How to Trace TCP Socket TCP_INFO tcpi_retrans_synack Abort Resets in Linux Using eBPF

In high-throughput Linux network infrastructure, subtle connection failures frequently occur when client connections abort prematurely during handshake negotiation. When a client closes the socket or terminates unexpectedly before completing the TCP three-way handshake, the kernel sends a TCP RST packet to tear down the half-open connection. Tracing these abort resets alongside tcpi_retrans_synack attempts provides critical visibility into whether failures stem from upstream timeouts or client application crashes.

Understanding TCP Handshake Abort Resets

During the connection establishment phase, an incoming SYN packet causes the kernel to create an inet_request_sock and issue a SYN-ACK. If the client sends an unexpected RST or if the server’s retransmission timer exceeds the maximum retries configured by tcp_synack_retries, the kernel executes tcp_v4_send_reset(). The kernel increments tcpi_retrans_synack for each retry prior to abortion. Tracing socket state at the moment of reset helps isolate network drops from client application errors.

Constructing the bpftrace Abort Reset Monitor

The following bpftrace script hooks into tcp_v4_send_reset and inet_rtx_syn_ack to correlate SYN-ACK retries with connection abort events:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing SYN-ACK abort resets... Hit Ctrl-C to finish.
");
}

kprobe:tcp_v4_send_reset
{
  $sk = (struct sock *)arg0;
  $skb = (struct sk_buff *)arg1;

  printf("[%s] TCP RST Sent during Handshake
",
    strftime("%H:%M:%S", nsecs));
  @abort_resets_total = count();
}

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

  if ($retries >= 2) {
    @synack_high_retries[$daddr] = count();
  }
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- SYN-ACK Abort Reset Summary ---
");
  print(@abort_resets_total);
  print(@synack_high_retries);
  clear(@abort_resets_total);
  clear(@synack_high_retries);
}

END
{
  clear(@abort_resets_total);
  clear(@synack_high_retries);
}

Running and Interpreting Kernel Measurements

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

sudo bpftrace synack_abort_resets.bt

The diagnostic console outputs real-time abort notifications:

[16:53:10] TCP RST Sent during Handshake
16:53:25 --- SYN-ACK Abort Reset Summary ---
@abort_resets_total: 18
@synack_high_retries[198.51.100.94]: 5

Remediation Strategies and Kernel Tuning

When recurrent handshake abort resets are identified:

• Verify Client Timeout Thresholds: Confirm that client connection timeouts exceed initial SYN-ACK RTO intervals to prevent premature client-side aborts.
• Tune SYN-ACK Retries: Review net.ipv4.tcp_synack_retries to balance resource retention against connection recovery time.
• Inspect Firewall Reset Rules: Verify that middleboxes or intrusion prevention systems (IPS) are not injecting spoofed TCP resets during handshakes.