How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP MD5 Signature Verification Drops in Linux Using eBPF

TCP MD5 signature authentication (RFC 2385) is widely used to protect Border Gateway Protocol (BGP) peering connections and enterprise routing sessions against spoofing attacks. The Linux kernel verifies MD5 digests in the TCP header options of incoming packets against pre-shared keys configured via TCP_MD5SIG socket options. When keys mismatch, MD5 options are stripped by middleboxes, or sequence numbers drift, the kernel silently discards the segment. During BGP session handshakes, these rejections cause listening sockets to repeatedly retransmit SYN-ACK segments, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate TCP MD5 signature drops in real time.

Kernel Mechanics of TCP MD5 Signature Validation

Inside the Linux network stack, MD5 signature checks are executed inside tcp_v4_inbound_md5_hash() and tcp_md5_do_lookup(). When an incoming SYN or ACK arrives for an MD5-protected socket, the kernel computes the expected MD5 digest using the socket’s secret key and compares it against the packet header:

If the hash does not match or if an expected MD5 option is missing, the kernel discards the segment without generating resets. Consequently, the listening socket enters exponential SYN-ACK backoff. Extended Berkeley Packet Filters allow systems engineers to monitor exact sequence disparities inside kernel tracepoints without overhead.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP MD5 validation and correlates verification failures with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_tcp_md5_drops.bt - Trace Linux TCP MD5 signature verification drops */

#include <net/sock.h>
#include <linux/tcp.h>

tracepoint:tcp:tcp_retransmit_synack
{
    $sk = (struct sock *)args->skaddr;
    $saddr = ntop(args->saddr);
    $daddr = ntop(args->daddr);
    $sport = args->sport;
    $dport = args->dport;

    printf("SYN-ACK Retransmit: %s:%d -> %s:%d (TCP MD5 Failure Candidate)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

kprobe:tcp_v4_inbound_md5_hash
{
    $sk = (struct sock *)arg0;
    @md5_checks = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP MD5 Verification Activity (5s Window) ---
");
    print(@md5_checks);
    print(@synack_retransmits);
    clear(@md5_checks);
    clear(@synack_retransmits);
}

Running the Trace on Production Linux Systems

Deploy the probe with root privileges on the target server:

# Install bpftrace and kernel dependencies
sudo apt-get update && sudo apt-get install -y bpftrace linux-headers-$(uname -r)

# Execute the trace
sudo bpftrace trace_tcp_md5_drops.bt

Kernel Optimization and Parameter Tuning

To inspect MD5 drop counters and evaluate socket configurations:

# Inspect kernel MD5 drop statistics
nstat -az TcpExtTCPMD5*

# Verify BGP socket configuration via ss
ss -ti '( dport = :179 or sport = :179 )'

By pairing eBPF kernel instrumentation with iproute2 diagnostics, network engineers can eliminate elusive SYN-ACK stalls while preserving strict cryptographic session security.