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

In carrier-grade routing infrastructure running BGP over Linux, TCP MD5 Signature Options (RFC 2385) protect peering sessions from spoofed packets. When MD5 password mismatches or corrupted authentication headers occur during connection negotiation, the kernel drops the packet without returning a diagnostic error, leading to repetitive SYN-ACK retransmissions. Tracing these security-related handshake drops via tcpi_retrans_synack provides vital visibility into misconfigured peering keys before sessions experience persistent flap outages.

Mechanisms of TCP MD5 Verification Drops

When a BGP peering socket is configured with TCP_MD5SIG, incoming segments must contain valid MD5 checksum options. If the client or peer router transmits an incorrect key digest, the kernel executes tcp_v4_inbound_md5_hash(), which returns an authentication failure, causing the kernel to drop the segment silently. The server continues retransmitting SYN-ACK packets, incrementing tcpi_retrans_synack until reaching retry limits. Tracing kernel MD5 verification routines unmasks silent drops instantly.

Constructing the bpftrace MD5 Verification Monitor

The following bpftrace script captures failed MD5 digest evaluations and correlates them with SYN-ACK retransmissions:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing TCP MD5 signature drops during handshake... Hit Ctrl-C to finish.
");
}

kprobe:tcp_v4_inbound_md5_hash
{
  $sk = (struct sock *)arg0;
  $skb = (struct sk_buff *)arg1;

  @md5_checks_total = count();
}

kretprobe:tcp_v4_inbound_md5_hash
{
  if (retval != 0) {
    printf("[%s] TCP MD5 Signature Mismatch / Drop Detected
",
      strftime("%H:%M:%S", nsecs));
    @md5_mismatches = count();
  }
}

kprobe:inet_rtx_syn_ack
{
  $req = (struct request_sock *)arg1;
  $daddr = ntop($req->ir_rmt_addr);
  $retries = $req->num_retrans;

  @synack_retries_under_md5[$daddr] = count();
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- TCP MD5 Verification & Retransmission Summary ---
");
  print(@md5_mismatches);
  print(@synack_retries_under_md5);
  clear(@md5_mismatches);
  clear(@synack_retries_under_md5);
}

END
{
  clear(@md5_checks_total);
  clear(@md5_mismatches);
  clear(@synack_retries_under_md5);
}

Running and Interpreting Kernel Measurements

Execute the script on your target server using administrative root capabilities:

sudo bpftrace synack_md5_drops.bt

The diagnostic console outputs real-time MD5 drop notifications:

[16:56:10] TCP MD5 Signature Mismatch / Drop Detected
16:56:25 --- TCP MD5 Verification & Retransmission Summary ---
@md5_mismatches: 14
@synack_retries_under_md5[198.51.100.99]: 6

Remediation Strategies and Configuration Auditing

When recurrent MD5 signature handshake drops are identified:

• Verify BGP Peering Passwords: Confirm that cryptographic passwords match identically across both autonomous system endpoints without extraneous whitespace.
• Audit TCP-AO Migration: Consider upgrading from legacy TCP MD5 (RFC 2385) to TCP Authentication Option (RFC 5925) for modern key management.
• Check Kernel Sysctl Flags: Ensure net.ipv4.tcp_l3mdev_accept = 1 is configured when peering across isolated VRF routing domains.