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 transport stacks interacting with custom protocol extensions, intermediate bridges may inadvertently set non-zero values in the 3-bit reserved field of the TCP header (offset 12). RFC 5925 explicitly requires that reserved bits are zeroed prior to generating and verifying the Message Authentication Code (MAC). Discrepancies between peer implementations lead to silent discard of packets.
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 reserved bit anomalies using eBPF and bpftrace.
The Reserved Bit Masking Dynamics in the Linux Kernel
Under Linux, tcp_ao_verify_header() copies the TCP header into a scratch buffer and applies a bitmask to ensure that reserved bits (th->res1) are zeroed before feeding the data into the cryptographic hashing algorithm. 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 reserved bits |
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 reserved bit 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 reserved bits 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 reserved bit discrepancy across peers.