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 high-throughput transport environments, sequence number wrap-around is managed via Sequence Number Extension (SNE). When SNE state slips between communicating peers or during active SYN-ACK retransmissions, the Linux kernel 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 SNE drift using eBPF and bpftrace.
The SNE Mechanism and SYN-ACK Retransmission Dynamics
TCP-AO calculates a Message Authentication Code (MAC) across each TCP segment using a Traffic Key derived from Master Key Tuples (MKT). To prevent replay attacks across sequence rollover boundaries, the MAC includes an extended 64-bit sequence counter consisting of the wire sequence number appended to the high-order 32-bit SNE counter.
During initial connection handshakes or passive connection recoveries, if a SYN-ACK must be retransmitted while the peer has already incremented its inbound SNE window, MAC verification fails. The kernel discards the segment at tcp_ao_verify_header() without returning an ICMP unreachable error.
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 SNE state |
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 SNE 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 SNE 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 an SNE synchronisation mismatch across peers.