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Filename : tcpdrop.bt
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#!/usr/bin/env bpftrace
/*
 * tcpdrop.bt   Trace TCP kernel-dropped packets/segments.
 *              For Linux, uses bpftrace and eBPF.
 *
 * USAGE: tcpdrop.bt
 *
 * This is a bpftrace version of the bcc tool of the same name.
 *
 * This provides information such as packet details, socket state, and kernel
 * stack trace for packets/segments that were dropped via kfree_skb.
 * It cannot show tcp flags.
 *
 * For Linux 5.17+ (see tools/old for script for lower versions).
 *
 * Copyright (c) 2018 Dale Hamel.
 * Licensed under the Apache License, Version 2.0 (the "License")
 *
 * 23-Nov-2018	Dale Hamel	created this.
 * 01-Oct-2022	Rong Tao	use tracepoint:skb:kfree_skb
 */

#ifndef BPFTRACE_HAVE_BTF
#include <linux/socket.h>
#include <net/sock.h>
#else
#include <sys/socket.h>
#endif

BEGIN
{
  printf("Tracing tcp drops. Hit Ctrl-C to end.\n");
  printf("%-8s %-8s %-16s %-21s %-21s %-8s\n", "TIME", "PID", "COMM", "SADDR:SPORT", "DADDR:DPORT", "STATE");

  // See https://github.com/torvalds/linux/blob/master/include/net/tcp_states.h
  @tcp_states[1] = "ESTABLISHED";
  @tcp_states[2] = "SYN_SENT";
  @tcp_states[3] = "SYN_RECV";
  @tcp_states[4] = "FIN_WAIT1";
  @tcp_states[5] = "FIN_WAIT2";
  @tcp_states[6] = "TIME_WAIT";
  @tcp_states[7] = "CLOSE";
  @tcp_states[8] = "CLOSE_WAIT";
  @tcp_states[9] = "LAST_ACK";
  @tcp_states[10] = "LISTEN";
  @tcp_states[11] = "CLOSING";
  @tcp_states[12] = "NEW_SYN_RECV";
}

tracepoint:skb:kfree_skb
{
  $reason = args.reason;
  $skb = (struct sk_buff *)args.skbaddr;
  $sk = ((struct sock *) $skb->sk);
  $inet_family = $sk->__sk_common.skc_family;

  if ($reason > SKB_DROP_REASON_NOT_SPECIFIED &&
      ($inet_family == AF_INET || $inet_family == AF_INET6)) {
    if ($inet_family == AF_INET) {
      $daddr = ntop($sk->__sk_common.skc_daddr);
      $saddr = ntop($sk->__sk_common.skc_rcv_saddr);
    } else {
      $daddr = ntop($sk->__sk_common.skc_v6_daddr.in6_u.u6_addr8);
      $saddr = ntop($sk->__sk_common.skc_v6_rcv_saddr.in6_u.u6_addr8);
    }
    $lport = $sk->__sk_common.skc_num;
    $dport = $sk->__sk_common.skc_dport;

    // Destination port is big endian, it must be flipped
    $dport = bswap($dport);

    $state = $sk->__sk_common.skc_state;
    $statestr = @tcp_states[$state];

    time("%H:%M:%S ");
    printf("%-8d %-16s ", pid, comm);
    printf("%39s:%-6d %39s:%-6d %-10s\n", $saddr, $lport, $daddr, $dport, $statestr);
    printf("%s\n", kstack);
  }
}

END
{
  clear(@tcp_states);
}