How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Key ID Mismatch Drops in Linux Using eBPF

The TCP Authentication Option (TCP-AO, RFC 5925) provides hitless key rollover for critical infrastructure peering sessions using designated Key IDs inside Master Key Tuples (MKT). During cryptographic key rotation or administrative configuration updates, if the sender and receiver disagree on the active Key ID, the receiver fails to locate a matching MKT inside its security database. The Linux kernel silently drops the inbound SYN or ACK segment, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate TCP-AO Key ID mismatch drop bottlenecks in real time.

Kernel Mechanics of TCP-AO Key ID Matching

Inside modern Linux kernels (Linux 6.7+), TCP-AO extracts the Key ID from incoming option headers inside tcp_ao_do_lookup(). The kernel iterates through configured MKTs matching the socket’s peer address and compares the Key ID:

If no matching active MKT corresponds to the incoming Key ID, the kernel returns null, triggering silent segment discard inside tcp_ao_verify_hash(). Consequently, listening sockets retransmit SYN-ACK segments repeatedly until timeout exhaustion. Using eBPF attached to kernel lookup routines gives infrastructure engineers exact per-socket visibility into key configuration discrepancies.

Authoring the Diagnostic Probe with bpftrace

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

#!/usr/bin/env bpftrace
/* trace_tcp_ao_key_id.bt - Trace Linux TCP-AO Key ID lookup 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 (TCP-AO Key ID Mismatch Candidate)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

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

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP-AO Key Lookup Activity (5s Window) ---
");
    print(@ao_lookups);
    print(@synack_retransmits);
    clear(@ao_lookups);
    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_key_id.bt

Kernel Optimization and Parameter Tuning

To inspect TCP-AO socket states and verify active Key IDs across MKTs:

# 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.