最近在linux做服务器开发的时候，发现了一个现象：服务器在启动的时候调用了 connect 函数，因为连接了一个不可用的端口，导致connect最后报出了 “Connection timed out” 的错误。但是这中间过了六十多秒的时间。
为何会等待这么长的时间才超时呢？这个时间又在哪里设置？
《UNIX网络编程（第一卷）——套接口 API 和 X/Open 传输接口 API》一书的4.3节有写到：
对于TCP套接口来说，函数 connect 激发TCP的三路握手过程，且仅在链接成功建立或出错时才返回，返回的错误可能有如下几种情况：
1. 如果TCP客户没有收到SYN分节的响应，则返回ETIMEDOUT。例如在4.4BSD中，当调用函数 connect 时，发出一个SYN，若无响应，等待6秒之后再发一个；若仍无响应，24秒钟之后再发一个。若总共等待了75秒钟之后仍未响应，则返回错误...
从书中可以看到 connect 建立TCP链接的过程中，会发送SYN包，如果没有收到SYN包的回包，内核会多次发送SYN包，并且每次重试的间隔会逐渐增加，避免发送太多的SYN包影响网络。
在CentOS上，这个重试次数是可以设置的：
$ sysctl net.ipv4 | grep tcp
net.ipv4.tcp_timestamps =
net.ipv4.tcp_window_scaling =
net.ipv4.tcp_sack =
net.ipv4.tcp_retrans_collapse =
net.ipv4.tcp_syn_retries = 5
net.ipv4.tcp_synack_retries =
net.ipv4.tcp_max_orphans =
net.ipv4.tcp_max_tw_buckets =
net.ipv4.tcp_keepalive_time =
net.ipv4.tcp_keepalive_probes =
net.ipv4.tcp_keepalive_intvl =
net.ipv4.tcp_retries1 =
net.ipv4.tcp_retries2 =
net.ipv4.tcp_fin_timeout =
net.ipv4.tcp_syncookies =
net.ipv4.tcp_tw_recycle =
net.ipv4.tcp_abort_on_overflow =
net.ipv4.tcp_stdurg =
net.ipv4.tcp_rfc1337 =
net.ipv4.tcp_max_syn_backlog =
net.ipv4.tcp_orphan_retries =
net.ipv4.tcp_fack =
net.ipv4.tcp_reordering =
net.ipv4.tcp_ecn =
net.ipv4.tcp_dsack =
net.ipv4.tcp_mem =
net.ipv4.tcp_wmem =
net.ipv4.tcp_rmem =
net.ipv4.tcp_app_win =
net.ipv4.tcp_adv_win_scale =
net.ipv4.tcp_tw_reuse =
net.ipv4.tcp_frto =
net.ipv4.tcp_frto_response =
net.ipv4.tcp_low_latency =
net.ipv4.tcp_no_metrics_save =
net.ipv4.tcp_moderate_rcvbuf =
net.ipv4.tcp_tso_win_divisor =
net.ipv4.tcp_congestion_control = cubic
net.ipv4.tcp_abc =
net.ipv4.tcp_mtu_probing =
net.ipv4.tcp_base_mss =
net.ipv4.tcp_workaround_signed_windows =
net.ipv4.tcp_challenge_ack_limit =
net.ipv4.tcp_limit_output_bytes =
net.ipv4.tcp_dma_copybreak =
net.ipv4.tcp_slow_start_after_idle =
net.ipv4.tcp_available_congestion_control = cubic reno
net.ipv4.tcp_allowed_congestion_control = cubic reno
net.ipv4.tcp_max_ssthresh =
net.ipv4.tcp_thin_linear_timeouts =
net.ipv4.tcp_thin_dupack =
net.ipv4.tcp_min_tso_segs =
net.ipv4.tcp_invalid_ratelimit =
其中的 net.ipv4.tcp_syn_retries 选项控制着SYN的重试次数，可以通过如下命令来查看和设置：
$ sysctl net.ipv4.tcp_syn_retries #查看
net.ipv4.tcp_syn_retries =
$ sudo sysctl -w net.ipv4.tcp_syn_retries=1 #设置
net.ipv4.tcp_syn_retries =
下面用一个简单的程序，来验证各种次数下的connect超时时间：
#include <iostream>
#include <sys/socket.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <errno.h>
#include <string.h>
#include <arpa/inet.h> long long GetCurrentMSecond()
{
struct timeval tv;
gettimeofday(&tv, NULL);
return tv.tv_sec * + tv.tv_usec / ;
} int main()
{
int fd = ;
struct sockaddr_in addr; fd = socket(AF_INET, SOCK_STREAM, ); socklen_t bufSize = * ;
int retCode = setsockopt(fd, SOL_SOCKET, SO_RCVBUF, &bufSize, sizeof(bufSize));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr("192.168.207.128");
addr.sin_port = htons(); //连接一个不用的端口，以保证会触发超时
long long llBeginTime = GetCurrentMSecond();
if (connect(fd, (struct sockaddr*)&addr, sizeof(addr)) == -)
{
long long llEndTime = GetCurrentMSecond();
std::cout << "connect failed, errno: " << errno << ", error: " << strerror(errno)
<< ", cost time: " << llEndTime - llBeginTime << std::endl;
return ;
}
std::cout << "connect success" << std::endl;
}
通过设置不同的重试次数，探究各种重试次数下的time out时间：
$ g++ connect.cpp -o main
$ sudo sysctl -w net.ipv4.tcp_syn_retries=1
net.ipv4.tcp_syn_retries = 1
$ ./main
connect failed, errno: 110, error: Connection timed out, cost time: 3000
$ sudo sysctl -w net.ipv4.tcp_syn_retries=2
net.ipv4.tcp_syn_retries = 2
$ ./main
connect failed, errno: 110, error: Connection timed out, cost time: 7000
| 重试次数 | 超时时间（单位：毫秒） |
| 1 | 3000 |
| 2 | 7000 |
| 3 | 14999 |
| 4 | 31000 |
| 5 | 63001 |
| 6 | 126999 |
从表格中可以看到，当前设置重试次数为5的时候，超时时间是63秒，可以通过修改重试次数的方式，来改变connect的超时时间。