn’t have to know that nft is now managing everything, rather than the older tools: iptables, ip6tables, arptables and ebtables.

OpenWrt Firewall Configuration


Why Netfilter? It reduces kernel code duplication, has more efficient execution, thus better performance, and adds atomic changes to filter rules.

Why OpenWrt? With the recent advent of malware targeting Small Office, Home Office [SOHO] routers, such as ZuoRAT, it is good to see an alternative to that old OEM router software, in a project which is active, and responding to security vulnerabilities.

nft has been around since 2014. Finally with the Netfilter address family of inet, IPv6 has equal status with IPv4. And with OpenWrt (22.03.x), your SOHO router will take advantage of a 21st Century firewall.

Photo Credit: NFT mania is here, and so are the scammers by Marco Verch under Creative Commons 2.0

Originally posted on (IPv6-only website)


Author: Craig Miller

IPv6 Advocate since 1998