The TCP Authentication Option (TCP-AO, RFC 5925) provides cryptographic segment protection for mission-critical routing sessions such as BGP peering across wide area networks. In complex multi-vendor backbones, subtle Key Derivation Function (KDF) algorithm discrepancies—such as HMAC-SHA-1 versus AES-128-CMAC KDF configurations—can cause initial handshakes to stall. When the Linux kernel encounters non-matching KDF outputs during passive SYN-ACK verification, it silently discards segments, incrementing internal retransmission statistics while logging minimal network-layer diagnostics.
Diagnosing these stealthy authentication failures requires low-overhead kernel tracing. By extracting metrics from struct tcp_sock and correlating the tcpi_retrans_synack counter from TCP_INFO, network engineers can pinpoint transient KDF drift using eBPF and bpftrace.
The KDF Verification Path in the Linux Kernel
TCP-AO derives per-connection traffic keys from configured Master Key Tuples (MKT). During socket initialisation, tcp_ao_compute_key() applies the designated KDF algorithm to generate traffic encryption keys. If the remote peer expects HMAC-SHA-1-96 while the local kernel generates keys via AES-128-CMAC-96, every inbound handshake packet produces an authentication failure at tcp_ao_verify_header().
The key kernel parameters and monitoring offsets are outlined below:
| Kernel Symbol | Source Location | Data Structure Offset | Diagnostic Purpose |
|---|---|---|---|
tcp_ao_verify_header |
net/ipv4/tcp_ao.c |
struct tcp_ao_key * |
Validates AO digest against current KDF traffic keys |
tcp_get_info |
net/ipv4/tcp.c |
tcpi_retrans_synack |
Reports cumulative SYN-ACK retry count |
tcp_v4_rcv |
net/ipv4/tcp_ipv4.c |
skb->cb |
Traces inbound segment discard points |
Attaching bpftrace Probes to TCP-AO Verification
Run the following bpftrace script to monitor passive handshake retries and highlight KDF verification failures in real time:
sudo bpftrace -e '
#include <net/sock.h>
#include <linux/tcp.h>
kprobe:tcp_ao_verify_header {
$sk = (struct sock *)arg0;
$tp = (struct tcp_sock *)arg0;
$dport = $sk->__sk_common.skc_dport;
// Filter for BGP port 179
if ($dport == 179 || $sk->__sk_common.skc_num == 179) {
@monitored_synacks[$sk] = $tp->retrans_out;
}
}
kretprobe:tcp_ao_verify_header {
if (retval != 0) {
time("%H:%M:%S ");
printf("TCP-AO KDF verification failed with code %d
", retval);
}
}
END {
clear(@monitored_synacks);
}'Reading tcpi_retrans_synack via ss Diagnostic Tooling
Complement kernel probes by inspecting active socket stats using ss:
# Query active TCP-AO enabled sockets with extended TCP_INFO metrics
ss -ti 'sport = :179 or dport = :179'Examine the output for high retrans figures accompanied by stagnant RTT estimators. When correlated with eBPF tcp_ao_verify_header failure return codes, this confirms a KDF configuration mismatch across peers.