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 heterogenous carrier networks, certain intermediate security gateways manipulate TCP flags (such as ECN bits, CWR, or URG flags) during transit. If flag modifications violate the zero-masking specifications required by RFC 5925, MAC verification fails. When the Linux kernel encounters header flag discrepancies 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 header flag anomalies using eBPF and bpftrace.
The Header Flag Masking Dynamics in the Linux Kernel
RFC 5925 mandates that specific TCP header fields—including checksum, flags subject to modification in transit, and dynamic options—must be zeroed out prior to computing the cryptographic digest. In Linux, tcp_ao_verify_header() copies the segment header to a scratch buffer, applies the zero-mask, and feeds the normalized block into the crypto subsystem. Discrepancies between peer masking implementations result in verification failure.
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 masked header flags |
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 header flag 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 header flag 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 asymmetric header flag discrepancy across peers.