通过 man 可以看到，connect 对于 udp, tcp 是不同的
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int
connect(int socket, const struct sockaddr *address, socklen_t address_len);
DESCRIPTION
The parameter socket is a socket. If it is of type SOCK_DGRAM, this call specifies the peer with which the socket is to be asso-
ciated; this address is that to which datagrams are to be sent, and the only address from which datagrams are to be received. If
the socket is of type SOCK_STREAM, this call attempts to make a connection to another socket. The other socket is specified by
address, which is an address in the communications space of the socket
udp 只是指定要连接的地址，而 tcp 要尝试三次握手建连，那具体流程呢？
Syscall 入口
和其它接口一样，系统调用 SYSCALL_DEFINE3, 最终调用 __sys_connect
SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr,
int, addrlen)
{
return __sys_connect(fd, uservaddr, addrlen);
}
整体实现
int __sys_connect(int fd, struct sockaddr __user *uservaddr, int addrlen)
{
struct socket *sock;
struct sockaddr_storage address;
int err, fput_needed;
sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (!sock)
goto out;
err = move_addr_to_kernel(uservaddr, addrlen, &address);
if (err < 0)
goto out_put;
err =
security_socket_connect(sock, (struct sockaddr *)&address, addrlen);
if (err)
goto out_put;
err = sock->ops->connect(sock, (struct sockaddr *)&address, addrlen,
sock->file->f_flags);
out_put:
fput_light(sock->file, fput_needed);
out:
return err;
}
参数是本机 socket fd, 对端要连接 socket 的地址 uservaddr. 先用 sockfd_lookup_light 根据 fd 找到 socket，然后有一个 move_addr_to_kernel 将用户空间地址拷贝到内核的过程，最后函数指针 ops->connect 回调具体协义动作，tcp 对应 inet_stream_ops.inet_stream_connect, udp 对应 inet_dgram_ops.inet_dgram_connect
udp实现
int inet_dgram_connect(struct socket *sock, struct sockaddr *uaddr,
int addr_len, int flags)
{
struct sock *sk = sock->sk;
int err;
if (addr_len < sizeof(uaddr->sa_family))
return -EINVAL;
if (uaddr->sa_family == AF_UNSPEC)
return sk->sk_prot->disconnect(sk, flags);
if (BPF_CGROUP_PRE_CONNECT_ENABLED(sk)) {
err = sk->sk_prot->pre_connect(sk, uaddr, addr_len);
if (err)
return err;
}
if (!inet_sk(sk)->inet_num && inet_autobind(sk))
return -EAGAIN;
return sk->sk_prot->connect(sk, uaddr, addr_len);
}
addr_len 判断长度，如果协义族是 AF_UNSPEC，直接断开连接。还有关于 BPF 的，暂时忽略。最后调用 sk_prot->connect 对应函数为 ip4_datagram_connect
int __ip4_datagram_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
{
struct inet_sock *inet = inet_sk(sk);
struct sockaddr_in *usin = (struct sockaddr_in *) uaddr;
struct flowi4 *fl4;
struct rtable *rt;
__be32 saddr;
int oif;
int err;
......
sk_dst_reset(sk);
oif = sk->sk_bound_dev_if;
saddr = inet->inet_saddr;
if (ipv4_is_multicast(usin->sin_addr.s_addr)) {
if (!oif || netif_index_is_l3_master(sock_net(sk), oif))
oif = inet->mc_index;
if (!saddr)
saddr = inet->mc_addr;
}
fl4 = &inet->cork.fl.u.ip4;
rt = ip_route_connect(fl4, usin->sin_addr.s_addr, saddr,
RT_CONN_FLAGS(sk), oif,
sk->sk_protocol,
inet->inet_sport, usin->sin_port, sk);
if (IS_ERR(rt)) {
err = PTR_ERR(rt);
if (err == -ENETUNREACH)
IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
goto out;
}
if ((rt->rt_flags & RTCF_BROADCAST) && !sock_flag(sk, SOCK_BROADCAST)) {
ip_rt_put(rt);
err = -EACCES;
goto out;
}
if (!inet->inet_saddr)
inet->inet_saddr = fl4->saddr; /* Update source address */
if (!inet->inet_rcv_saddr) {
inet->inet_rcv_saddr = fl4->saddr;
if (sk->sk_prot->rehash)
sk->sk_prot->rehash(sk);
}
inet->inet_daddr = fl4->daddr;
inet->inet_dport = usin->sin_port;
sk->sk_state = TCP_ESTABLISHED;
sk_set_txhash(sk);
inet->inet_id = jiffies;
sk_dst_set(sk, &rt->dst);
err = 0;
out:
return err;
}
-
sk_dst_reset清空路由 cache -
ip_route_connect查找路由相关，如果找不到报错返回，并更新统计 - 如果 saddr 未定指，还要根据路由信息来确定源地址
- 更新 socket 状态为 ESTABLISHED, 最后
sk_dst_set设置下一跳路由
从这可以看到，udp connect 不会真正的建连
tcp实现
tcp_v4_connect
/* This will initiate an outgoing connection. */
int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
{
struct sockaddr_in *usin = (struct sockaddr_in *)uaddr;
struct inet_sock *inet = inet_sk(sk);
struct tcp_sock *tp = tcp_sk(sk);
__be16 orig_sport, orig_dport;
__be32 daddr, nexthop;
struct flowi4 *fl4;
struct rtable *rt;
int err;
struct ip_options_rcu *inet_opt;
struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
nexthop = daddr = usin->sin_addr.s_addr;
inet_opt = rcu_dereference_protected(inet->inet_opt,
lockdep_sock_is_held(sk));
if (inet_opt && inet_opt->opt.srr) {
if (!daddr)
return -EINVAL;
nexthop = inet_opt->opt.faddr;
}
orig_sport = inet->inet_sport;
orig_dport = usin->sin_port;
fl4 = &inet->cork.fl.u.ip4;
rt = ip_route_connect(fl4, nexthop, inet->inet_saddr,
RT_CONN_FLAGS(sk), sk->sk_bound_dev_if,
IPPROTO_TCP,
orig_sport, orig_dport, sk);
if (IS_ERR(rt)) {
err = PTR_ERR(rt);
if (err == -ENETUNREACH)
IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
return err;
}
if (rt->rt_flags & (RTCF_MULTICAST | RTCF_BROADCAST)) {
ip_rt_put(rt);
return -ENETUNREACH;
}
if (!inet_opt || !inet_opt->opt.srr)
daddr = fl4->daddr;
if (!inet->inet_saddr)
inet->inet_saddr = fl4->saddr;
sk_rcv_saddr_set(sk, inet->inet_saddr);
if (tp->rx_opt.ts_recent_stamp && inet->inet_daddr != daddr) {
/* Reset inherited state */
tp->rx_opt.ts_recent = 0;
tp->rx_opt.ts_recent_stamp = 0;
if (likely(!tp->repair))
tp->write_seq = 0;
}
inet->inet_dport = usin->sin_port;
sk_daddr_set(sk, daddr);
inet_csk(sk)->icsk_ext_hdr_len = 0;
if (inet_opt)
inet_csk(sk)->icsk_ext_hdr_len = inet_opt->opt.optlen;
tp->rx_opt.mss_clamp = TCP_MSS_DEFAULT;
/* Socket identity is still unknown (sport may be zero).
* However we set state to SYN-SENT and not releasing socket
* lock select source port, enter ourselves into the hash tables and
* complete initialization after this.
*/
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;
}
/* OK, now commit destination to socket. */
sk->sk_gso_type = SKB_GSO_TCPV4;
sk_setup_caps(sk, &rt->dst);
rt = NULL;
if (likely(!tp->repair)) {
if (!tp->write_seq)
tp->write_seq = secure_tcp_seq(inet->inet_saddr,
inet->inet_daddr,
inet->inet_sport,
usin->sin_port);
tp->tsoffset = secure_tcp_ts_off(sock_net(sk),
inet->inet_saddr,
inet->inet_daddr);
}
inet->inet_id = tp->write_seq ^ jiffies;
if (tcp_fastopen_defer_connect(sk, &err))
return err;
if (err)
goto failure;
err = tcp_connect(sk);
if (err)
goto failure;
return 0;
}
-
ip_route_connect寻找路由，主要是找到下一跳，如果报错那么返回 -
tcp_set_state将 socket 状态设置为 SYN_SENT -
inet_hash_connect将 socket 扔到 hash 表里，这里面会判断有没有指定源 port，如果没指定自动选择一个，涉及 reuseport, 暂时不看，以后有机会再分析 - 下面还有路由的，还有一段关于 SKB_GSO 分片的，暂时也不看了
- write_seq 初始化序列号，这里可以看到是安全的，具体算法也忽略
-
fastopen也忽略，最后调用tcp_connect来干活
tcp_connect
/* Build a SYN and send it off. */
int tcp_connect(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *buff;
int err;
tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB, 0, NULL);
if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
return -EHOSTUNREACH; /* Routing failure or similar. */
tcp_connect_init(sk);
if (unlikely(tp->repair)) {
tcp_finish_connect(sk, NULL);
return 0;
}
buff = sk_stream_alloc_skb(sk, 0, sk->sk_allocation, true);
if (unlikely(!buff))
return -ENOBUFS;
tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
tcp_mstamp_refresh(tp);
tp->retrans_stamp = tcp_time_stamp(tp);
tcp_connect_queue_skb(sk, buff);
tcp_ecn_send_syn(sk, buff);
tcp_rbtree_insert(&sk->tcp_rtx_queue, 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);
if (err == -ECONNREFUSED)
return err;
/* We change tp->snd_nxt after the tcp_transmit_skb() call
* in order to make this packet get counted in tcpOutSegs.
*/
tp->snd_nxt = tp->write_seq;
tp->pushed_seq = tp->write_seq;
buff = tcp_send_head(sk);
if (unlikely(buff)) {
tp->snd_nxt = TCP_SKB_CB(buff)->seq;
tp->pushed_seq = TCP_SKB_CB(buff)->seq;
}
TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);
/* 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);
return 0;
}
-
tcp_connect_init初始，设置滑动窗口，超时时间杂七杂八的 -
sk_stream_alloc_skb生成 skb，这个很重要，四层协义栈的数据都写在这里发送出去。调用tcp_init_nondata_skb初始化 sdk - 判断是否是 fastopen，当前没有数据，调用
tcp_transmit_skb发送数据，实际也是发到队列里 - 最后
inet_csk_reset_xmit_timer注册 timer, 如果发送 SYN 超时继续重发
小结
暂时只看大致的流程，一些初始化的细节暂时没细看。一句话总结，udp connect 不会发送数据，状态直接设置 ESTABLISHED，而 tcp connect 要发送 SYN 包，并且设置各种初始窗口，序列号等等，状态标记为 SYN_SENT
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