In the world of high-performance networking, latency is currency. Every millisecond countsâespecially when you're pushing packets at scale. Thatâs where Linux Fast Path comes into play.
If you're running routers, firewalls, or SD-WAN appliances on general-purpose Linux machines, Fast Path is the difference between sluggish forwarding and wire-speed performance.
What Is Linux Fast Path?
Fast Path refers to the set of mechanisms in Linux that allows packets to bypass the traditional, CPU-heavy kernel networking stack and instead take a much faster, streamlined processing route.
In the default Linux model, every packet undergoes a series of checks: iptables rules, routing decisions, QoS classification, connection tracking, etc. Thatâs great for flexibility, but horrible for performance.
Fast Path aims to optimise this by:
Skipping the slow parts of the kernel
Bypassing iptables and conntrack
Short-circuiting L3/L4 routing when possible
Reducing context switches and cache flushes
It's used in high-performance systems like:
Fast Path Mechanisms in Linux
There isnât one âFast Pathâ switchârather, several overlapping technologies provide Fast Path-like behaviour:
XDP (eXpress Data Path) â Early packet processing at the driver level, before kernel involvement.
DPDK (Data Plane Development Kit) â Bypasses kernel entirely, uses user space NIC drivers.
IP Fast Forwarding / Flow Caching â Simplified kernel path using connection tracking and caching.
Nftables Flowtable â Linux 5.x feature that enables fastpath forwarding for connection-tracked flows.
Letâs focus on XDP and nftables flowtables, as they are the most accessible on Debian.
Implementing Fast Path on Debian
Step 1: Prerequisites
Debian 11 or later (or Ubuntu 20.04+)
Kernel version ⼠5.4 (for nftables flowtables)
nftables, iproute2, and optionally bpftool for XDP work
sudo apt update
sudo apt install nftables iproute2 bpftool
This creates a fast path for known flows.
sudo nft add table inet myfilter
sudo nft add flowtable inet myfilter ftable { hook ingress priority 0\; devices = { eth0 } \; }
sudo nft add chain inet myfilter forward {
type filter hook forward priority 0;
policy accept;
ip protocol tcp flow offload @ftable;
ip protocol udp flow offload @ftable;
}
Youâve now told Linux to push matched TCP/UDP flows into the flowtable. These bypass the normal stack after initial tracking.
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Benefits:
Reduced latency
Lower CPU usage
Higher throughput
Step 3: Using XDP for Even Faster Processing
XDP runs in the kernel at driver level and can drop or redirect packets before they even hit iptables or routing.
sudo apt install libbpf-dev clang llvm
You can then load a basic XDP program:
ip link set dev eth0 xdp obj ./xdp_prog_kern.o sec xdp
Or use xdp-tools to apply ready-made examples.
Note: XDP requires NIC driver support (e.g. Intel ixgbe, i40e, mlx5). Not all NICs support XDP natively.
Common Pitfalls
No visibility: Once flows are offloaded, tools like tcpdump or conntrack may no longer see packets.
Broken connection tracking: Improper flowtable config can interfere with NAT/firewall rules.
NIC driver compatibility: Not all NICs or virtual interfaces support XDP or offload.
Wrap
Fast Path is no longer a niche hackâitâs a requirement for any high-performance Linux network appliance.
If youâre running routing or SD-WAN on Debian and not using nftables flowtables or XDP, youâre leaving performance on the table. The tools are there. The kernel supports it. The only question left is: why arenât you using it?
