linux下connect超時時間探究

最近在linux作服務器開發的時候，發現了一個現象：服務器在啓動的時候調用了 connect 函數，由於鏈接了一個不可用的端口，致使connect最後報出了「Connection timed out」的錯誤。可是這中間過了六十多秒的時間。linux

爲什麼會等待這麼長的時間才超時呢？這個時間又在哪裏設置？ios

《UNIX網絡編程（第一卷）——套接口 API 和 X/Open 傳輸接口 API》一書的4.3節有寫到：編程

　　對於TCP套接口來講，函數 connect 激發TCP的三路握手過程，且僅在連接成功創建或出錯時才返回，返回的錯誤可能有以下幾種狀況：windows

　　1. 若是TCP客戶沒有收到SYN分節的響應，則返回ETIMEDOUT。例如在4.4BSD中，當調用函數 connect 時，發出一個SYN，若無響應，等待6秒以後再發一個；若仍無響應，24秒鐘以後再發一個。若總共等待了75秒鐘以後仍未響應，則返回錯誤...服務器

從書中能夠看到 connect 創建TCP連接的過程當中，會發送SYN包，若是沒有收到SYN包的回包，內核會屢次發送SYN包，而且每次重試的間隔會逐漸增長，避免發送太多的SYN包影響網絡。cookie

在CentOS上，這個重試次數是能夠設置的：網絡

$ sysctl net.ipv4 | grep tcp net.ipv4.tcp_timestamps = 1 net.ipv4.tcp_window_scaling = 1 net.ipv4.tcp_sack = 1 net.ipv4.tcp_retrans_collapse = 1 net.ipv4.tcp_syn_retries = 5 net.ipv4.tcp_synack_retries = 5 net.ipv4.tcp_max_orphans = 262144 net.ipv4.tcp_max_tw_buckets = 262144 net.ipv4.tcp_keepalive_time = 7200 net.ipv4.tcp_keepalive_probes = 9 net.ipv4.tcp_keepalive_intvl = 75 net.ipv4.tcp_retries1 = 3 net.ipv4.tcp_retries2 = 15 net.ipv4.tcp_fin_timeout = 60 net.ipv4.tcp_syncookies = 1 net.ipv4.tcp_tw_recycle = 0 net.ipv4.tcp_abort_on_overflow = 0 net.ipv4.tcp_stdurg = 0 net.ipv4.tcp_rfc1337 = 0 net.ipv4.tcp_max_syn_backlog = 2048 net.ipv4.tcp_orphan_retries = 0 net.ipv4.tcp_fack = 1 net.ipv4.tcp_reordering = 3 net.ipv4.tcp_ecn = 2 net.ipv4.tcp_dsack = 1 net.ipv4.tcp_mem = 639168       852224  1278336 net.ipv4.tcp_wmem = 4096        16384   4194304 net.ipv4.tcp_rmem = 4096        87380   4194304 net.ipv4.tcp_app_win = 31 net.ipv4.tcp_adv_win_scale = 2 net.ipv4.tcp_tw_reuse = 0 net.ipv4.tcp_frto = 2 net.ipv4.tcp_frto_response = 0 net.ipv4.tcp_low_latency = 0 net.ipv4.tcp_no_metrics_save = 0 net.ipv4.tcp_moderate_rcvbuf = 1 net.ipv4.tcp_tso_win_divisor = 3 net.ipv4.tcp_congestion_control = cubic net.ipv4.tcp_abc = 0 net.ipv4.tcp_mtu_probing = 0 net.ipv4.tcp_base_mss = 512 net.ipv4.tcp_workaround_signed_windows = 0 net.ipv4.tcp_challenge_ack_limit = 1000 net.ipv4.tcp_limit_output_bytes = 262144 net.ipv4.tcp_dma_copybreak = 4096 net.ipv4.tcp_slow_start_after_idle = 1 net.ipv4.tcp_available_congestion_control = cubic reno net.ipv4.tcp_allowed_congestion_control = cubic reno net.ipv4.tcp_max_ssthresh = 0 net.ipv4.tcp_thin_linear_timeouts = 0 net.ipv4.tcp_thin_dupack = 0 net.ipv4.tcp_min_tso_segs = 2 net.ipv4.tcp_invalid_ratelimit = 500

其中的 net.ipv4.tcp_syn_retries 選項控制着SYN的重試次數，能夠經過以下命令來查看和設置：app

$ sysctl net.ipv4.tcp_syn_retries 　　　　　　#查看 net.ipv4.tcp_syn_retries = 5 $ sudo sysctl -w net.ipv4.tcp_syn_retries=1 #設置 net.ipv4.tcp_syn_retries = 1

下面用一個簡單的程序，來驗證各類次數下的connect超時時間：socket

#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 * 1000 + tv.tv_usec / 1000; } int main() { int fd = 0; struct sockaddr_in addr; fd = socket(AF_INET, SOCK_STREAM, 0); socklen_t bufSize = 128 * 1024; 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(13500); //鏈接一個不用的端口，以保證會觸發超時
    long long llBeginTime = GetCurrentMSecond(); if (connect(fd, (struct sockaddr*)&addr, sizeof(addr)) == -1) { long long llEndTime = GetCurrentMSecond(); std::cout << "connect failed, errno: " << errno << ", error: " << strerror(errno) << ", cost time: " << llEndTime - llBeginTime << std::endl; return 0; } std::cout << "connect success" << std::endl; }

經過設置不一樣的重試次數，探究各類重試次數下的time out時間：tcp

$ 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

重試次數	超時時間（單位：毫秒）
1	3000
2	7000
3	14999
4	31000
5	63001
6	126999

從表格中能夠看到，當前設置重試次數爲5的時候，超時時間是63秒，能夠經過修改重試次數的方式，來改變connect的超時時間。