How to Trace TCP Socket TCP_INFO tcpi_retrans_synack SYN Cookie Fallbacks in Linux Using eBPF

In high-throughput Linux network infrastructure, SYN flood attacks and sudden connection spikes can exhaust the kernel’s SYN backlog queue. When the queue overflows, the kernel falls back to SYN cookies, bypassing state allocation until the final handshake ACK arrives. However, when packets drop during this fallback phase, tracing connection retries via tcpi_retrans_synack becomes essential for distinguishing benign traffic bursts from malicious resource exhaustion. Extended Berkeley Packet Filter (eBPF) provides non-invasive, wire-speed visibility into these kernel transitions.

Mechanisms of SYN Cookie Activation and SYN-ACK Stalls

When the SYN backlog table fills up and net.ipv4.tcp_syncookies = 1 is set, the kernel invokes cookie_v4_init_sequence() to generate a cryptographically encoded initial sequence number (ISN). Because no request_sock state is stored in the backlog, retransmissions follow specialized codepaths. Tracking SYN cookie triggers alongside inet_rtx_syn_ack events pinpoints exactly which services are operating under backlog exhaustion.

Constructing the bpftrace SYN Cookie Monitor

The following bpftrace script hooks into SYN cookie generation and SYN-ACK retransmissions to track fallback occurrences per destination port:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing SYN cookie fallback activations and retransmissions... Hit Ctrl-C to finish.
");
}

kprobe:cookie_v4_init_sequence
{
  $skb = (struct sk_buff *)arg0;
  printf("[%s] SYN Cookie Generated (Backlog Overflow Detected)
",
    strftime("%H:%M:%S", nsecs));
  @syn_cookies_generated = count();
}

kprobe:inet_rtx_syn_ack
{
  $req = (struct request_sock *)arg1;
  $daddr = ntop($req->ir_rmt_addr);
  $retries = $req->num_retrans;

  @synack_retries_under_cookie[$daddr] = count();
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- SYN Cookie & Retransmission Summary ---
");
  print(@syn_cookies_generated);
  print(@synack_retries_under_cookie);
  clear(@syn_cookies_generated);
  clear(@synack_retries_under_cookie);
}

END
{
  clear(@syn_cookies_generated);
  clear(@synack_retries_under_cookie);
}

Running and Interpreting Kernel Measurements

Execute the script on your target server using administrative root capabilities:

sudo bpftrace synack_cookies.bt

The diagnostic console outputs real-time fallback notifications:

[16:51:10] SYN Cookie Generated (Backlog Overflow Detected)
16:51:25 --- SYN Cookie & Retransmission Summary ---
@syn_cookies_generated: 48
@synack_retries_under_cookie[198.51.100.91]: 12

Remediation Strategies and Kernel Tuning

When recurrent SYN cookie fallbacks are identified:

• Increase SYN Backlog: Scale net.ipv4.tcp_max_syn_backlog = 8192 or higher to accommodate legitimate connection bursts without cookie fallback.
• Scale Socket Listen Backlog: Ensure application server listen limits match kernel settings by tuning net.core.somaxconn = 8192.
• Inspect Firewall Rate Limits: Confirm that network border firewalls are not dropping SYN-ACK responses with spoofed cookie ISNs.