How to Trace TCP Socket TCP_FASTOPEN_NO_COOKIE in Linux Using eBPF

Standard TCP Fast Open (RFC 7413) requires a client to perform a prerequisite handshake to acquire a cryptographic TFO cookie before it can transmit data inside initial SYN packets. In trusted private cloud environments, Kubernetes service meshes, and dedicated data center interconnects where IP spoofing and SYN flood attacks are mitigated by network infrastructure, cookie acquisition introduces unnecessary RTT latency overhead. The Linux kernel provides the TCP_FASTOPEN_NO_COOKIE socket option via setsockopt(), enabling immediate zero-RTT data delivery on the very first connection without cookie negotiation. Tracing cookieless TFO handshakes with eBPF gives systems architects deep visibility into private network latency optimization.

Mechanics of Cookieless TCP Fast Open

In standard TFO, if a client dispatches data with a SYN segment without a cookie, the server rejects the data payload and responds with a standard SYN-ACK containing a new cookie. When TCP_FASTOPEN_NO_COOKIE is enabled on a listening socket in Linux:

/* Enable cookieless TFO on server listening socket */
int no_cookie = 1;
setsockopt(listen_fd, IPPROTO_TCP, TCP_FASTOPEN_NO_COOKIE, &no_cookie, sizeof(no_cookie));

The kernel network stack processes data payloads attached to SYN segments immediately upon arrival—even if no cookie is present or if the cookie field is omitted entirely. The server application receives the payload on its very first read() after accept(), achieving genuine zero-RTT execution.

Authoring a Custom bpftrace Cookieless TFO Script

To inspect when cookieless TFO segments arrive and trace incoming SYN data payloads, write an eBPF script using bpftrace:

#!/usr/bin/env bpftrace

#include <net/sock.h>
#include <linux/tcp.h>

kprobe:tcp_try_fastopen
{
  $sk = (struct sock *)arg0;
  $skb = (struct sk_buff *)arg1;

  @tfo_attempts = count();
  if ($skb->len > 0) {
    @syn_data_payload_bytes = hist($skb->len);
  }
}

kprobe:tcp_fastopen_no_cookie
{
  $sk = (struct sock *)arg0;
  time("%H:%M:%S: Cookieless TFO accepted on sock %p\n", $sk);
  @cookieless_tfo_accepted = count();
}

interval:s:10
{
  print(@tfo_attempts);
  print(@cookieless_tfo_accepted);
  print(@syn_data_payload_bytes);
  clear(@tfo_attempts);
  clear(@cookieless_tfo_accepted);
  clear(@syn_data_payload_bytes);
}

Save this script as trace_tfo_nocookie.bt. It targets tcp_try_fastopen() and tcp_fastopen_no_cookie() to capture cookieless acceptance events and incoming payload byte distributions.

Executing and Evaluating Kernel Telemetry

Run the bpftrace monitoring script with administrative privileges:

sudo bpftrace trace_tfo_nocookie.bt

Generate outbound synthetic RPC traffic with SYN data payloads. Review the output metrics:

• Cookieless acceptance: A matching count between @tfo_attempts and @cookieless_tfo_accepted confirms that SYN data packets are accepted without rejection.
• Payload distribution: Verify that @syn_data_payload_bytes reflects complete HTTP request headers or RPC frames delivered on the first segment.
• Security isolation: Ensure cookieless TFO is never exposed directly to untrusted public Internet interfaces to avoid amplification risks.

Configuring Host Parameters for Cookieless TFO

To enable and tune cookieless Fast Open across private Linux clusters:

1. Enable TCP Fast Open server mode: sysctl -w net.ipv4.tcp_fastopen=3.
2. Configure TCP_FASTOPEN_NO_COOKIE on internal gRPC and microservice listening sockets.
3. Ensure ingress firewalls (nftables or iptables) do not flag SYN packets with payloads as malformed traffic.