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. Under the RFC 5925 specification, the standard 16-bit TCP Checksum field must be replaced with zeros prior to calculating the Message Authentication Code (MAC). If hardware checksum offload engines or intermediate NIC ring buffers calculate the MAC over an already populated checksum value, the remote Linux kernel rejects the frame. When the kernel encounters checksum zero-fill 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 checksum zero-fill anomalies using eBPF and bpftrace.
The Checksum Zero-Fill Verification Dynamics in the Linux Kernel
In Linux, tcp_ao_verify_header() copies the incoming packet header into an internal buffer and zeroes out the 16-bit th->check field before computing the HMAC or CMAC digest. If an unaligned hardware offload engine failed to perform this normalization before transmitting the initial segment, verification fails 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 zeroed checksum field |
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 checksum zero-fill 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 checksum zero-fill 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 checksum zero-fill discrepancy across peers.