How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Authentication Option Drops in Linux Using eBPF

The TCP Authentication Option (TCP-AO, RFC 5925) provides cryptographic message authentication for BGP peering sessions and mission-critical TCP connections, replacing legacy TCP MD5 signatures. However, when authentication keys mismatch, Master Key Tuple (MKT) IDs expire, or middleboxes corrupt TCP-AO option headers, the Linux kernel silently discards inbound SYN or ACK segments. When these cryptographic rejections occur during connection handshakes, sockets stall into exponential SYN-ACK backoff, escalating tcpi_retrans_synack. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate TCP-AO cryptographic drop bottlenecks in real time.

Kernel Mechanics of TCP-AO Verification

Inside modern Linux kernels (Linux 6.7+), TCP-AO verification occurs inside tcp_ao_verify_hash(). When an incoming SYN or ACK arrives, the kernel extracts the Key ID and validates the MAC using the configured cryptographic key.

If key negotiation fails or option headers are mangled, the kernel discards the segment immediately without sending resets. Consequently, listening sockets retransmit SYN-ACK segments repeatedly until timeout exhaustion, incrementing tcpi_retrans_synack in struct tcp_info. Using eBPF attached to kernel TCP-AO verification routines gives infrastructure engineers exact per-socket visibility into cryptographic handshake drops.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP-AO verification and correlates failed handshakes with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_tcp_ao_drops.bt - Trace Linux TCP-AO authentication 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 Drop Suspect)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

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

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP-AO Authentication Activity (5s Window) ---
");
    print(@ao_verifications);
    print(@synack_retransmits);
    clear(@ao_verifications);
    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_drops.bt

Kernel Optimization and Parameter Tuning

To inspect TCP-AO socket states and active Master Key Tuples:

# 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.