深入理解TCP协议及其源代码

#dpkg-reconfigure wireshark-common
“Should non-superusers be able to capture packages?”

//选择Yes （默认是no）

sudo vim /etc/group

gdb server gdb client

tcp_set_state(sk, TCP_SYN_SENT);
    err = inet_hash_connect(tcp_death_row, sk);
    if (err)
        goto failure;

    sk_set_txhash(sk);

    rt = ip_route_newports(fl4, rt, orig_sport, orig_dport,
                   inet->inet_sport, inet->inet_dport, sk);
    if (IS_ERR(rt)) {
        err = PTR_ERR(rt);
        rt = NULL;
        goto failure;

/* Build a SYN and send it off. */
int tcp_connect(struct sock *sk)
{
...
     /* Reserve space for headers. */
      skb_reserve(buff, MAX_TCP_HEADER);

       tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
      tp->retrans_stamp = tcp_time_stamp;
      tcp_connect_queue_skb(sk, buff);
       tcp_ecn_send_syn(sk, buff);

    /* Send off SYN; include data in Fast Open. */
      err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
            tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
...
      /* Timer for repeating the SYN until an answer. */
   inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
...

/*
 * This will accept the next outstanding connection.
 */
struct sock *inet_csk_accept(struct sock *sk, int flags, int *err)
{


 /* We need to make sure that this socket is listening,
 * and that it has something pending.
 */
 error = -EINVAL;
 if (sk->sk_state != TCP_LISTEN)
 goto out_err;

 /* Find already established connection */
 if (reqsk_queue_empty(queue)) {
 long timeo = sock_rcvtimeo(sk, flags & O_NONBLOCK);
...
 error = inet_csk_wait_for_connect(sk, timeo);
...
}
EXPORT_SYMBOL(inet_csk_accept);

/*
* Wait for an incoming connection, avoid race conditions. This must be called
 * with the socket locked.
 */245static int inet_csk_wait_for_connect(struct sock *sk, long timeo)
{
... //无限for循环，一旦有请求则跳出
 for (;;) {
 prepare_to_wait_exclusive(sk_sleep(sk), &wait,
 TASK_INTERRUPTIBLE);
...
}

static const struct net_protocol tcp_protocol = {
 .early_demux = tcp_v4_early_demux,
 .handler = tcp_v4_rcv,
 .err_handler = tcp_v4_err,
 .no_policy = 1,
 .netns_ok = 1,
 .icmp_strict_tag_validation = 1,
};
...
static int __init inet_init(void)
{
...
 /*
 * Add all the base protocols.
 */

 if (inet_add_protocol(&icmp_protocol, IPPROTO_ICMP) < 0)
 pr_crit("%s: Cannot add ICMP protocol
", __func__);
 if (inet_add_protocol(&udp_protocol, IPPROTO_UDP) < 0)
 pr_crit("%s: Cannot add UDP protocol
", __func__);
if (inet_add_protocol(&tcp_protocol, IPPROTO_TCP) < 0)
 pr_crit("%s: Cannot add TCP protocol
", __func__);
#ifdef CONFIG_IP_MULTICAST1719 if (inet_add_protocol(&igmp_protocol, IPPROTO_IGMP) < 0)
 pr_crit("%s: Cannot add IGMP protocol
", __func__);
#endif
...
}

/* The socket must have it's spinlock held when we get
 * here.
 *
 * We have a potential double-lock case here, so even when
 * doing backlog processing we use the BH locking scheme.
 * This is because we cannot sleep with the original spinlock
 * held.
 */
int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
{
...
 if (sk->sk_state == TCP_LISTEN) {
 struct sock *nsk = tcp_v4_hnd_req(sk, skb);
 if (!nsk)
 goto discard;

 if (nsk != sk) {
...
 return 0;
 }
 } else
 sock_rps_save_rxhash(sk, skb);

 if (tcp_rcv_state_process(sk, skb, tcp_hdr(skb), skb->len)) {
 rsk = sk;
 goto reset;
 }
...
}
EXPORT_SYMBOL(tcp_v4_do_rcv);

static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,const struct tcphdr *th, unsigned int len)
{

..
 tcp_send_ack(sk);
...
}