How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Key Derivation Function Drops in Linux Using eBPF

The TCP Authentication Option (TCP-AO, RFC 5925) utilizes a cryptographic Key Derivation Function (KDF) to generate independent traffic keys for incoming and outgoing connection directions from a Master Key Tuple (MKT). However, if the endpoints disagree on the KDF algorithm parameters (such as HMAC-SHA-1 vs AES-128-CMAC KDF variants) or if initial sequence numbers (ISN) drift during derivation calculations, the derived traffic keys diverge. Inbound SYN or ACK packets fail verification, causing the Linux kernel to drop segments silently and escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate TCP-AO KDF drop bottlenecks in real time.

Kernel Mechanics of TCP-AO Key Derivation

Inside modern Linux kernels (Linux 6.7+), TCP-AO initiates traffic key derivation in tcp_ao_calc_key(). The kernel combines the MKT pre-shared secret, connection 4-tuple, and the initial sequence numbers exchanged during the handshake:

If middleboxes alter ISN sequences or if the peer executes an incompatible KDF iteration, tcp_ao_verify_hash() computes an invalid MAC digest. Consequently, listening sockets retransmit SYN-ACK segments repeatedly until timeout exhaustion. Using eBPF attached to kernel key derivation routines gives infrastructure engineers exact per-socket visibility into cryptographic derivation failures.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP-AO KDF execution and correlates derivation failures with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_tcp_ao_kdf.bt - Trace Linux TCP-AO Key Derivation Function drops */

#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 (TCP-AO KDF Failure Candidate)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

kprobe:tcp_ao_calc_key
{
    $sk = (struct sock *)arg0;
    @ao_kdf_derivations = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP-AO KDF Activity (5s Window) ---
");
    print(@ao_kdf_derivations);
    print(@synack_retransmits);
    clear(@ao_kdf_derivations);
    clear(@synack_retransmits);
}

Running the Trace on Production Linux Systems

Deploy the probe with root privileges on the target server:

# 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_tcp_ao_kdf.bt

Kernel Optimization and Parameter Tuning

To inspect TCP-AO socket states and verify KDF parameters:

# Inspect socket TCP-AO configuration via iproute2
ss -ta --ao

# Check kernel SNMP network statistics
nstat -az TcpExtTCPAO*

By pairing eBPF kernel instrumentation with iproute2 diagnostics, network engineers can eliminate elusive SYN-ACK stalls while preserving strict cryptographic session security.