TCP_KEEPALIVE選項只是一個開關，Linux中默認的Keepalive的選項如下：
$sudo sysctl -a | grep keepalive net.ipv4.tcp_keepalive_time = 7200 net.ipv4.tcp_keepalive_probes = 9 net.ipv4.tcp_keepalive_intvl = 75
上文中的keepalive選項表示如果一個連接上7200s后沒有任何數據發送，則設置了這個選項的本端向對端發送keepalive保活報文，它會有如下三種結果：
- 對端回復ACK。則本端TCP認為該連接依然存活。繼續等7200s后再發送keepalive報文。
- 對端回復RESET。說明對端進程已經重啟，本端的應用程序應該關閉該連接。
- 沒有對端的任何回復。則本端做重試，如果重試9次（前后重試間隔為75秒）仍然不可達，則向應用程序返回錯誤信息，ETIMEOUT（無任何應答）或EHOST
如果應用程序向改變keepalive的默認行為，該怎么辦呢？答案就是利用 TCP_KEEPIDLE、TCP_KEEPINTVL、TCP_KEEPCNT這幾個TCP選項，首先看看如何使用：
int setKeepAlive(int fd, int interval)
{
int val = 1;
if (setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &val, sizeof(val)) == -1) {
printf("setsockopt SO_KEEPALIVE: %s", strerror(errno));
return -1;
}
/* Send first probe after `interval' seconds. */
val = interval;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &val, sizeof(val)) < 0) {
printf("setsockopt TCP_KEEPIDLE: %s\n", strerror(errno));
return -1;
}
/* Send next probes after the specified interval. Note that we set the
* delay as interval / 3, as we send three probes before detecting
* an error (see the next setsockopt call). */
val = interval/3;
if (val == 0) val = 1;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPINTVL, &val, sizeof(val)) < 0) {
printf("setsockopt TCP_KEEPINTVL: %s\n", strerror(errno));
return -1;
}
/* Consider the socket in error state after three we send three ACK
* probes without getting a reply. */
val = 3;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPCNT, &val, sizeof(val)) < 0) {
printf("setsockopt TCP_KEEPCNT: %s\n", strerror(errno));
return -1;
}
return 0;
}
- TCP_KEEPDILE 設置連接上如果沒有數據發送的話，多久后發送keepalive探測分組，單位是秒
- TCP_KEEPINTVL 前后兩次探測之間的時間間隔，單位是秒
- TCP_KEEPCNT 關閉一個非活躍連接之前的最大重試次數
用tcpdump抓包就可以看到設置了上面選項的那端的詳細行為。