Software fastpath with nftables flowtable


Firewalld gained support for nftables flowtable. This is a software fastpath that may significantly improve forwarding performance.

Nftables flowtable makes use of the kernel’s connection tracking to bypass much of the network stack. This accelerates data packets of established connections.

What It Looks Like

This feature can be enabled by setting NftablesFlowtable in /etc/firewalld/firewalld.conf. This setting defaults to off. To enable flowtable support set this value to your list of interfaces for which you want flowtable to be enabled, e.g. NftablesFlowtable=eth0 eth1.

This can be done manually or with a sed expression.

Example to enable eth0 and eth1:

# sed -i 's/^NftablesFlowtable=.*/NftablesFlowtable=eth0 eth1/' /etc/firewalld/firewalld.conf
# firewall-cmd --reload

When this feature is enabled firewalld adds the below additional nftables rules. It’s one additional rule and one flowtable object.

table inet firewalld {
        flowtable fastpath {
                hook ingress priority filter + 10
                devices = { eth0, eth1 }
	chain filter_FORWARD {
		type filter hook forward priority filter + 10; policy accept;
		ct state { established, related } meta l4proto { tcp, udp } flow add @fastpath  <--- new rule
		ct state { established, related } accept

Performance Tests

This is the test topology used for gather performance test results.

flowchart LR
    subgraph DUT
        subgraph firewalld
    subgraph traffgen
        subgraph net_namespace
            subgraph iperf3_server
        subgraph iperf3_client

The device under test was artificially limited to two CPU cores. This was done specifically to stress the forward path.

The traffic generation uses 16 iperf3 instances run in parallel with 128 parallel streams for 60 seconds. This simulates 2048 concurrent connections. The benchmark is run 10 times to normalize the results and produce a standard deviation.

Below is a graph of the results of NftablesFlowtable disabled vs enabled. The absolute numbers are less important. The important takeaway is the relative performance improvement.

performance graph

More information

Nftables flowtable can accelerate TCP and UDP flows. Control packets will still take the traditional network path, i.e. they will take the slow path.

Firewalld supports source based zones with --add-source. These can also be accelerated, but keep in mind that flowtable is enabled on the interface. So you must make sure that traffic from that source is received on the interface that was added to NftablesFlowtable. If in doubt, always use --add-interface.


Nftables flowtable brings a significant performance improvement for forwarded traffic. This is applies to use cases like: network firewall, home router, and even container/VM traffic.