How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Master Key Tuple Lifecycle Expiry Drops in Linux Using eBPF

The TCP Authentication Option (TCP-AO, RFC 5925) manages session security via Master Key Tuples (MKT) that define cryptographic pre-shared secrets, lifetime validity windows, and active state flags. In automated key-rotation environments, if an active MKT reaches its expiration timestamp before network administrators configure a successor key, the Linux kernel invalidates the key. Inbound packets matching the expired tuple fail MAC verification, leading the kernel to drop segments silently and escalating tcpi_retrans_synack. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate TCP-AO MKT lifecycle expiration drop bottlenecks in real time.

Kernel Mechanics of MKT Expiration and Invalidation

Inside modern Linux kernels (Linux 6.7+), MKT lifecycle checks occur in tcp_ao_mkt_is_active(). When an incoming SYN or ACK arrives, the kernel evaluates whether the matching MKT has exceeded its configured lifetime or has been marked inactive:

If the key is expired, the kernel rejects the tuple, leaving the socket with no valid cryptographic configuration. Consequently, the listening socket repeatedly retransmits SYN-ACK segments until timeout exhaustion. Using eBPF attached to kernel MKT validation functions gives infrastructure engineers exact per-socket visibility into key expiration stalls.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP-AO MKT lifecycle checks and correlates expiration events with SYN-ACK retransmits:

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

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

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP-AO MKT Lifecycle Activity (5s Window) ---
");
    print(@mkt_active_checks);
    print(@synack_retransmits);
    clear(@mkt_active_checks);
    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_mkt_expiry.bt

Kernel Optimization and Parameter Tuning

To inspect TCP-AO socket states and verify MKT expiration timestamps:

# 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.