tcp_send_active_reset()
    -> skb = alloc_skb(MAX_TCP_HEADER, priority);
    -> tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk), TCPHDR_ACK | TCPHDR_RST);
    -> tcp_transmit_skb()

tcp_v4_send_reset()
        if (th->ack) {
                // 这里对应的就是上图中为何出现Seq==Ack
                rep.th.seq = th->ack_seq; 
        } else {
                // 极小概率，如果出现，那么RST包的就没有Seq序列号
                rep.th.ack = 1;                                                   
                rep.th.ack_seq = htonl(ntohl(th->seq) + th->syn + th->fin +    
                                       skb->len - (th->doff << 2));               
        }

sudo bpftrace -e 'k:tcp_send_active_reset { @[kstack()] = count(); }'

sudo bpftrace -e 'k:tcp_v4_send_reset { @[kstack()] = count(); }'

   CPU 0                           CPU 1
   -----                           -----
tcp_v4_rcv()                  syn_recv_sock()
                            inet_ehash_insert()
                            -> sk_nulls_del_node_init_rcu(osk)
__inet_lookup_established()
                            -> __sk_nulls_add_node_rcu(sk, list)

// iptables A port -> B port
iptables ... -p tcp --port 1111 -j REDIRECT --to-ports 80
iptables ... -p tcp --port 1112 -j REDIRECT --to-ports 80

1. saddr:12345 -> daddr:80 // 正常连接
2. saddr:12345 -> daddr:1112 -> daddr:80 // NAT参与转化
（对内核细节不感兴趣的同学可以跳过此段）

// 2.2.2.2是去敏后的server端ip地址，另外两个是client的ip
sk info: 1.1.1.1:1111 <-> 2.2.2.2:80 // 我们可以知道真实的socket的建立是使用了80端口
skb info: 1.1.1.2:2222 <-> 2.2.2.2:1112 // 异常的skb未成功将1112端口转化为80端口

#!/usr/bin/env python

from __future__ import print_function
from bcc import BPF
import argparse
from time import strftime
from socket import inet_ntop, AF_INET, AF_INET6
from struct import pack
import ctypes as ct
from time import sleep
from bcc import tcp

# arguments
examples = """examples:
    ./tcpdrop           # trace kernel TCP drops
"""
parser = argparse.ArgumentParser(
    description="Trace TCP drops by the kernel",
    formatter_class=argparse.RawDescriptionHelpFormatter,
    epilog=examples)
parser.add_argument("--ebpf", action="store_true",
    help=argparse.SUPPRESS)
args = parser.parse_args()
debug = 0

# define BPF program
bpf_text = """
#include <uapi/linux/ptrace.h>
#include <uapi/linux/tcp.h>
#include <uapi/linux/ip.h>
#include <net/sock.h>
#include <bcc/proto.h>

BPF_STACK_TRACE(stack_traces, 1024);

struct ipv4_data_t {
    u32 pid;
    u64 is_sknull;
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u8 state;
    u8 tcpflags;
    u32 stack_id;
};
BPF_PERF_OUTPUT(ipv4_events);

struct active_data_t {
    u32 pid;
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 stack_id;
};
BPF_PERF_OUTPUT(active_events);

static struct tcphdr *skb_to_tcphdr(const struct sk_buff *skb)
{
    // unstable API. verify logic in tcp_hdr() -> skb_transport_header().
    return (struct tcphdr *)(skb->head + skb->transport_header);
}

static inline struct iphdr *skb_to_iphdr(const struct sk_buff *skb)
{
    // unstable API. verify logic in ip_hdr() -> skb_network_header().
    return (struct iphdr *)(skb->head + skb->network_header);
}

// from include/net/tcp.h:
#ifndef tcp_flag_byte
#define tcp_flag_byte(th) (((u_int8_t *)th)[13])
#endif

int trace_tcp_v4_send_reset(struct pt_regs *ctx, struct sock *sk, struct sk_buff *skb)
{
    u8 is_sk_null = sk ? 0 : 1;
    u8 state = sk ? (u8)sk->__sk_common.skc_state : 1;
    u32 pid = bpf_get_current_pid_tgid();
    struct iphdr *ip = skb_to_iphdr(skb);
    u32 daddr = ip->daddr;
    u32 saddr = ip->saddr;

    // pull in details from the packet headers and the sock struct
    u16 family = sk->__sk_common.skc_family;
    u16 sport = 0, dport = 0;
    struct tcphdr *tcp = skb_to_tcphdr(skb);
    u8 tcpflags = ((u_int8_t *)tcp)[13];
    sport = tcp->source;
    dport = tcp->dest;
    sport = ntohs(sport);
    dport = ntohs(dport);

    if (family == AF_INET &&
        (saddr == 16777343 && daddr == 16777343) &&
        (sport == 8004 || dport == 8004)) {
        struct ipv4_data_t data4 = {};
        data4.pid = pid;
        data4.saddr = saddr;
        data4.daddr = daddr;
        data4.dport = dport;
        data4.sport = sport;
        data4.state = state;
        data4.tcpflags = tcpflags;
        data4.stack_id = stack_traces.get_stackid(ctx, 0);
        ipv4_events.perf_submit(ctx, &data4, sizeof(data4));

    }

    return 0;
}

int trace_tcp_send_active_reset(struct pt_regs *ctx, struct sock *sk, unsigned int priority)
{
    u32 pid = bpf_get_current_pid_tgid() >> 32;
    u32 saddr = 0, daddr = 0;
    u16 family = AF_INET;
    u16 sport = 0, dport = 0;

    // sport is not right
    sport = sk->__sk_common.skc_num;
    dport = sk->__sk_common.skc_dport;
    dport = ntohs(dport);

    saddr = sk->__sk_common.skc_rcv_saddr;
    daddr = sk->__sk_common.skc_daddr;

    if (family == AF_INET && (saddr == 16777343 && daddr == 16777343)) {
        struct active_data_t data4 = {};
        data4.pid = pid;
        data4.saddr = saddr;
        data4.daddr = daddr;
        data4.dport = dport;
        data4.sport = sport;
        data4.stack_id = stack_traces.get_stackid(ctx, 0);
        active_events.perf_submit(ctx, &data4, sizeof(data4));
    }

    return 0;
}
"""

if debug or args.ebpf:
    print(bpf_text)
    if args.ebpf:
        exit()

# event data
class Data_ipv4(ct.Structure):
    _fields_ = [
        ("pid", ct.c_uint),
        ("is_sknull", ct.c_ulonglong),
        ("saddr", ct.c_uint),
        ("daddr", ct.c_uint),
        ("sport", ct.c_ushort),
        ("dport", ct.c_ushort),
        ("state", ct.c_ubyte),
        ("tcpflags", ct.c_ubyte),
        ("stack_id", ct.c_ulong)
    ]

class Data_active(ct.Structure):
    _fields_ = [
        ("pid", ct.c_uint),
        ("saddr", ct.c_uint),
        ("daddr", ct.c_uint),
        ("sport", ct.c_ushort),
        ("dport", ct.c_ushort),
        ("stack_id", ct.c_ulong)
    ]

# process event
def print_ipv4_event(cpu, data, size):
    event = ct.cast(data, ct.POINTER(Data_ipv4)).contents
    if event.is_sknull is 1:
        print("%-8s %-7d %-20s > %-20s %s (%s)" % (
            strftime("%H:%M:%S"), event.pid,
            "%s:%d" % (inet_ntop(AF_INET, pack('I', event.saddr)), event.sport),
            "%s:%s" % (inet_ntop(AF_INET, pack('I', event.daddr)), event.dport),
            "sk-is-null", tcp.flags2str(event.tcpflags)))
    else:
        print("%-8s %-7d %-20s > %-20s %s (%s)" % (
            strftime("%H:%M:%S"), event.pid,
            "%s:%d" % (inet_ntop(AF_INET, pack('I', event.saddr)), event.sport),
            "%s:%s" % (inet_ntop(AF_INET, pack('I', event.daddr)), event.dport),
            tcp.tcpstate[event.state], tcp.flags2str(event.tcpflags)))
    for addr in stack_traces.walk(event.stack_id):
        sym = b.ksym(addr, show_offset=True)
        print("\t%s" % sym)
    print("")

def print_active_event(cpu, data, size):
    event = ct.cast(data, ct.POINTER(Data_active)).contents
    print("%-8s %-7d %-20s > %-20s" % (
        strftime("%H:%M:%S"), event.pid,
        "%s:%d" % (inet_ntop(AF_INET, pack('I', event.saddr)), event.sport),
        "%s:%d" % (inet_ntop(AF_INET, pack('I', event.daddr)), event.dport)))

    for addr in stack_traces.walk(event.stack_id):
        sym = b.ksym(addr, show_offset=True)
        print("\t%s" % sym)
    print("")

# initialize BPF
b = BPF(text=bpf_text)
if b.get_kprobe_functions(b"tcp_v4_send_reset"):
    b.attach_kprobe(event="tcp_v4_send_reset", fn_name="trace_tcp_v4_send_reset")
else:
    print("ERROR: tcp_drop() kernel function not found or traceable. "
        "Older kernel versions not supported.")
    exit()

if b.get_kprobe_functions(b"tcp_send_active_reset"):
    b.attach_kprobe(event="tcp_send_active_reset", fn_name="trace_tcp_send_active_reset")
else:
    print("ERROR: tcp_v4_send_reset() kernel function")
    exit()

stack_traces = b.get_table("stack_traces")

# header
print("%-8s %-6s %-2s %-20s > %-20s %s (%s)" % ("TIME", "PID", "IP",
    "SADDR:SPORT", "DADDR:DPORT", "STATE", "FLAGS"))

# read events
b["ipv4_events"].open_perf_buffer(print_ipv4_event)
#b["active_events"].open_perf_buffer(print_active_event)
while 1:
    try:
        b.perf_buffer_poll()
    except KeyboardInterrupt:
        exit()