How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP SYN-ACK ACK Spoof Filtering in Linux Using eBPF

In high-density Linux web clusters, stateful edge firewalls and kernel netfilter rules actively monitor TCP handshakes to mitigate blind spoofing and off-path connection hijacking. However, when middleboxes modify TCP sequence numbers or when asymmetric routing configurations misalign connection states, return ACK packets fail kernel sequence validation. Consequently, the kernel silently discards the invalid ACK, forcing the listening socket into exponential SYN-ACK backoff and incrementing tcpi_retrans_synack. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate handshake spoof filtering and packet drop bottlenecks in real time.

Kernel Mechanics of TCP ACK Validation and Filtering

During the final phase of the standard three-way handshake, the Linux kernel processes the client’s returning ACK inside tcp_check_req() and tcp_v4_syn_recv_sock(). The kernel strictly validates that the acknowledgement number matches the socket’s transmitted initial sequence number (ISN) plus one (rcv_nxt == req->rcv_isn + 1).

If an upstream NAT gateway, proxy, or spoofed packet presents an out-of-window ACK sequence, the validation fails. Rather than establishing the socket, the kernel retains the request sock in its half-open queue, retransmitting SYN-ACK packets until timeout exhaustion. Extended Berkeley Packet Filters allow systems engineers to monitor exact sequence disparities inside kernel tracepoints without overhead.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP request socket validation and correlates failed ACKs with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_ack_spoof_filter.bt - Trace Linux TCP ACK validation failures */

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

tracepoint:tcp:tcp_retransmit_synack
{
    $sk = (struct sock *)args->skaddr;
    $saddr = ntop(args->saddr);
    $daddr = ntop(args->daddr);
    $sport = args->sport;
    $dport = args->dport;

    printf("SYN-ACK Retransmit: %s:%d -> %s:%d (Handshake Incomplete)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

kprobe:tcp_check_req
{
    $sk = (struct sock *)arg0;
    $skb = (struct sk_buff *)arg1;
    $th = (struct tcphdr *)($skb->head + $skb->transport_header);

    @ack_checks = count();
}

kretprobe:tcp_check_req
{
    if (retval == 0) {
        @invalid_acks_rejected = count();
    }
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP Handshake ACK Validation Activity (5s) ---
");
    print(@ack_checks);
    print(@invalid_acks_rejected);
    print(@synack_retransmits);
    clear(@ack_checks);
    clear(@invalid_acks_rejected);
    clear(@synack_retransmits);
}

Executing the Kernel Trace

Deploy the probe with root privileges on your Linux host:

# Install bpftrace and kernel dependencies
sudo apt-get update && sudo apt-get install -y bpftrace linux-headers-$(uname -r)

# Execute the trace
sudo bpftrace trace_ack_spoof_filter.bt

When illegitimate or out-of-order ACKs reach the server, @invalid_acks_rejected increments in sync with @synack_retransmits, exposing middlebox sequence rewriting or network transmission flaws.

Kernel Optimization and Parameter Tuning

To evaluate if system firewall rules or conntrack tables are dropping invalid handshakes, inspect Linux network statistics:

# Check invalid connection state drops in iptables/nftables
sudo iptables -nvL -t filter | grep -E "INVALID|DROP"

# Verify netfilter conntrack TCP timeout loose settings
sysctl net.netfilter.nf_conntrack_tcp_loose

Ensuring that nf_conntrack_tcp_loose = 1 allows conntrack to track connections even when packets arrive out of window, preventing premature segment drops during asymmetrical network handshakes.